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Structural collapse   basic shoring principles

Structural collapse basic shoring principles






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    Structural collapse   basic shoring principles Structural collapse basic shoring principles Presentation Transcript

    • Str uctur al CollapseBasic ShoringPrinciples FEMA/Corps of Engineers Overview FEBRUARY 2006
    • BUILDING MATERIAL &STRUCTURAL SYSTEMSObjectives:•Discuss the FEMA Response Team•Review the basics of how various building materials resist forces,the importance of Ductile vs. Brittle behavior, the concepts ofVertical and Lateral Load Resisting Systems, and StructuralRedundancy.•Discuss the Urban Search & Rescue Structure Marking System.•Take a look at shoring principles and techniques knowing thehow the forces and materials will perform in a collapse situation.
    • TYPES OF FORCESIndividual LOADS, usually referred to as FORCES can be divided into four types: Tension. Compression, Bending, and Shear.When a FORCE is applied to an individual member, itproduces STRESSES, which are defined as the FORCEdivided by the cross-sectional area on which it acts. Example: If a 1000lb FORCE acting in Tension on a 2 inch x 2 inch steel bar, will produces a 250 lbs per square inch (psi)
    • TENSION FORCESTENSION FORCES stretch members of steel or wood. Concreteand masonry have no reliable tension strength.When a moderate tension force is applied a steel bar willlengthen, and when the force is removed, the bar will return toits original length. This is called ELASTIC Behavior and canbe repeated many times in competent steel or woodmembers.If a much larger force is applied to the steel bar it will start tolengthen more rapidly. When this rapid lengthening begins,one can observe that the cross-section of the bar will start toget smaller (neck down). When the force is removed, the barwill not return to it’s original length, since it has experiencedpermanent yielding (DUCTILE Behavior)The DUCTILE behavior of steel in tension provides the specialproperty of forgiveness (warning of failure) and responsewhich makes it especially desirable in resisting dynamicloading.
    • TENSION FORCESTENSION FORCES (continued)· Ductile behavior is the ability of amaterial to stretchand/or bend without suddenly breaking,and after the loadis removed it can remain stretched/bentand then be reloaded.· EXAMPLE: one can bend a hook on arebar, and evenunbend it without breaking· Brittle behavior means that the materialwill break without warning (CatastrophicFailure)
    • COMPRESSIONFORCES push on members and canCOMPRESSION FORCESlead to crushing of materials when the members areshort and relatively fat. ( small length to width ratios,L/D)At bearing surfaces between wood or concrete beamsand columns, crushing can also occur.The crushing failures tend to give warning, such aslocal splitting of concrete and noisy, slow, compressionof wood fibersWhen long, slender members are loaded incompression, they can fail suddenly by BUCKLING(bowing)This type of sudden failure wants to be avoided
    • BENDING FORCESBENDING FORCES occur mostly as a result ofVertical Loads from gravity that are applied tofloor slabs and beams. They also occur insloped roof rafters and sloped slabs in rubblepiles.Bending causes the bottoms of simple beamsto become stretched in TENSION and the topsof beams to be pushed together inCOMPRESSION.Continuous beams and cantilever beams havetension forces at the top + compression at thebottom near their supports.In mid span the forces are in the samelocations as for simple beams and slabs.
    • BENDING FORCESCON’T•Vertical cracks develop near the mid-span of concrete, sincethe Tension Force causes the concrete to crack in order forthe Reinforcing Steel (Rebar) to resist the Tension Force.•This cracking can be observed in damaged structures tomonitor and determine the potential for collapse.Stable, hairline cracks are normal, but widening cracksindicate impending failure•Structural steel and reinforced concrete, moment resistantframes experience tension and compression stresses onopposite faces (similar to continuous beams). Thesestressescan reverse during earthquakes and high winds.
    • SHEAR FORCESSHEAR FORCES occur in all beams, and are greatest adjacentto supports. Shear stress can be described as the tendency to tear thebeams surfaces apart.Example: Consider a beam that is made from a group ofindividual books as they sit on a bookcase, with a long threadedrod extending all the way through them, tightened with nuts ateach end. If this beam is placed so that it spans between 2 tablesand one attempts to push one of the books down to the floorbelow, a SHEAR FORCE will be exerted on the surface of thebooks immediately adjacent to the one that is being pushed outIn concrete beams these shear stresses develop diagonaltension cracks, since concrete is very weak in tension.This cracking can also be monitored in a damagedstructure.Wood beams are strong in tension and compression, but areparticularly weak in shear along the horizontal plane of thesofter spring wood.
    • SHEAR FORCESPUNCHING SHEAR occurs where a two-way concrete flat slabis connected to a column and it is the tendency of the slab to dropas a unit around the column. The column appears to punch through the slab.The cracking that indicates the over-stress leading to thistype of collapse is most visible on the top surface of theslab, which is often covered by debris during US&Ractivities.BOLT SHEAR is the tendency of steel pin-like connector (bolt,nail, and screw) to break across its cross section.Example: A roll of coins is Sheared off as each coin slips pastthe other. This type of failure can be sudden.Nail failures in wood structures, which involve some degree of pullout, canoccur with enough deformation to give warning.
    • VERICAL LOADINGVERTICAL LOAD SYSTEMSStructural members in these systems can be divided into twotypes, those that form horizontal (or sloped roof) planes andthose that provide the vertical support for these planes.Vertical Load Systems• Concept of gravity load path• Loads must be transferred fromsource to the ground• Top down approach – Plumbingsystem analogy• Framed and Un-Framed• Connections are particularlyvulnerable
    • TRUSSESTRUSSES are special vertical load resistant members that usegreater depth for structural efficiency, but require more positivelateral bracing of compression members.Trusses are usually made from wood and/or steel, althoughconcrete has been used for economy in some areas of the world.Individual members are stressed in either tension orcompression, although stress may reverse in some membersdue to changes in live load (people, vehicles, and rain/snow).Compression members are normally governed by bucklingand tension members are normally governed by theirconnections.
    • VERTICALLOADINGsuch as:HORIZONTAL MEMBERS support floor and roof planes and arenormally loaded in bending •Wood - rafters, joists, beams, girders. •Steel - corrugated sheets (filled with concrete), joist, purlins, beams, girders. •Reinforced concrete floor systems may be of many types. All have some relationship to the economy of providing adequate structural depth with available forming materials. •Pre-cast concrete floors may contain, planks, cored slabs, single or double tees, beams and girders. Most modern systems in California combine a cast-in-place overlay slab to provide adequate interconnection of individual members and overall planar stability. •These individual members need to be interconnected to their supported planes in order to provide the lateral stability to resist the extreme fiber compression forces associated with bending, which occur on the top or bottom of the members.
    • VERTICAL LOADINGVERTICAL SUPPORT MEMBERS are normally configured asbearing walls or columns.In wood and light framed steel systems the bearing walls are made usingclosely spaced columns (studs at 16-24" o.c.) that must be interconnectedby a skin in order to provide the lateral stability that will allow the individualmembers to be loaded in compression without buckling.Concrete and masonry bearing walls are proportioned to carry heavy verticalloads depending on their height to thickness ratio.Individual column (posts) normally carry large compression forces and maybe made of wood, steel, or reinforced concrete. In all cases the load capacityis based on the members slenderness ratio (l/r, l/d) as well as the adequacyof the connection between the column and the horizontal system.All vertical load systems need some system to provide forlateral stability (i.e., the proper alignment of vertical load path).
    • STRUCTURALREDUNDANCYREDUNDANCYEspecially in Seismic Zones, it is important for the LateralLoad System to possess some degree of Redundancy.Redundancy in a structure means that there is more than onepath of resistance for Lateral Forces------Multi Elements •Box Buildings •Can be achieved by having a Moment Resistant Frame with many columns and beams, all with ductile connections, •This can be achieved by having a Dual System, like Shear walls plus a Moment Resistant Frame aka-Collapse Preventor’s
    • WOOD PROPERTIES•Is tough, light fibrous, fire supporting,•Has defects like knots, splits and non-straight grain that cause stress concentration.•The growth pattern of fast growing spring wood vs. slower growing summer wood leads to structural problems.•Connections are best made by bearing one member on it’s supporting member, however, metal connection devices can be successfully used.•Nailed connections perform well as long as splitting is avoided, and bolting may be successful if adequate spacing and edge distances are provided.Properly proportioned wood structures can exhibit Ductility -When wood posts are kept short and bear on the cross grain surfaces of beams or sole plates, slow crushing of the cross grain can be observed to warn of failure.•Box Cribbing will exhibit this same failure mode since all the load is transferred in cross grain bearing.
    • WOOD PROPERTIES• Douglas Fir or Southern Pine are the most common types of structural timber used in the U.S.• Average values for these species are – Compression parallel to grain = 1100 PSI – Compression perpendicular to grain = 600 PSI• The capacity of header beam and sole plate is determined by bending an/or horizontal shear strength.• Average values for Douglas Fir and Southern Pine – Fb = extreme fiber bending stress = 1500 PSI – Fh = horizontal shear stress = 90 PSI
    • STEEL PROPERTIES•Is tough, light, strong, ductile, and formable into any shape, but needs to be fireproofed.•It starts to lose strength above 700° Fahrenheit.•It has magical property of ductility. That is, it can be stressed beyond it’s Elastic Limit and severely bent, but still have enough strength to resist failure.•Ideal structural material, in that it gives warning of collapse (has forgiveness).•Steel is strong in Tension, Compression, and Shear•Steel beams must be laterally braced so as not to buckle about their weak axis, especially if the ductile performance required for earthquake resistance is expected.•Steel-framed structures must be properly proportioned in order to avoid the over loading of columns.•Steel can be very efficiently connected by bolting or welding (older structures used rivets instead of bolts).•Welded joints must be properly designed and constructed or they can lead to a brittle failure.
    • CONCRETEPROPERTIES•Is essentially cast rock, sand & cement •Strong in COMPRESSION, weak in TENSION and SHEAR •Steel bars are cast into concrete to provide for the longitudinal tension force and enclosing type steel ties and stirrups are added for confinement and shear resistance. •Sufficient steel can be added to provide adequate toughness for seismic resistance, enabling reinforced concrete to exhibit ductile properties similar to structural steel. Concrete shrinks, cracks, and creeps under normal circumstances, and this normal behavior needs to be differentiated from the cracking and spalling that indicates failure. •Concrete Construction •Cast in Place •Pre-cast •Pre-Tensioned •Post Tensioned
    • MASONARY -UNREIFORCED•Very brittle material •Walls were constructed with a thickness made from three or more bricks being laid long ways, side by side, for five or six layers high (courses) and then a layer was placed with the bricks at 90 degrees (header course), and so on. •The strength and seismic performance of un-reinforced masonry is highly dependent on the mortar strength. •The shear strength of mortar can vary from 15 PSI to over 150 PSI, and is determined by the proportion of lime to Portland cement and the workmanship. •Lime produces a nice buttery mortar, but if too much is used a low strength will result. •Lime can also be leached out of the mortar by water over time. •Used as decorative veneers
    • MASONARY -REINFORCED •Is made from clay brick or hollow concrete blocks formed into walls using mortar joints and concrete grout filling of interior cavities. •Masonry properties are similar to concrete - reinforcing steel bars are normally added to provide tension and shear resistance. •Highly dependent on the workmanship to provide adequate mortar and grout strength •Can exhibit very good ductility when properly designed and constructed.
    • MASONRY -REINFORCEDSolid Brick Unit Masonry - Two single brick thick outer layers (wythes) are laid up, then rebar and grout are placed between the layers. - The wythes are connected together with large wire to prevent blow-out when the grout is poured. - Small heavy wire ladder type reinforcing is used at the joints in some cases.Concrete Hollow Unit Masonry (CMU) - Each block comes with preformed cavities. - As the units are laid up, horizontal reinforcing (small rebar or large wire) is placed in the joints. - After the wall reaches a predetermined height, vertical rebar is placed in specified cells and grout is poured to bond the reinforcing steel to the concrete units.
    • WEIGHTS OF COMMON BUILDING MATERIALS. •Wood = 35 PCF •Steel = 490 PCF •Concrete = 150 PCF PCF = lbs per cubic ft •Masonry = 125 PCF PSF = lbs per square ft •Concrete/Masonry Rubble=10PSF PER INCH (of thickness)WEIGHTS OF COMMON BUILDING CONSTRUCTION •Concrete floors weigh from 90 to 150 PSF •Steel beam w/ concrete-filled metal deck = 50-70PSF •Wood floors weigh from 10 to 25 PSF (floors w/ thin concrete fill are 25 PSF or more) •Add 10 to 15 PSF for wood or metal stud interior walls, each floor level •Add 10 PSF or more for furniture/contents each floor (more for storage, etc.) •Add 10 to 20 PSF for Rescuers · 10 PSF on large slab that spreads out load · 20PSF on wood floors to allow for concentrations
    • SHORING BASICSBASIC DEFINITION AND PRINCIPALSShoring is normally the temporary support of structures duringconstruction, demolition, reconstruction, etc. in order to provide thestability that will protect property as well as workers and the public.SHORING PLACEMENTTwo Main Objectives•Maintain the integrity of all structurally unstable elements•Properly transmit or redirect the collapse loads to stable ground or other suitable structural elements capable of handling the additional loads.
    • SHORING BASIC CON’Tlike double funnel. It needs to collect theA Shoring system isload with headers/sheathing, deliver it into the post/struts, and thento distribute it safely into the supporting structure below. A heavilyloaded wood post can punch thru a concrete slab etc.Shoring should be built as a system that has the following:•Header beam, wall plate, other element collects load•Post or other load carrying element that has adjust ability and positive end connections•Sole plate, bearing plate, or other element to spread the load into the ground or other structure below.•Lateral bracing to prevent system from racking (becoming parallelogram), and prevent system from buckling (moving sideways).•Built-in forgiveness (will give warning before failure)
    • T-SPOT SHOREThe main purpose of the “T” shore is to initially stabilizedamaged floors, ceilings or roofs, so that the more substantialshoring can be constructed at less risk.The T Shore is basically unstable. •That is if the supported load is not centered directly over the Shore, it will tend to tip over • The header beam is deliberately kept short so as to minimize to effect of tipping.The size of lumber most commonly used in the T shore is 4 X 4 Douglasfir. The estimated weight of the floor and its contents will help todetermine the number of shores that will be required.
    • RAKER SHORESYSTEMTHE RAKER SHOREThe main purpose of the raker shore is to support leaning orunstable walls and columns by transferring additional weightdown the raker, to the ground or other structural supportingmembers, and away from the wall or column.
    • RAKER SHOREMEASUREMENTThe length of a 45-degree angle raker shore: •Height of the raker shore support point in feet multiplied by 17 will give the length of the raker, tip to tip, in inches. (8 ft x 17 = 136” or 11’- 4”).The length of a 60-degree angle raker shore: •Height of the raker shore support point in feet multiplied by 14 will give the length of the raker, tip to tip, in inches. (8 ft x 14 = 112” or 9’- 4”). NOTE: WHEN CUTTING RAKERS YOU NEED TO HAVE THE ANGLE ON BOTH ENDS
    • CRIBBING•Multi member lay-up of 4x4 to 8x8 lumber in two or three member per layer configuration.•Capacity is determined by perpendicular to grain load on sum of all bearing surfaces.•Stability is dependent on height to width of crib and should not exceed 3 to 1.•Need to overlap corners a minimum of 4” to guard against splitting offcorners of individual pieces that can negatively impact overall stability.•Cribs used by contractors (or in short-term emergencies) often rely only on the friction between bearings for lateral strength, notsufficient for aftershocks or lateral movement.•Individual pieces may be notched like Lincoln logs, to provide lateralresistance in addition to the friction between pieces.
    • CRIBBING – SLOPED SURFACES•For conditions where the shore height is less than 3 feet.•Slope for crib supported floor should not exceed 15%. - Cribs can be built into the slope, but care must be taken to properly shim the layers in order to maintain firm, complete bearings. - Notched crib members could be used since they can transfer more lateral load than the usual friction interconnection.
    • SLOPED SURFACESSHORINGREMEMBER – Sloped Floor/Surfaces are complicated, not simple. No one or two solutions/shoring systems will work for all conditions. •One needs to carefully assess the situation, to determine which way the floor will move. •To shore sloped wood floors the header needs to be placed perpendicular to the joist.