6 6.1
Portal-frame Buildings
Introduction
Because of their clean lines, good overhead clearance and relatively low cost, portal- frame buildings have become very popular. They make up a large percentage of the small to medium size single-storey industrial buildings in current use.
6.2
I-section portal frames
The rafters and columns of I-section portal frames consist of rolled I-sections, with the rafter ends being haunched and site-bolted to the columns, as was shown in detail (c) of Fig 5.1. The column section is heavier than the rafter section, so that the relative columnhaunch rafter strengths roughly follow the shape of the bending moment diagram up the column and across the rafter. The frames are usually designed plastically, so the moments referred to are the plastic moments under gravity loading, with plastic hinges developing either in the column top or in the rafter at the haunch and in the rafter near the apex. With the reduction in roof live loading specified in SABS 0160-1989 (viz. 0,3 kPa instead of 0,5 kPa), and with the more favourable load combination factor for dead load, the design loading for the dead plus live load combination for a typical portal frame is now only about 65 per cent of what it was previously. This means, of course, that lighter rafter and column sections can be used, but if as a consequence these sections are also made shallower, 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 case not be taken of the reduction in load and the required plastic modulus would be considerably more than 65 per cent of the old value. The above comments apply to dead plus live loading, which is usually the load combination that dictates the choice of column and rafter sizes from a strength point of view. Dead plus wind load, however, is often the combination that governs from a deflection point of view and it may be necessary to increase member sizes to bring deflections within allowable limits. For the above reasons portal frames are more likely to be designed elastically than plastically in future. In any case, an elastic analysis is necessary to check the deflections.
6.1
With latticed trusses, on the other hand, deflection is seldom critical, so that better advantage of the new loading specifications can be taken and their efficiency rating by comparison with portals can be improved.
6.3
Eaves and apex connections
The types of eaves and apex haunches shown in Fig 6.1 are the ones almost universally used because of their relative simplicity and the ease with which the frame can be erected. The critical design condition is usually gravity loading with the rafter-to-column connection having to sustain a high negative moment and the apex connection a smaller positive moment.
(b) B
A
B
(a)
B
(ai)
(c)
Fig 6.1: Portal haunch and apex connections
6.2
The moment at the eaves produces a high tensile force in the upper flange of the rafter that is transmitted through the upper tension bolts and the end plate to the inner flange of the column. The compressive force in the lower flange of the haunch is transferred in bearing 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 so the forces are reversed. The haunch and apex regions are vitally important parts of the frame and must be carefully proportioned. It is possible to achieve economy through simplification 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 to accommodate the two topmost bolts, but may be dispensed with in smaller portals, as shown in detail (b). If the roof sheeting line interferes with the column top it will be necessary for the column to be trimmed as shown dotted in detail (a). This obviously involves extra expense. The use of the stiffening plate 'A' is seldom necessary and should be 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 of Structural Steelwork Connections – Limit States Design (Ref. 7). The downward extension of the end plate, as shown dotted, is only necessary when a high positive moment is induced under wind load. The end of the haunch flange, where it butts against the end plate, is often bevelled as shown in detail (a) of Fig 6.1 to receive a full penetration groove weld. As the force here is high, it would be difficult in a detail such as shown in (ai) to ensure full bearing of the flange end against the plate. If the force were to be transferred by the welds then the throat thickness of the lower weld would tend to be too small. Where the other end of the haunch flange meets the underside of the rafter it should always be cut, as shown, to allow an adequate fillet weld to be laid. The depth hh of the haunch should be the maximum attainable from the section used, viz. h - tf - r1. The downward extension of the end plates below the apex haunch shown in detail (c) can be dispensed with in the case of small moments, while the upward extension, shown dotted, 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 flanges against the tensile or compressive force in the flanges of the haunch, but they can be dispensed 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 as described 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 transfer shear forces in friction grip. They should, however, be well tightened to ensure proper bearing between the contact surfaces. An alternative rafter-to-column connection is shown in detail (a) of Fig 6.2, which can be used for portal frames where the moment at the rafter-to-column junction is not too severe. The rafter and column have the same section size and are shop-welded with
6.3
their flanges bevelled to receive complete penetration groove welds. This is a simple and cheap connection and is supplemented by a site-bolted splice some way up the rafter, at a point of reduced bending moment. The location of the splice should be such that the length L1 of the column-rafter component, as appropriate, is within transport limitations. A variation of the rafter site splice is shown in detail (b) where a combination of shop welding and site bolting is used, making for much easier erection. The apex joint is shopwelded. The length L2 of the rafter to the opposite splice should meet transport requirements.
(a)
(b)
Fig 6.2: Portal connections - alternative details
Detail (a) of Fig 6.3 shows rafter-to-column and rafter apex splices incorporating a division plate and applies to connections where the flanges require stiffening. Where frames have equal column and rafter sections, but where the corner moment is higher, the details shown in (b) of the figure may be used. Here haunches are attached by shop welding to accommodate the higher moments and provide increased stiffness.
6.4
(a)
Rafter splices as in Fig 6.2
(b)
Fig 6.3: Portal connections - alternative details
6.4
Lateral restraint to portal frames
As there are high bending moments at the column-to-rafter junction it is necessary to provide adequate lateral-torsional restraint in this region. This may be done by means of a strut connected into the column web within the depth of the rafter haunch. A typical detail is 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 the butt-welded end plate it is also able to offer torsional restraint to the column in this highlystressed area. The strut is tied into the vertical bracing system(s) at one or both ends of the building, as discussed in Chapter 11, which deals with bracing systems.
6.5
Where the column requires additional lateral restraint within its height a similar strut may be used, or the inner flange of the column may be kneebraced to a girt as shown in detail (b) of Fig 6.4. Lateral restraint for the rafter is also required and this is usually provided by connecting conventional rafter bracing to the top flange or within the depth of the rafter section. The form that this bracing might take is also discussed in Chapter 11.
6.5
(a)
Bracing to compression flanges of rafters Kneebraces
Where the lower flanges of the rafters are in compression, either near the columns when under gravity loading or further up the slope when under wind uplift loading, they require to be restrained laterally to prevent buckling. This is usually done by fitting angle braces to the purlins, as shown in Fig 6.5. When angles are used they may be positioned on either one or both sides of the rafter. It is, however, important that the section used is able to act in compression and tension.
(b)
Fig 6.4: Stabilising of column
6.6
Purlins
In a typical portal-frame building the main structural components are the portal frames themselves and the purlins. To achieve maximum economy it is necessary to optimise the combined cost of these two items by choosing the correct spacing for the frames. Various spanning arrangements can be used for the purlins, e.g. single, double or multispan, or Zsections overlapped over the rafters, so it is not possible to give definitive guidelines as to optimum spacing of the frames. What is clear is that the spacing between the purlins should be as large as the spanning capacity of the roof cladding will allow. Thereafter, for a given span of portal frame, the designer should do comparative checks for varying spacings, allowing for the purlin splicing arrangement applicable. The subject of purlins is dealt with in greater detail in Chapter 12.
6.7
Camber
As mentioned earlier, I-section portal frames are prone to high deflections because of their slender proportions. For a typical frame with a rafter slope of 15º under dead plus live load, the vertical deflection at the apex would be of the order of span 200 and the outward deflections at the tops of the columns about sin 15º times this amount.
6.6
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.
Preset
a a
Preset
b
b
Inner flange of column
Nominal shape Preset shape
Nominal shape, ie with frame under full dead load
(b)
(a)
Fig 6.6: Presetting of portal frame
6.7
Outer flange of rafter
6.8
Column bases
The great majority of portal frames are designed with nominally pinned bases. This is for reasons of economy and simple design. Not only are fixed bases more expensive because of the need for thicker and larger base plates and the stiffening that is necessary, 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 the columns, is it necessary to resort to fixed bases. These should be kept as simple as possible, as discussed in Chapter 10. An alternative method is to use partially fixed bases that can develop a specified moment that 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 frames
Where buildings are not designed for future lengthwise extension, there is no need for portal frames to be provided at the ends. A more economical alternative is to supply a light I- or channel section rafter spanning across the tops of the gable posts and tied laterally into the rafter bracing system, as shown in Fig 6.7. Both the rafter and the corner columns can be much lighter than that of a portal, but more importantly the high cost of the portal eaves and apex haunches can be saved. It is necessary, though, to provide lateral support and this can be done by means of a simple bracing system such as that shown in the figure. Because of the double bracing panels the diagonal members need be designed for tension only.
6.10
Multispan portals
Multibay buildings were discussed in Section 5.4 of Chapter 5 in the discussion on the advantages of the double-slope versus the multi-slope profile. In the multi-slope profile shown in detail (a) of Fig 5.6 it would be structurally desirable to design the internal columns with fixed tops, i.e. with the columns forming part of the portal frames. In the case of the double-slope profile shown in detail (b) of that figure, however, the internal columns might be more economically designed as pin-ended, since on account of their greater height they would be somewhat less effective in providing lateral stiffness to the building. Also, because of the absence of lateral restraint in both the xx and yy directions over their full height a more suitable cross section might be an H-section or a square RHS. This was discussed in Section 5.2 of Chapter 5 with reference to detail (j) of Fig 5.1.
6.11
Standard portal frames
Some years ago a standardised design for medium to large span low-pitch portal frames suitable for repetitive production was developed in the United States of America under the name of Butler building frames. Unlike the plastically designed I-section portals already described, these frames have their rafter and column sections made of welded plate girder section, employing minimum thickness material. The slender proportions of the
6.8
cross-section elements render the frames unsuitable for plastic design treatment and they are thus analysed elastically.
A A
B
B
(i)
B = Bracing planes
(ii)
Alternative Sections A - A
Fig. 6.7: Gable framing Because of their economy, versatility and attractive appearance these buildings soon became very popular, their use spreading rapidly to countries outside of the United States. They are produced in South Africa under the name of Superframe Systems. This type of construction is, however, generally only economically justified where large-span buildings are required.
6.9
A typical single-span frame of this design is shown in Fig 6.8. It will be seen that the columns and rafters are tapered to match the general shape of the gravity bending moment diagram and the high moments at the column-rafter junction and at the apex can thus be accommodated by the deeper section. Uniform flange and web thicknesses can be used, resulting in a frame with a minimum steel content. Roof slope 1:12
Fig 6.8: Standardised portal frame
The higher fabrication cost of the tapered, welded construction is more than offset by the much reduced material content. The mass can be as little as 75 per cent of a conventional rolled-steel portal frame of similar size. Web thicknesses are as small as 5 mm and flange thicknesses 8 mm. Such thin-webbed sections require non-conventional design and fabrication procedures and the specialist fabricators use computer-aided design and detailing routines and automated shop assembly methods. The standard spans range from 12,0 m to 30,0 m in 3,0 m increments and then up to 54,0 m in 6,0 m increments. Eaves heights range from 4,0 m to 8,0 m in 1,0 m increments. 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
•
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
