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Wolfgang Schueller, high-rise systems

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The Design of Building Structures (Vol.1, Vol. 2), rev. ed., PDF eBook by Wolfgang Schueller, 2016: last chapter

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Wolfgang Schueller, high-rise systems

  1. 1. THE DESIGN OF BUILDING STRUCTURES WOLFGANG SCHUELLER
  2. 2. Sec. 10.1 / General Introduction to High-Rise Structure Systems 817 Today. a new generation of skyscrapers is rising in Asia. The l-‘$76-ft high twin Petronas Towers in the Malaysian capital of Kuala Lumpur will be 22 feet taller than Chicago's Sears Tower when finished in 1996: Cesar Pelli is the architect and Thom- ton-Tomasetti are the structural engineers. The tallest reinforced-concrete building structure in the world is currently the I228-ft-high. 78-story Central Plaza office tower with a triangular-shaped plan in Hong Kong ( 1992 ); architects were Ng Chun Man and Associates and consulting engineers Ovc Arup Partners. 10.1 GENERAL INTRODUCTION TO HIGH-RISE STRUCTURE SYSTEMS A building structure can be visualized as consisting of horizontal planes or floor fram- ing and the supponing vertical planes of walls and~"or frames. The horizontal planes tie the vertical planes together to achieve a box effect and a certain degree of compact- ness. It is obvious that a slender. tall tower building must be a compact. three-dimen- sional closed structure where the entire system acts as a unit. The tubular, core- interactive. and staggered truss buildings are typical examples of three-dimensional structures. On the other hand, a massive building block only needs some stiff. stabiliz- ing elements that give lateral support to the rest of the building. In this sense, the build- ing structure represents an open system where separate vertical planar structure systems. such as solid walls. rigid frames. and braced frames. are located at various places and fonn stand-alone systems that provide lateral stability. Every building consists of the load-bearing structure and the nonload-bearing portion. The main load-bearing structure, in turn. is subdivided into the gravity struc- ture. which carries only the gravity loads. and the lateralflirce-re. ti. sting . 'lrm. 'tm'e. which supports gravity forces but also must provide stability to the building. The con- dition where the lateral bracing only resists horizontal forces but does not can'y gravity loads, with the exception of its own weight. is considered a secondwjv smu. -mre . Fail- ure of secondary members is not as critical as that for main members. where an immc-_ diatc collapse of a building portion may occur depending on the redundancy of the structure. The nonload-bearing structural building elements include wind bracing as well as the membranes and skins. that is, the curtains. ceilings, and partitions that cover the structure and subdivide space. The lateral force-resisting structure in a building tower may be concentrated entirely in the central core. for instance when an optimal view and thus a light perim- eter structure is required. Conversely. rather then hiding the lateral force-resisting stntcturc in the interior, it may be exposed and fomi the perimeter structure. as for tubes. The structure represents an assembly system that consists of components and their linkages. The basic elements are lines (columns. beams). grids (floor framework. fmmcsl. surfaces (slabs. walls. plates). spatial units (cells. tubes), and any combination of these. The interaction or degree of continuity between these elements depends on the type oflinkage (hinged. semirigid, or rigid). These basic components can be com- bined in an endless variety to form a building. Before discussing fundamental concepts of structure behavior. typical structure systems are introduced. but purely from a geometrical point of view. Although build- ings arc three dimensional. their support structures can often be treated from a behav- ioral point of view as an assembly oftwo-dimensional vertical planar elements in each major direction of the building. In other words. structures can usually be subdivided into a few simpler assemblies. since structural elements are rarely placed randomly in plan. The most common high-rise strttctttrc systems are identified in Fig. 10.2. They are shown simply as planar. two-dimensional structures. although they may act in combination with each other and. in context of the buildin". may fonn spatial
  3. 3. 818 Chap.10 I High Rise Building Structures Sll. '’l’l*It‘-'5 ION (. ‘0.‘ll’l'l-I55 I OH (j(J. "lli 1 l‘€: '~. l" i (l. ‘»' (ZHRE : . l')l"lYl"(ICF. R l FR_~‘aI‘lE. L E ‘. "lEIREl»lDEE ; . ()L"l'Rl. GGER i-. ':lf. LVVAIFVALVVA I-2:‘ I>A“'4I : ~t VALVVALVVAE lllllll-Illlll LVVALVVALVYA Illllll-I Ill VALVPZLVVAE lllllll-I LVVABVALVVA KIEVAIFVAF LVVALVVALVVA V4L‘V4L‘74LV K‘7AL‘7JL"A V4LVV4L‘7Ab3 HEEEVALV suntan’; ;VEL‘.7A; V lllllll-I Illllll-I fIlllll—llIII| lllllll-llllll a 9 S a it E"riRF(')RA'l'E; ‘ t~. '.i«tl. l. ". "Rl'S. ‘iEl) l~. '.-i. l. SU PIE l‘R. ~‘«. ‘lE l"lU~. .-l l-J. -‘tl, I, liliavl Figure 10.2 High-rise structure systems. structures. They range from pure structure systems. such as skeleton and wall con- struction. and systems requiring transfer structures to composite systems and mega- structures. As the buildings increase in height. different structure systems are needed for reasons of efficiency (i. e.. least weight). The following classification ofthc various systems is roughly in accordance with these cffieienc_' considerations. as discussed in the next section. ' Tn'0-rlimensfmml. rI/ 't/ t'ItII'es Bearing wall structures: combinations of single walls and connected walls. cross walls. long walls. two-way walls. stacked boxes Light framing construction (e. g.. wood platform framing for three- to four- story buildings) Skeleton (frame) stntcturcs: rigid frame. braced frame. braced rigid frame. truss. llat slab. Vierendcel wall beam tintcrspatial, bridge type) Connected walls and frames Core structures: they may be considered three-dimensional from a structural point of view. but do not necessarily‘ integrate the entire building
  4. 4. Sec. 10.1 / ‘ General Introduction to High-Rise Structure Systems 819 shape: cantilevered slab. bridge structures (multieore). cores with outrig- gers on top (suspension). at the bottom. and at intenncdiate levels Combinations of these systeirts ' T/ tree-dinrcn. rimml . 'fI‘tl(‘mI'(‘. ‘ Staggered wall beams Cores plus outriggers plus belt trusses: single-. double—. and multiple-outrig- ger systems Tubes: Vierendeel tube. deep-spandrel tube. perforutetl wallfshell tube. trussed tube. tube with belt trusses and head. etc. Nlegttstrueturc: superframe. superdiagonals Hybrid structures Typical combinations of structure systems are the following: Walls + coretst Frames + eorc(s) andfor walls Tube + fr: rme(s) or wall( 5) Tube + core (tube-in-tube) Tube + tube (bundled tubes) Other combinations include the following: Vertical stacking ofstrtrcttrres: connected towers of the bridge type Series ofsuperframes lntcmally braced structures Cellular structures Stayed structures Other mixed systems The selection ol'a structure system is not a simple undertaking. Among other cri- teria, it depends on the overall geometry. the vertical profile. height restrictions. the slenderness (that is, the building height-to-width ratio). and the plan configuration (depth-to-width ratio, degree of regularity. etc. ) and is a function of strength, stiffness. and possibly ductility demands in response to loading conditions. Selection also depends on building base conditions. site conditions. and construction coordination. including prcconstruction and construction time. In order not to give the impression that the preceding pure structure systems are imposed upon the architecture and do not allow any flexibility in the form-giving process. various building cases are presented in Fig. 10.3. They demonstrate some of the endless possible combinations of the struc- ture systems for low- and mid-rise buildings, realizing that the smaller-scale buildings allow more freedom than large-scale towers. It is shown that the structures respond to setbacks. cavities. changing spans. varying story heights, altering bay proportions. sudden changes of stiffness. sloping site conditions. space inclinations, and so on. Most structures are treated as planar. with the exception of the central core-type build- ings with diagonal outriggers connected to the comer columns or that are stabilized by a tensile network along the perimeter. To gain a better understanding ofthe structure systems in Fig. 10.2. they must be seen within the building space: hence their location must be known. For this reason. solid surface elements have been placed into the unifomi beam-column grid of the various plans in Fig. 1.12. They represent the lateral force-resisting structure systems ofwalls, cores. frames. tubes. or any other combination; they may form either planar or spatial assemblies.
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  6. 6. Sec. 10.1 I General Introduction to High-Ftise Structure Systems 821 Considerations of Efficiency A building must resist the primary loads of gravity and lateral force action. With respect to gravity loads. the weight of the structure increases almost linearly with the number of stories. In this context. it is the weight of the vertical structural elements (columns. walls) that increases roughly linearly with height. since their weight is pro- portional to the axial stresses that determine the vertical member sizes. while the weight for each floor remains constant. It is also interesting to note that the floor weight. and hence floor cost. constitutes more than one-half of the cost of the entire structure for buildings not higher than the 30- to 40-story range. However. with an increase in building height and slenderness. the importance oflateral force action rises in a much faster nonlinear fashion as compared to the gravity loads and becomes dom- inant. Therefore, thc cantilever action becomes more critical than the column action. or the rotation M/ S is more important than the axial action N/ A . The section modulus S (or the moment of inertia I ). rather than the cross-sectional area .4. controls the stress and becomes the determinant of form. Hence. the material needed for the resistance of lateral forces increases as the square of the height, that is. at a drastically accelerating rate. For typical medium-rise structures in the 20- to 30-story range. the vertical load resistance nearly offsets the effect of the lateral forces: only about l0‘/ o of the total structure material is needed for lateral force resistance. The late eminent structural engineer Fazlur Khan of SOM has shown that it may be economical to select (for a tall building) a structure system in which the bending stresses. due to lateral force action. do not exceed one-third of the axial gravity stresses so that the effect of the lateral forces can be ignored. At a certain height. however. the lateral sway of a building becomes critical. so that considerations of stiffness. rather than the strength of the structural material. control the design. The degree of stiffness depends on the building shape and the spatial organization of the structure. It has been a challenge to building designers to minimize the effect of the hori- zontal forces by developing optimum lateral force-resisting structure systems. The efficiency of a particular system is directly related to the quantity of material used. at least for steel structures. It is measured as the weight per square foot (psi), that is. the weight of the total building structure divided by the total square footage of the gross floor area. Therefore. optimization ofa structttre for given spatial requirements should yield the maximum strength and stiffness with the least weight. This results in innova- tive structure systems applicable to certain height ranges. Naturally. it must be kept in mind that not only material costs. but also fabrication costs and erection time. must be considered. Fazlur Khan argued. in the mid-19605 (and was supported by the development of computer simulations). that the rigid frame that had dominated high-rise building con- stntction was not the only system : Lssociatcd with tall buildings. Due to a better under- standing of the mechanics of materials and how materials and members interact. the structure could now be treated as a whole or the building form as a three-dimensional unit. Khan later proposed a range of structure systems for office buildings of ordinary proportions and shapes that are appropriate for certain heights. He showed that the weight of steel structure systems range front about 30 psf for a l00-story building to 6 psf for a I0-story structure (Table l0.l ). This effect of scale is known from nature. where animal skeletons become much bulkier with an increase in size. since the weight increases with the cube. while the supporting area only increases with the square. For example. the bones of a mouse make up only approximately 8% of the total mass. in contrast to about 18% for the human body. Furthennore. Frank Lloyd Wright's experiments. as early as the l920s, with slender. ucclikc. concrete cantilever structures that were opposite in nature to the tra- ditional skeleton construction. should not be forgotten. This approach has recently lead
  7. 7. 822 TABLE 10.1 Chap.10 I High Rise Building Structures Typical Weight of Historically lmponant High-Rise Steel Structures Building Heightl Year Stories Width psf Structure System Empire State Building, New York John Hancock Center, Chicago World Trade Center. New York Sears Tower. Chicago Chase Manhattan. New York U. S. Steel Building. Pittsburgh I. D.S. Center. Minneapolis Boston Company Building. Alcoa Building, San Francisco Housing. Brockton, Massachusetts 1931 102 9.3 42.2 Braced rigid frame 1968 100 7.9 29.7 Trussed tube 1972 110 6.9 37.0 Framed tube 1974 109 6.4 33.0 Bundled tubes 1963 60 7.3 55.2 Braced rigid lrame 1971 64 6.3 30.0 Shear walls + outriggers 1971 57 6.1 17.9 + belt trusses Boston 1970 41 4.1 21.0 K-braced tube 1969 26 4.0 26.0 Latticed tube 1971 do 5.1 6.3 _— to buildings more than 50 stories high. where large concrete cores alone provide lateral force resistance. Many of the concrete structures of the 19605 exposed the cores in order to articulate the strength of the three-dimensional support structure (often of the bridge type) and to express the senicing as clearly separated from the served spaces. in this case. the design philosophy is very different from most of the steel skeleton structures of the same period. The efficiency of a concrete structure is evaluated (to a great extent) in tenns of the process of construction. in addition to the quantities of materials used (roughly between 0.5 to 1.0 1'? /fig concrete and reinforcing steel of 2 to 4 psi). in contrast to steel, which considers only the quantity ofmaterial used. Many ofthe tall buildings today no longer represent the pure shapes of the l9(il)s and early l970s. Compound. hybrid building shapes have become fashionable. With the aid of computers. in response to complex geometric shapes. a wealth of new stntc- ture layouts has been made possible, which basically consist of combinations of the fundamental structure systems. in addition. wind tunnel studies have become very accurate in evaluating the response ofa building to wind flow. Despite this increased sophistication of structural analysis and design, however. the fundamental fact should not be overlooked that the material and layout of the structure should not provide the stiffness solely by themselves; the jbrm of the structure must also be searched for with the help of computers so as to ctticiently reduce the use of materials. The new structure systems reflect optimum solutions for given complex building shapes. which include composite structures and the mixed construction ofconcrete and steel. Large buildings are broken down into smaller zones: megaframes give support to the supertall buildings of the future. It is hoped that the architect will use the potential richness of structure to express its power and its purpose as support, rather than just letting the engineer plug the stmcture into a fonn that was derived independently of its nature. 10.2 FORCE FLOW IN HIGH-RISE BUILDING STRUCTURES The horizontal and venical structural building planes must disperse the cxtemal and intcmal forces to the ground. Some basic concepts of vertical and lateral load transmis- sion for various structure systems are discussed in a simplified fashion in Figs. l0.-1 and 4.12. Visualize a gravity load acting on the slab and transferred by the floor framing in bending (Fig. 10.4. top left) to one of the venical structure building planes, which may transmit the load axially directly to the ground. The type and pattem of force flow
  8. 8. Sec. 10.2 I Force Flow in High-Rise Building Structures 823 depend on the arrangement of the vertical structural planes as indicated at the top of Fig. l0.-'4 for two-dimensional structures. The columns may be vertical or inclined. continuous or staggered: they may be evenly distributed or concentrated in the center or along the periphery. possibly to form cores. The path of the force flow maybe con- tinuous along the columns or may be suddenly interrupted and transferred horizontally to another vertical line. The transmission of the loads may be short and direct or long and indirect with a detour. such as for a suspension building. From an efficiency point of view. the vertical loads should be carried along the shortest path possible to the foundations. When columns are inclined. gravity will cause lateral thrust, which increases as the column moves away from the vertical supporting condition. The cases at the bot- tom ofFig. l0.4 indicate that the horizontal floor beams at the top act as ties in tension when the columns lean outward but as struts in compression at the bottom. For a sym- metrical structure. the thrust due to the dead load will self-balance. but the horizontal forces due to asymmetrical live loads must still be resisted. as for an asymmetrical building. vhcrc the weight also causes thrust. Hence not only wind and earthquake. besides the centrifugal outward effect of cars upon curved bridges. but also gravity together with the respective geometry may cause lateral force action on a building. Optimum. frcc ground-level space with a minimum ofcolumns is often required for high-rise buildings. Exainples range from grand entrances and wide lobby spaces. loading docks. and parking aisles to open public plazas. For these conditions the upper building mass must be linked to the ground by using a different structure system. The geometrical pattern of the building structure cannot extend to the foundation walls: it becomes discontinuous and is replaced by another structure system. For preliminary design purposes, the upper and lower structure portions can be analyzed separately. Various transition types are shown in the central part of Fig. l0.4. They range from suspension buildings and lifting an entire building up on frames or stilts. to changing the column spacing to a wider pattcm by using transfer systems within the framed tube grid. The latter may be accomplished. for example. by increasing the spandrel beam sizes toward the main columns or by changing the column sizes in a trcelike fashion, thereby generating a natural, archlike. gradual transition of the loads. The load transi- tion can also be achieved by heavy transfer systems. such as girders. trusses. wall beams. arches (direct or indirect action), or V- and Y-shaped tree columns (two-. and three-forked columns) to collect the columns above. These V-columns are effective in resisting wind and earthquake forces. but unfortunately respond to asymmetrical grav- ity loading with horizontal thrust. as previously discussed. The reader may want to study the behavior of the various inclined column cases in Fig. 10.4. The horizontal forces are transmitted along the floor and roof planes. which act as deep. flat beams spanning between the vertical lateral foree—resisting structures. as described in Fig. 4.12. Once the lateral forces are distributed to the resisting vertical structure planes. these systems must act as venical cantilevers to carry the forces down to the ground. The various two- and three-dimensional structure systems in Fig. 4.13 demonstrate how the overturning moment due to wind action is resisted at the base by different cantilever types. consisting of solid walls or skeleton construc- tion. The skeletons may act either as fle. mrul systems when they are rigid frames (where the members react in bending) or they may be predominately a. v:'aI . sj'. ‘! L'ln. ‘, as for braced frames where the forces are effectively resisted directly in tension and compression. Conflicts of force flow are generated when plan forms or structure systems change. possibly at locations of setbacks often found at the base. top. or intermediate levels of buildings. For example. when a triangular plan changes to an L-shaped base or when a perimeter stnteture such as a tube cannot be continued to the base, an exten- sive horizontal IraI1.j/ er structure is necessary not only to redirect the vertical forces. but also to act as a diapluagm to transfer the horizontal forces (see also Fig. 10.6).
  9. 9. Chap.10 I High Rise Building Structures 824 . l=. ==-z ‘DEEED -_= =I. n . ‘llIliIEll| llEl'JlElHfl3ll mm mm mm :4 II]. {-1 YLFI -w v . ... . I -mun unu--3.--= E5135!! !’ "" mm: ‘ . |_, »II. ; l‘o: ZC"l'l3€'Z§‘. ":’. ':Q 4' {E3335 Jsamza 5 Figure 10.4 Vertical force flow (Reproduced with permission from The Vertical Building Structure. Wolfgang Schuelle Reinhold. ) . .l. __i. .______. ... ._n. _nEmm. W-u__n____§§fi wmw_. m"§§__§&__ . §____§§__a§ r, copyright © 1990 by Van Nostrand
  10. 10. Sec. 10.3 / Introduction to Basic Behavior of High-Rise Building Structures 825 10.3 INTRODUCTION TO BASIC BEHAVIOR OF HIGH-RISE BUILDING STRUCTURES Strength and stiffness are the primary characteristics activated when the building structure responds to the load action described in Fig. I0.5: in areas of strong seismic activity. ductility also becomes an imponant criterion. Considerations of stability and other load-related clTeets have already been briefly introduced elsewhere. in structural analysis, the real structure is replaced with an idealized model. its response to loading is based on analytical theory derived from material properties and member behavior. First. the reaction of the structure to lateral loading is conceptually investigated. It is shown in Fig. -4.|2 that. with respect to horizontal force action. the floor structure acts as a rigid diaphragm supported on elastic vertical elements. These lateral force- resisting elements in Fig. 10.5 are interior eores. core plus walls (Fig. 10.5)). and a perimeter tube (Fig. l0.5m). The cores are of an open or closed type and may be located at the centroid of the building. or their location may be eccentric. The building 2%? ! JOHSIONAL SHEAR BENDING srness t, :v~A i, ,=M5 . -‘Vi I ‘ Figure 10.5 The building response to load action. (Reproduced with permission from The venical Building Structure. Wolfgang Schueller. copyright E31990 by Van Nostrand Reinhold. )
  11. 11. 826 Chap.10 I High Rise Building Structures fonn dctennines where the resultant wind pressure acts. and the anangement of the building masses defines the location of the resultant seismic force. which the core must resist. Since lateral stiffness requirements reduce in the upper portion of the building. the core walls can be dropped off or stepped back at the termination of the low- and mid-rise elevator banks. The tall buildings in Fig. 4. I 6 respond to lateral forces primarily as_/ Iexural can- Ii/ ct'er. s‘ if the resisting structure consists of shear walls or braced frames. The behavior of these systems is controlled by rotation rather than shear: they have a high shear stiff- ness provided by the solid wall material or axial capacity of the diagonals. so the shear deformations can be neglected. Tall buildings act as xhuur cantilevers when the resist- ing elements are rigid frames since the shear can only be resisted by the girders and columns in bending. In this context. the elTcct of rotation (i. e., axial shortening and lengthening of columns) is secondary and maybe ignored for preliminary design pur- poses. The combined action of different structure systems. such as rigid frames together with a braced core (depending on the relative stiffness of each system) may have the appearance of a flat S-curve with a shear-type frame building sitting on top of a flexural cantilever-type structure. In the preceding discussion. it has been assumed that the structure systems were for tall buildings and were of the same height. It is apparent that when the shear wall or braced frame is no longer shallow and slender. as for the extreme case of a horizon- tal panel in a low-rise building. such systems do behave like shear cantilevers and not flexural cantilevers. As known from basic mechanics of materials (Section 2.6), flexural resistance to lateral loads is expressed by the axial bending action M/ S and an average shear action of WA for certain conditions of symmetry (Fig. l0.5d). For the given unifonn lateral loading case, the shear increases linearly toward the base (V we H). while the moment grows much faster following. a second-degree parabola ( M o: H3). Since this special condition of simple bending due to symmetry is often not present because of asymme- try of the resisting structure and. /or the eccentric action of the resultant lateral force. some general concepts of structural behavior ofbending members are briefly reviewed first. Furthcmrorc. thin-walled beam behavior. as for tubular structures. is ignored in this context. In general. to determine the stresses due to pure bending of an unsymmetrical section with no arcs of . r_rnrnren; r requires complex calculations. First. the principal u. re. v. which are always mutually perpendicular and about which the moments of iner- tia are maximum and minimum. respectively, must be located, then the direction ofthe neutral axis has to be found. All these axes together with the cemroidal arcs pass through the centroid of the cross section. For this general condition. the simple bend- ing fonnula f, , = Mc/ I = M/ S, which applies only to symmetrical bending. cannot be used! ln addition. the loads tnust act through the shear center or center oftwist. which is located at the intersection of the shear a. res. in order to not generate torsion in addi- tion to unsymmetrical bending. Therefore. this shear center must be located: it does not coincide with the centroid ofthe cross section. We may conclude that. when the load is applied at the centroid. the member will twist as it bends. Lack of symmetry results in eccentric loads. unsymmetrical bending. and torsion! Fortunately. cross sections usually have a certain degree of symmetry (i. e.. one axis of symmetry). which simplifies understanding of the behavior and the stress calculations. remembering that an axis of symmetry is always a principal axis. Forces are not always transferred in a straightforward fashion as by the pure structure systems discussed previously. For example, the 500-ft-high concrete struc- ture of the Metro Dadc Center in Miami. Florida ( 1983). designed by LeMessurier. is a hybrid structure that consists of frame eonstmction with two shear walls at each end and huge 60-fl-deep spandrel girders near the base. as shown in Fig. 10.6. Here. the
  12. 12. Sec. 10.4 I Brief Investigation of Common High-Rise Structures 827 gravity loads are carried by the columns and end walls. The exterior columns along the broad faces. in tum. transfer the loads to the spandrel girders that bridge the space between the end walls. The wind against the broad building face is transferred by the floor slabs to the two coupled shear walls at the building ends. which act as cantilevers above the span- drel girders. Below this level the walls act similarly to end cores or open tubes. where the tension due to rotation is suppressed by the gravity loads. In other words. gravity loads are used as a stabilizing agent. The lateral force action on the short face is resisted in the upper building portion by the combined action of the cantilevering end shear walls and the exterior rigid frames and at the base by the huge portal frames. as shown in Fig. 10.6. 10.4 BFIIEF INVESTIGATION OF COMMON HIGH-RISE STRUCTURES The most common structure types. identified conceptually in Fig. 10.2. are briefly dis- cussed in this section. Bearing Wall SlI'tl<‘Im‘L'. ‘. ' The bearing wall was the primary support stnicture for high-rise buildings before the steel skeleton and the curtain wall were introduced in the l880s in Chicago. The traditional tall masonry buildings were massive gravity struc- tures where the walls were perceived to act independently; their action was not seen as part ofthe entire three-dimensional building fomt. It was not until alter World War ll that engineered thin-walled masonry constmction was introduced in Europe. Bearing wall construction is used mostly for building types that require frequent subdivision of space. such as for residential application. The bearing walls may either be closely spaced. e. g.. l2 to I8 ft and directly define the rooms. or they may be spaced. for instance. 30 ft apart and use long-span floor systems that support the partition walls subdividing the space. Bearing wall buildings of IS stories or more in brick. concrete block. precast large-panel concrete. or cast-in-place reinforced concrete are common- l. /‘l'ERAL (ZR; -'aVl’1'Y ABSORUS THE FORCE TENSION DUE TO ACTION l. r‘tTlZRAL FORCE 03-‘ THE ACTl0.‘ ° 17 SL‘PPRESSES LONG FACE 'l'ENSIl. E STRESSES ill TIIE FOUIID/ t'| ‘ l OHS LATE RM. I I I I FORCE ACTION _, ‘ I W“ E l l : on THE (. .‘. NTI J: .'. . I I I I SHORT FACE "°“°" | I l : l I -1lr ‘*IF Q : RIGID FR. 't. ‘lES cast‘ in I. _ + J» ‘. t utttn * ‘V’ ¢ £. 'lD sue. -at i. utLLs . -tan PORTAL riwir: i me/ tot: .t: '1‘ ION I Figure 10.6 Hybrid structure.
  13. 13. 828 Chap.10 I High Rise Building Structures place today; they have been built up to the 26-story range. The bearing wall principle is adaptable to a variety of building forms and layouts. Plan fonns range from slab- type buildings and towers of various shapes to any combination. The wall arrange- ments can take many different fomis. such as the cross-wall. long-wall. double cross- wall. tubular. cellular. and radial systems. An endless variety ofhybritl systems is pos- sible by combining these cases. The walls may be continuous in nature and in line with each other. or they may be staggered: they may intersect or they may function as sep- arate elements to fonn individual wall columns. Bearing walls usually carry both grav- ity and lateral forces. (‘are S! l'llC! llI'l. '.'. ' Many multicore buildings with their exposed service shafts have been influenced by the thinking of the Metabolists of the l960s. who clearly separated the venical circulation along cores and the served spaces. According to Kenzo Tange. “buildings grow like organisms in a metabolic way. ” Their urban clusters consisted of vcr1ical service towers linked by multilevel bridges. which. in turn. contained the cel- lular subdivisions. Other examples of urban-type megastrttctures can be found in hos- pital planning of the 1970s. The linear bearing wall structure works well for residential buildings where functions are fixed and energy supply can be easily distributed veni- cally. ln contrast. office and commercial buildings require maximum flexibility in lay- out. calling for large open spaces subdivided by movable partitions. Here. the venical circulation and the distribution of other services must be gathered and contained in shafts and then channeled horizontally at every floor level. These vertical cores may also act as lateral stabilizers for the building. There is an unlimited variety of possibil- ities related to the shape. number. arrangement. and location ofcores. They range from single-core structures (core with cantilevered floor framing, core with massive base cantilever. core with large top and/ or intennediate cantilevers. core with other struc- ture systems) to multiple core structures. Bridge Structures. ‘ The idea ofthe bridge structure was vitalized by the designers of the l960s. who were coneemed with large-scale urban architecture and wanted to sep- arate the ground and services from social activities. These megastructurcs or urban structures were proposed by the Mctabolists in Japan, Archigram in England. and designers such as Yona Friedman in France and Eckhard Schulze-Fielitz in Germany. who used horizontal space-frame structures. The long span from venical support to vertical support can be achieved through an endless number of possibilities. as has been expressed in architecture. Closely related to the bridge concept is the core Sll'tlC- ture. where many of the buildings formed megaframcs to support. in bridgclike fash- ion. sccondary building packages. Similarly, several of the suspension structures are based on the bridge principle. as are supertall buildings that use megaframes or super- diagonals to gather the building weight to certain points for the purpose of stability. Space can be bridged by using one of the following structural concepts: Vierendeel trusses. trusses. arches. suspended arches. and wall beams. Stl. ‘[)c'IISi0It BuiIrlr‘ngs. ' The application of the suspension principle to high-rise build- ings rather than to roof structures is essentially a phenomenon of the late l950s and l960s. although experiments with the concept go back to the l920s. The structuralists of this period discovered a wealth of new support stmcturc systems in the search to minimize the material and to express antigravity. that is. lightness of space and open- ncss of the facade. allowing no visual obstruction with heavy structural members. The fact that hanging the floors on cables required only about one-sixth of the material compared to columns in compression. as in skeleton construction, provided a new challenge to designers. In addition. this type of structure allowed a column-free space at the base. The treelike buildings with a large central tower. from which giant arms are cantilevered at the top to support the tensile columns at their ends, are common today. The floors or spatial units (c. g.. capsules. entire building blocks) are suspended from the support sturctures by using either vertical or diagonal tensile members. Typical
  14. 14. “ 5 ' ilfiiw *‘ I 15111:’ ‘V , ,1.'1:. 'i'%g. .. D": ', ‘ . g . _ : :""| §.‘7'7'I‘*", ".T. »7.~ . :A ‘" ’ ‘ ' ’ ’ A. -’fiI. '.-:1 . H: “ % »- +-»: %.= "'l*’= h[“*‘% r'*%; ~.~: :A;5=: .'11v€*. . I . " . .‘. “yg. .‘, "I ' ‘Li " -1‘ ‘ft ' ' 4 I ‘ - . U'. ', ffi{fi¢1} .7,. x;. ,~‘, ,_ I ‘. ~. $5‘ ‘ " ' i . mT"C l“ 0 Lil “ ' '7‘ ‘ L ESE ’ -' Q1,‘ m1‘, ::n. . zagnég , “ E -vfifiéifiifilafi _' -41 ~ » _. - I 3 i ‘wan _ _ fin Ilgiii . - 5423:‘ " 1 . - F , .v < afl tuna __? ’{l4 9‘ E/ :_ hill IIII‘ ~1 ""_<mf 5‘ 1" ‘V. I ~<: ;3'~_l* fifiifigy. < - _. _-: ,. .w - . ~._'.31. ... “-‘. _ _ v - - 5 .514 ' _r ‘‘_I', . 5§§n%*§ '-V I. ’-' s ‘ ‘- . .L; . 7 , . 5 -Qmora ZFAFW‘ ,9 . |-- _ . "1 " . m_: ;2*g§‘mI;5§; _w> ,1}: . ._. ,7 . [1 I. x» . l'k? h.‘H. . ' . '( ‘ gm-"—I ‘ ‘ kn fiV' “. .'; .__‘T‘ E u H - ‘ . .(-q , .__ 4 . . , . .; ‘{¢'h _ . . A g 53. 1|" -" (3 ‘- s’ ‘”': g“‘--¢, »a‘z: ;¢_'. as: l§_s: t1I&? !‘L§‘l£ : - , ' ‘r : ('*"-. ; ms». .. . . . : - . av. " . , 2 . '.‘_a.1«? £nd A -" ' '. - r ' 4. . “‘ ll 5‘! ‘». .; ’lv, ‘!. ,;; .)Il4k" WM‘ »' _ 1 . ]‘_'_f_s, .,_, ;,. - ~ ‘tv_(= :-_ . _ ‘pm ‘ _. .—ur— — E: "'1' * . ‘f . . I, ' M! ‘ ‘C H'. “,“'3'V'. , : .$'. - ‘n, 11"‘ ‘m MB ‘ - ‘-3 Fe’ _ ‘wk 3 " 5". -3?? ’ ’n 1?4': »:: asz= ¢o_«; au, :: -__ ‘ ; = f'? 'f’“". ’:: ‘T't7?fiT{? Tj‘~‘ , 1, ‘-- h . | . 'If . ' ”''‘“<»'‘‘*'‘‘'{1"”~‘vL£;5?' —. -‘ ‘» é§*_. '-‘_, ‘_ -'_p , . -, >. : ' fl" : ~—-__, --* - ' . .». -a . . . .. ._. ., ,. . , . ._ 1.‘! , _4 ‘i"'~‘v_ <, ,,, ., . '.. —.A . . - . '1 ~A. ‘.*M. z-; ,.’ «T. ' -. " 1' IE‘! -,—r, ;., ..‘ . .‘r. ] -w-. . » Lin. .. V 7/ .
  15. 15. 830 Chap.10 I High Rise Building Structures suspension systems use the rigid core principle (single or multiple cores with outrig- gers or beams. megaframes, treelike frames. etc. ), the guycd mast principle. and the tensegrity or space-net principle. For example, the architect Albeno Galardi uses the suspension principle for the Olivetti building in Florence, Italy (1973, Fig. 10.7) to form a bridge and thus allow open space at the base. The exterior. preslrcssed concrete hangars supporting the four floors are hung from the roof structure, which. in tum, is carried by two towerlike con- crete cores. Staggered Wall Beam Structures. ‘ In this innovative structure system. developed in the mid-19603 by a team of architects and engineers at M. l.T. , story-high wall beams span the full width of the building on alternate floors ofa given bay and are supported by columns along the exterior walls; there are no interior columns. The wall beams are usually steel trusses. but can also be pierced reinforced-concrete members. The steel trusses are concealed within the room walls. In the interstitial system, wall beams are used at every other floor to allow for uninterrupted free flow in the floor space between. while in the staggered wall beam system. the wall beams are used at every floor level, but arranged in a staggered fashion between adjacent floors. The arrange- ment of the story-high members depends on the layout of the functional units. We can visualize apartment units to be contained between the wall beams and to be vertically stacked to resemble masonry bond pattems. As the unit sizes change. the spacing of the wall beams may be adjusted or additional openings may be provided. The most common system of organization is the mnning bond or checkerboard pattcm. Skeleton Slrtlclttrcx: When William Le Baron Jenney. in the l0-story Home life Insurance Building in Chicago (1885), used iron framing for the first time as the sole support structure canying the masonry facade walls. the all-skeleton construction was bom. The tradition of the Chicago Frame was revived after World War II when the skeleton again became a central theme of the modem movement in its search for merg- ing technology and architecture. Famous landmarks became SOM‘s Lever House in New York (1952) and Mies van der Rohe‘s two 860-880 Lake Shore Drive Apartment Buildings ( l95l. Fig. l0.8). These landmarks have been most influential to the subse- quent gcncration of designers; they symbolized at the time. with their simplicity of expression. the new spirit of structure and glass. Although the pure. boxy shapes of the 19605 are closely associated with skeleton construction, as derived from Miesian min- imalism. other high-rise building skeletal fonns. based on different design philoso- phies. have been built. for example. the unusual hammer-shaped Velasca Tower in Milan. Italy ( I957). Today. there seems to be no limit to the variety of building shapes: the skeleton as an organizing element for this new generation of hybrid forms has been extensively experimented with. Odd-shaped towers. possibly with tapered frames. rellect the change of irregular plan forms with height: skeleton buildings may he stepped at various floor levels where large setback terraces may be fully landscaped. in the Lloyd’s of London Building (1986) by Richard Rogers, the braced perimeter concrete frame is surrounded by six satellite service towers. while the internal perim- eter columns carry the elaborate central atrium structure. Kisho Kurokawa articulated the regularity of the three-dimensional grid and its adaptability to growth and change by constructing the Takara Beautillion for Osaka’s Expo ‘70 from single six-pointed spatial cross units. Facade framing ranges from long-span, deep girder systems and Vierendeel frames to perforatetl walls. The open, airy skeleton is contrasted with the framed tubular wall. Frames may be organized as continuous rigid frames. hinged frames. and any combination. The behavior of moment-resisting frames (i. e., rigid frames). which resist both gravity and lateral forces. is investigated briefly in Fig. 5.23. F / at Slab Building Structures: F lat slab buildings. developed during the mid-19405 in New York. consist of horizontal planar concrete slabs directly supported on col- umns, thus eliminating the need for floor framing. This results in a minimum story
  16. 16. IIIIII[IIIllllllllllfllllllil IIIIIIII . -IIIllllllllllllllllIIIIIIIIIIIIIIIII . -IIIIlllllllllllglfllllflfllallIII Plliflllllllilifliflll}lnnI. i {I'l_IjIIIIlIlIIIIIll| lII‘IIIIIIIlllIIIl . -TIIIIIIIIIIlllllllllllllllllIIIIIII ifllllllllllllllllllallIIIIIIJ/ llllllll —t 1 _ 111 ‘iii ‘I11 ‘-111 :1:—— $$—$$—— In 11“ Ijilnjf 4 I I Z — Z — —'Z —, — ! ‘— II l-IIilll! !I? IlIlIl! !l! l?lIIIIIIflllllllllll ‘ ' " Figure 10.8 Skeleton building. 831
  17. 17. 832 Chap.10 I High Rise Building Structures height. an obvious economic benefit that is especially advantageous for apartment buildings. Drop panels and/ or column capitals are frequently used because of high shear concentrations around the columns. Slabs without drop panels are commonly called_/ Ia! plates. This system is adaptable to an irregular support layout. From a behavioral point of view. flat slabs are highly complex structures. The intricacy of the force flow along art isotropic plate. in response to uniform gravity action. is rcflcctcd by the principal ntoment contours (Figs. l.4. 2.l ). Here. the main moments around the column support are negative and have circular and radial directions. while the positive field moments basically connect the columns linearly. The pattcms remind one of organic structures. such as the branching grids oflcaves, the delicate network ofinsect wings. radial spider webs, and the contour lines of conical tents. realizing a similar relationship between cable response and loading as well as the corresponding moment diagram. Pier Luigi Ncrvi. for the Gatti Wool Factory (1953) in Rome. Italy. actually followed the principal bending moments with the layout of the floor ribs. Centuries earlier. however. the late medieval master builders had already intuitively developed pattems for ribbed vaulting predicting these tensile trajectories: the fan vatrlts of the Tudor period in England are a convincing example. Braced Frame S! I'lI(‘lllI'¢'. ‘. ' Tltc concept of resisting lateral forces through bracing is the most common construction method; it is applied to all types of buildings. ranging from low-rise structures to skyscrapers. At a certain height, depending on the building proportions and the density of frame layout. the rigid frame structure becomes too mushy and may be uneconomieal. so it must be stiffened by. for example, steel bracing or concrete shear walls. The basic bracing types for frames are single diagonal bracing, cross-bracing. K-bracing. lattice bracing. eccentric bracing (single diagonal or rhom- bic pattcm). knee bracing. and combinations. When the diagonal members are kirtked for the placement of openings. they must be stabilized by additional members. The architects Burnltant and Root developed the concept of vertical shear wall (or the vertical truss principle) in the 20-story Masonic Temple Building (1892) in Chicago. The spirit of a braced frame structure is investigated conceptually in Fig. l0.9. In braced frames. the frames carry the gravity loads. whereas the bracing resists the lateral loads. In contrast. in braced rigid frames the frame not only carries the grav- ity loads but also. together with the bracing. resists lateral loads. Trrrssed Frame Srrucrur'cs. ' Trusses not only constitute support structures hidden within the building, but may also be revealed on the exterior. One of the earliest exam- ples of braced skeleton buildings is the Chocolate Factory at Noisiel-sur-Mame near Paris by Jules Saulnicr l I872). where the walls consist of exposed trussed iron frame- work. This method of construction was surely inspired by trussed bridge construction. as well as by the timber framing. that first occurred in Europe during the Middle Ages. Here. each region developed its own distinct pattcm of braced wall heavy timber fram- ing. with space bctween the timber members inftllcd with masonry or other material mixtures. An early example of high-rise braced frame construction is Gustave Eiffel's interior-braced iron skeleton for the I51-ft-high Statue ofLibcrty (I886) in New York. He also designed the braced skeleton wrought iron structure of the Eiffel Tower( I889.) in Paris. at almost 1000 it the tallest building of its time: this first modem tower became a symbol for a new era with its daring lightness ofconstruction. In contrast to braced frames. trussed frames are bearing wall structures that cany both gravity and lateral loads. In other words. the diagonal members also can'y gravity loads. During the early part of this century. the elaborate tops of skyscrapers required complex bracing systems. For example, a high spire structure with a needlelike termi- nation was designed to surmount the dome ofthe Chrysler Building ( 1930. Fig. I0. I ). Currently. postmodern building tops with their spires and pinacles revive omamcnta- tiort and the architectural styles of the past. Intricate braced frames are required for the various roof shapes. such as pyramids. domes. spirals artd gabled. stepped. folded. or
  18. 18. Sec. 10.4 I Brief Investigation of Common High-Rise Structures 833 . -%‘%3§ = === Il= ‘‘ iii. ‘ = === =I= W‘ : §-= -i= E= .%E'%EE . . -= =IIF "i V ‘ Ila- , , / [N . IIlIlII—— : 5* N ‘-. .. Figure 10.9 Braced frame structure. arched fonns. These structural complexities are not only found in the roof spires. but also in lobby entrances and atria of high-rise buildings. Shear Walls with Outriggers. ‘ At a certain height the braced frame will become uneconomicttl. particularly when the shear core is too slender to resist excessive drifi. Here, the efficiency of the building structure can be greatly improved by using story- higli or deeper outrigger arms that cantilever from the core at one or several levels and tie the perimeter structure to the core by either connecting directly to individual col- umns or to a belt truss. This interaction activates the participation of the perimeter col- umns as struts and ties, thus redistributing the stresses and eccentric loading. Pier Luigi
  19. 19. 834 Chap.10 I High Rise Building Structures Nervi applied the outrigger concept to the 47~story Place Victoria (1964. Fig. 10.3. bottom left) in Montreal. the first reinforced—concrete building to utilize the principle. Tubular Srrucrurcx The development of tubular structures is closely associated with SOM during the l960s: the 38-story Brunswick Building (1964). the 100-story John Hancock Building (I968), and the 1 10-story Sears Tower (1974) with 1454 it. all in Chicago. are famous early examples. Much credit must be given to the eminent struc- tural engineer Fazlur Khan. a partner of SOM. who invented the concept in the search for optimizing structures with the use of computers. As the building increases in height in excess of roughly 60 stories. the slender interior core and the planar frames are no longer sufficient to effectively resist the lateral forces. Now the perimeter structure of the building must be activated to provide this task by behaving as a huge cantilever tube. Here. the outer shell may act as a three-dimensional hollow structure. that is, as a closed box beam where the exterior walls are monolithically connected around the corners and intemally braced by the rigid horizontal floor diaphragms. The concept evolved from the three-dimensional action of structure as found in nature and in the monocoque design of automobiles and aircraft. The dense column spacing and the deep spandrell beams also tend to equalize the gravity loads on all the exterior col- umns, similar to :1 bearing wall. thereby minimizing column sizes. In addition, the closed perimeter tube provides excellent torsional resistance. In the 1960s, the tubular concept revived the bearing wall for tall building construction. but in steel. concrete. and composite construction rather than in masonry. Now. window lights can be placed directly between the columns of the punched wall: hence the need for a separate cur- lain wall is eliminated. The pure tubular concepts include single perimeter tubes (punched, framed. or trussed walls). tube-in-tube. and bundled tubes. Modified tubes include interior braced tubes, partial tubes, and hybrid tubes. The well-known structural engineer Leslie E. Robertson of New York developed a unique tubular structure for the 72-story Bank of China Building (1988) in Hong Kong designed by l. M. Pei. as shown in the conceptual drawing of Fig. 10. ll). consist- ing of four adjacent triangular prisms ofdillercnt heights rising out of the square base. The I209-ft high tower is a space’-. /i‘anu. ~ hmcud tube organized in 13-story miss mod- ules. where the I70-ft square plan at the bottom of the building is divided by diagonals into four triangular quadrants. The space truss resists the lateral loads and transfers almost the entire building weight to the four supercolumns at the corners: the column at the center of the four quadrants is discontinued at the twenty-fifth floor. where it transmits the loads to the top of the tetrahedron. which canies them to the supercol- umns. Midway through the l3-story truss modules. transverse trusses wrap around the building to transfer the gravity loads from the intemal columns to the supcrcolumns at the comers: the horizontal trusses are not expressed in the facade. The loading condi- tions in Hong Kong, in contrast to the United States. are much more severe: the live loads and wind loads are twice those in New York. and the earthquake load is four times higher than in San Francisco. The superdiagonals are not directly attached to each other at the corners to fonn complex spatial connections, but are. instead. anchored in the massive concrete columns. thereby forcing, the concrete to behave as a shear transfer mechanism. The mixed construction of the primary structure consists of the separate steel columns at the comers (to which the diagonals are connected). which are encased and bonded together by the massive concrete columns. The giant diagonal truss members are steel box columns filled with concrete. The open space at the base of the building did not allow the diagonals to continue to the bottom: at the fourth level. a specially reinforced floor diaphragm was required to transfer the lateral shear to stecl—plalcd core walls. which were designed as three-cell shear tubes. Composite and . -llixca’ Ste(rI~-COI1c'I'cIc Buildings: The integral interaction of rein- forced concrete and steel can be seen not only in the popular composite metal deck and floor framing system. but also on a much larger scale. It is not the composite action of
  20. 20. >: o"oz¢'. o;. o A §’$“ ( ‘§&V ~ ‘ Figure 10.10 Space-trame braced tube. 835
  21. 21. 836 Chap.10 I High Rise Building Structures the structure members—the slabs. beams. and columns that are of interest here. but rather the combination or interaction of these members that are blended into a single structure system. Typical composite building ! _17Jc. s. which have developed over the last decade or so. are composite framed tubes. composite steel frames. composite panel-braced steel frames. composite interior core-braced systems. composite mega- frames. and hybrid composite structures. Recently. niircd . s1ecl—concrcre buildings have also become popular; the combining of major structure components ofconeretc. steel. or composite buildings is a relatively new development. For example. it may now be economical to place a steel building on top of a concrete building. or vice versa; altcmativcly. a central concrete core may be slip-fomtccl to a predetennined height anti then the steel frame built around it. -/ (‘guS! l'llL'! III'£'. V.’ In this context. the tcmt megastructure does not refer to the vision- ary concepts of the 19605. expressing the comprehensive planning of a cotnmunity or even an entire city. but solely to the support structure of a building. However. this mcgastructure is still formulated on the basic concept of a primary structure that stip- ports and services secondary structures or smaller individual building blocks. In the early l970s. Fazlur Khan ol'SOM proposed to replace the multicolumn concept by the tour nru. m'vc corner culmnn . mp/ mrring super; /i-unis by using supertransfer tntsses at every 20 floors or so on the interior and exterior of the building. thereby allowing all gravity loads to flow to the four sttpcrcolumns. The principle can be traced to Khan's studies ol‘superframes for multiuse urban skyscrapers. with the John Hancock Center in Chicago representing the forerunner of this idea. Surely. one of the most important first examples for the new breed ofmegastructures is the 59-story C iticorp Center in New York ( l9"/7). In this building the renowned structural engineer William J. LcMes- surier introduced a unique structure and a new way ofthinking about structure. This is also reflected by LeMcssuricr‘s ingenious support structure. which is not, however. integrated in articulating the building form for Helmut Jahn‘s Bank of the Southwest proposal (I982. Fig. l0.l l) in Houston. an obelisklike. 82-story square tower with chamfered comers. The slender. I220-ft-ltigh structure tapers from a I65-ft square base to a 135-ft square plan at the top. The entire building is supported by eight super- columns. two on each side, which reduce in size from 10 x 15 it at the bottom to S x 5 ft at the top of the building. Interior steel superdiagonals straddling the core cross the plan to connect the massive perimeter concrete columns on the opposite sides. Similar to a Greek cross configuration. they gather and then transfer the gravity loads at the base of each module. as well as act as the web with respect to wind shear. The chevron configuration of the primary interior bracing is organized in nine-story modules. [Ii-brid Srrm-nu'es. ' The current trend away from pure building shapes toward irreg- ular complex ones. that is. hybrid solutions, as expressed in geometry. material. struc- ture layout. and building use. is apparent. In the search for more efficient structural solutions. especially for very tall buildings. a new generation of systems has developed with the aid ofcomputers. which. in turn, have an exciting potential for architectural expression. These new structures do not necessarily follow the traditional classifica- tion of the previous sections. Now. the selection of 2t structure system as based on the pi imary variables ofmaterial and I. hC type and location of stmcture is no longer a sim- ple choice between a limited number of possibilities. Mathematical modeling with computers has made mixed construction possible. which may vary with building height. thus allowing nearly endless possibilities that could not have been imagined only £1 few years ago. The computer simulates the effectiveness of a support system so that the structure layout can be optimized and nonessential members can be eliminated to obtain the stiffest structure with a minimum amount of material. Naturally. other tlesign considerations besides structure will have to be included. but the design con- cepts can be tested quickly and efficiently by the computer.
  22. 22. Sec.4.1 I Lateral Load Action 337 system and will provide some lateral resistance. The height limits in seismic zones 3 and 4 vary from 65 to 240 it. In buildings that employ the braced nlrilllellt-I'€Sis! irIgflvmli: ' . '_l‘. '! (’lil (which is a typical example ofa dual s_v. vtenr). the frame carries the gravity loads while the lateral forces are resisted by the frame together with the shear walls or diagonal bracing. The moment-resisting ductile frame must be capable of resisting at least 25% of the total lateral load. according to the UBC. in order to act as a backup system (as based on its ductile character) in case of failure of the bracing elements. Because of the varying degree of ductility of the dual systems. that is. depending on the combination of the structure types. the Ru coefficients range from high values of l2 for eccentn'cally braced ductile steel frames and ductile rigid frames braced by concrete shear walls to low values for other systems. TABLE 4.2 Partial List of FL, Coefficients tor Various Structure Systems‘ Structural Systems R, ., H (It) Bearing wall system Light-framed walls with shear panels Plywood walls tor structures three stories or less 3 55 All other light-framed walls 5 55 Shear walls Concrete 5 150 Masonry 6 160 Braced trame system (using trussing or shear walls) Steel eccentrically braced ductile trame _ 10 240 Light-lramed walls with shear panels Plywood walls lor structures three stories or less 9 65 All other light-framed walls 7 65 Shear walls Concrete 8 240 Masonry 8 160 Concentrically braced frames Steel 8 160 Concrete (only for zones 1 and 2) 8 — Heavy timber 8 65 Moment-resisting trame system Special moment-resisting lrames (SMRF) Steel 12 N. L. Concrete 12 N. L. Concrete intennediate moment-resisting frames (IMRF) (only for zones 1 and 2) 8 — Ordinary moment-resisting lrames (OMFIF) Steel 6 160 Concrete (only for zone 1) 5 — Dual systems (selected cases are for ductile rigid lrames only) Shear walls Concrete Masonry 12 N. L. Steel eccentrically braced ductile trame 8 160 Steel Concentrically braced trame 12 N. L Steel Concrete (only tor zones 1 and 2) 10 N. L. 9 . _ ‘Partial reproduction from Unilarm Bum-‘no Coda. 199i Edition. courtesy oi tntomatlonal Contoronce at Building Otllciats

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