17372162 economics-for-structural-steel-portal-frames (1)


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17372162 economics-for-structural-steel-portal-frames (1)

  1. 1. 6 Portal-frame Buildings6.1 IntroductionBecause of their clean lines, good overhead clearance and relatively lowcost, portal- frame buildings have become very popular. They make up alarge percentage of the small to medium size single-storey industrialbuildings in current use.6.2 I-section portal framesThe rafters and columns of I-section portal frames consist of rolled I-sections, with therafter ends being haunched and site-bolted to the columns, as was shown in detail (c) ofFig 5.1. The column section is heavier than the rafter section, so that the relative column-haunch rafter strengths roughly follow the shape of the bending moment diagram up thecolumn and across the rafter. The frames are usually designed plastically, so themoments referred to are the plastic moments under gravity loading, with plastic hingesdeveloping either in the column top or in the rafter at the haunch and in the rafter near theapex.With the reduction in roof live loading specified in SABS 0160-1989 (viz. 0,3 kPa insteadof 0,5 kPa), and with the more favourable load combination factor for dead load, thedesign loading for the dead plus live load combination for a typical portal frame is nowonly about 65 per cent of what it was previously. This means, of course, that lighter rafterand column sections can be used, but if as a consequence these sections are also madeshallower, the deflection of the frame could be greater than it was for the heavier loading.Where deflection was critical under the old loading, full advantage could in such a casenot be taken of the reduction in load and the required plastic modulus would beconsiderably more than 65 per cent of the old value.The above comments apply to dead plus live loading, which is usually the loadcombination that dictates the choice of column and rafter sizes from a strength point ofview. Dead plus wind load, however, is often the combination that governs from adeflection point of view and it may be necessary to increase member sizes to bringdeflections within allowable limits.For the above reasons portal frames are more likely to be designed elastically thanplastically in future. In any case, an elastic analysis is necessary to check the deflections. 6.1
  2. 2. With latticed trusses, on the other hand, deflection is seldom critical, so that betteradvantage of the new loading specifications can be taken and their efficiency rating bycomparison with portals can be improved.6.3 Eaves and apex connectionsThe types of eaves and apex haunches shown in Fig 6.1 are the ones almost universallyused because of their relative simplicity and the ease with which the frame can beerected. The critical design condition is usually gravity loading with the rafter-to-columnconnection having to sustain a high negative moment and the apex connection a smallerpositive moment. (b) B A B (a) B (ai) (c) Fig 6.1: Portal haunch and apex connections 6.2
  3. 3. The moment at the eaves produces a high tensile force in the upper flange of the rafterthat is transmitted through the upper tension bolts and the end plate to the inner flange ofthe column. The compressive force in the lower flange of the haunch is transferred inbearing through the end plate onto the column flange and into the web.The transfer of moment at the apex is similar, except that here the moment is positive sothe forces are reversed. The haunch and apex regions are vitally important parts of theframe and must be carefully proportioned. It is possible to achieve economy throughsimplification of the connections, but only when every aspect of the transfer of direct,moment and shear forces has been carefully considered.The upward extension of the haunch end plate in detail (a) of Fig 6.1 is often necessary toaccommodate the two topmost bolts, but may be dispensed with in smaller portals, asshown in detail (b). If the roof sheeting line interferes with the column top it will benecessary for the column to be trimmed as shown dotted in detail (a). This obviouslyinvolves extra expense. The use of the stiffening plate A is seldom necessary and shouldbe avoided where this is possible, even at the expense of slightly thickening the end plate.A method for designing extended end-plate moment connections is given in Section 7.6 ofStructural Steelwork Connections – Limit States Design (Ref. 7). The downward extensionof the end plate, as shown dotted, is only necessary when a high positive moment isinduced under wind load.The end of the haunch flange, where it butts against the end plate, is often bevelled asshown in detail (a) of Fig 6.1 to receive a full penetration groove weld. As the force here ishigh, it would be difficult in a detail such as shown in (ai) to ensure full bearing of theflange end against the plate. If the force were to be transferred by the welds then thethroat thickness of the lower weld would tend to be too small. Where the other end of thehaunch flange meets the underside of the rafter it should always be cut, as shown, toallow an adequate fillet weld to be laid. The depth hh of the haunch should be themaximum attainable from the section used, viz. h - tf - r1.The downward extension of the end plates below the apex haunch shown in detail (c) canbe dispensed with in the case of small moments, while the upward extension, showndotted, is only required in the case of very high negative moments.The stiffening plates B to the column are often required to stiffen the web and or flangesagainst the tensile or compressive force in the flanges of the haunch, but they can bedispensed with in smaller portals or when the column section is sufficiently stocky.Checks on the strength of all of the regions discussed above should be carried out asdescribed in Section 7.6 of the Steel Construction Handbook (Ref. 5).The bolts used in eaves and apex connections should be Grade 8.8S (friction-grip type)because of the high-tension forces induced, but need not be fully torqued to transfershear forces in friction grip. They should, however, be well tightened to ensure properbearing between the contact surfaces.An alternative rafter-to-column connection is shown in detail (a) of Fig 6.2, which can beused for portal frames where the moment at the rafter-to-column junction is not toosevere. The rafter and column have the same section size and are shop-welded with 6.3
  4. 4. their flanges bevelled to receive complete penetration groove welds. This is a simple andcheap connection and is supplemented by a site-bolted splice some way up the rafter, ata point of reduced bending moment. The location of the splice should be such that thelength L1 of the column-rafter component, as appropriate, is within transport limitations. Avariation of the rafter site splice is shown in detail (b) where a combination of shopwelding and site bolting is used, making for much easier erection. The apex joint is shop-welded. The length L2 of the rafter to the opposite splice should meet transportrequirements. (a) (b) Fig 6.2: Portal connections - alternative detailsDetail (a) of Fig 6.3 shows rafter-to-column and rafter apex splices incorporating adivision plate and applies to connections where the flanges require stiffening.Where frames have equal column and rafter sections, but where the corner moment ishigher, the details shown in (b) of the figure may be used. Here haunches are attached byshop welding to accommodate the higher moments and provide increased stiffness. 6.4
  5. 5. (a) Rafter splices as in Fig 6.2 (b) Fig 6.3: Portal connections - alternative details6.4 Lateral restraint to portal framesAs there are high bending moments at the column-to-rafter junction it is necessary toprovide adequate lateral-torsional restraint in this region. This may be done by means of astrut connected into the column web within the depth of the rafter haunch. A typical detailis shown in (a) of Fig 6.4, where a circular hollow section with an end plate is used.This section is light, as well as strong in both tension and compression; because of thebutt-welded end plate it is also able to offer torsional restraint to the column in this highly-stressed area. The strut is tied into the vertical bracing system(s) at one or both ends ofthe building, as discussed in Chapter 11, which deals with bracing systems. 6.5
  6. 6. Where the column requires additional lateralrestraint within its height a similar strut may beused, or the inner flange of the column may bekneebraced to a girt as shown in detail (b) ofFig 6.4.Lateral restraint for the rafter is also requiredand this is usually provided by connectingconventional rafter bracing to the top flange orwithin the depth of the rafter section. The formthat this bracing might take is also discussed inChapter 11. (a)6.5 Bracing to compression flanges of rafters KneebracesWhere the lower flanges of the rafters are incompression, either near the columns whenunder gravity loading or further up the slopewhen under wind uplift loading, they require tobe restrained laterally to prevent buckling. Thisis usually done by fitting angle braces to the (b)purlins, as shown in Fig 6.5. When angles areused they may be positioned on either one orboth sides of the rafter. It is, however, importantthat the section used is able to act incompression and tension. Fig 6.4: Stabilising of column6.6 PurlinsIn a typical portal-frame building the main structural components are the portal framesthemselves and the purlins. To achieve maximum economy it is necessary to optimise thecombined cost of these two items by choosing the correct spacing for the frames. Variousspanning arrangements can be used for the purlins, e.g. single, double or multispan, or Z-sections overlapped over the rafters, so it is not possible to give definitive guidelines as tooptimum spacing of the frames. What is clear is that the spacing between the purlinsshould be as large as the spanning capacity of the roof cladding will allow. Thereafter, fora given span of portal frame, the designer should do comparative checks for varyingspacings, allowing for the purlin splicing arrangement applicable. The subject of purlins isdealt with in greater detail in Chapter 12.6.7 CamberAs mentioned earlier, I-section portal frames are prone to high deflections because oftheir slender proportions. For a typical frame with a rafter slope of 15º under dead pluslive load, the vertical deflection at the apex would be of the order of span 200 and theoutward deflections at the tops of the columns about sin 15º times this amount. 6.6
  7. 7. It may be necessary to provide a camber or pre-set in the frame to compensate for the dead load and possibly for some part of the live load to ensure that the columns will be within the required tolerances for plumbness when erected. Fig 6.5: Braces to rafter bottom flange he pre-set is achieved very simply by adjusting the angles between the columns and the rafters, and between the rafters at the apex, as shown in Fig 6.6. a Preset a Preset Outer flange of rafterb b Inner flange of column Nominal shape, ie with Nominal shape frame under full dead load Preset shape (a) (b) Fig 6.6: Presetting of portal frame 6.7
  8. 8. 6.8 Column basesThe great majority of portal frames are designed with nominally pinned bases. This is forreasons of economy and simple design. Not only are fixed bases more expensivebecause of the need for thicker and larger base plates and the stiffening that isnecessary, but the foundations require to be much larger to resist the base moments.Only in cases of large lateral deflection, or possibly where brick walls are built into thecolumns, is it necessary to resort to fixed bases. These should be kept as simple aspossible, as discussed in Chapter 10.An alternative method is to use partially fixed bases that can develop a specified momentthat is less than the fully-fixed moment to keep lateral deflections within acceptable limits;such bases are obviously cheaper than fully fixed bases.6.9 Gable framesWhere buildings are not designed for future lengthwise extension, there is no need forportal frames to be provided at the ends. A more economical alternative is to supply alight I- or channel section rafter spanning across the tops of the gable posts and tiedlaterally into the rafter bracing system, as shown in Fig 6.7. Both the rafter and the cornercolumns can be much lighter than that of a portal, but more importantly the high cost ofthe portal eaves and apex haunches can be saved. It is necessary, though, to providelateral support and this can be done by means of a simple bracing system such as thatshown in the figure. Because of the double bracing panels the diagonal members need bedesigned for tension only.6.10 Multispan portalsMultibay buildings were discussed in Section 5.4 of Chapter 5 in the discussion on theadvantages of the double-slope versus the multi-slope profile. In the multi-slope profileshown in detail (a) of Fig 5.6 it would be structurally desirable to design the internalcolumns with fixed tops, i.e. with the columns forming part of the portal frames. In thecase of the double-slope profile shown in detail (b) of that figure, however, the internalcolumns might be more economically designed as pin-ended, since on account of theirgreater height they would be somewhat less effective in providing lateral stiffness to thebuilding. Also, because of the absence of lateral restraint in both the xx and yy directionsover their full height a more suitable cross section might be an H-section or a squareRHS. This was discussed in Section 5.2 of Chapter 5 with reference to detail (j) of Fig Standard portal framesSome years ago a standardised design for medium to large span low-pitch portal framessuitable for repetitive production was developed in the United States of America under thename of Butler building frames. Unlike the plastically designed I-section portals alreadydescribed, these frames have their rafter and column sections made of welded plategirder section, employing minimum thickness material. The slender proportions of the 6.8
  9. 9. cross-section elements render the frames unsuitable for plastic design treatment and theyare thus analysed elastically. A A B B (i) B = Bracing planes (ii) Alternative Sections A - A Fig. 6.7: Gable framingBecause of their economy, versatility and attractive appearance these buildings soonbecame very popular, their use spreading rapidly to countries outside of the UnitedStates. They are produced in South Africa under the name of Superframe Systems. Thistype of construction is, however, generally only economically justified where large-spanbuildings are required. 6.9
  10. 10. A typical single-span frame of this design is shown in Fig 6.8. It will be seen that thecolumns and rafters are tapered to match the general shape of the gravity bendingmoment diagram and the high moments at the column-rafter junction and at the apex canthus be accommodated by the deeper section. Uniform flange and web thicknesses canbe used, resulting in a frame with a minimum steel content. Roof slope 1:12 Fig 6.8: Standardised portal frameThe higher fabrication cost of the tapered, welded construction is more than offset by themuch reduced material content. The mass can be as little as 75 per cent of aconventional rolled-steel portal frame of similar size. Web thicknesses are as small as5 mm and flange thicknesses 8 mm.Such thin-webbed sections require non-conventional design and fabrication proceduresand the specialist fabricators use computer-aided design and detailing routines andautomated shop assembly methods.The standard spans range from 12,0 m to 30,0 m in 3,0 m increments and then up to54,0 m in 6,0 m increments. Eaves heights range from 4,0 m to 8,0 m in 1,0 mincrements. The rafter slope is 1:12 (4,76º).6.12 Summary• I-section portal frames designed to the current codes, viz. SABS 0160-1989 and SABS 0162-1:1993, are lighter than those designed to the earlier codes as regards strength requirements and deflection is thus likely to be a critical design factor. Portal frames will therefore tend to be designed elastically in future.• Simple bolted connections with Grade 8.8S bolts should be used for eaves and apex joints. Stiffeners to the column and rafter web, and end plate extensions, should be omitted where feasible.• For frames with equal column and rafter sections the members may be joined by welding, with bolted site splices located a short way up the rafters. For high corner moments welded haunches may be added. 6.10
  11. 11. • The rafter-to-column joint requires stabilisation against twist and lateral buckling because of the high negative moment. This may be achieved with a circular hollow section strut with end plates welded on and knee-braces if necessary.• It is important that all bracing members, including purlins and girts, used to effect lateral torsional stability to the rafter and column have sufficient stiffness to restrain the appropriate points.• The bottom flanges of rafters, when under flexural compression, need to be stabilised against buckling. The means usually employed is angle bracing from the purlins to the bottom flange.• For the sake of overall economy in material content and labour input, the spacing of the portal frames should be considered carefully. A layout having greater purlin spans and fewer, but slightly heavier frames, is usually best.• To counteract deflection under gravity loading, portal frames should be preset to provide a suitable camber. This is done by adjusting the angles between the columns and the rafter, and between the rafters at the apex.• Column bases should be pinned wherever possible. If fixed bases are used the stiffening details should be kept simple.• In buildings not designed for lengthwise extension, gable frames should be substituted for the end portals. These frames can be very simple and light, but must be braced within their plane.• Multispan portal buildings having two roof slopes only (i.e. no valleys), may have the interior columns pinned top and bottom for maximum economy and reduced foundation loading. 6.11