economics-for-structural-steel

1. 6 Portal-frame Buildings
6.1 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 column-
haunch 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

2. 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

3. 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

4. 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 shop-
welded. 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

5. (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 highly-
stressed 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

6. 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. (a)
6.5 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 (b)
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.
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 Z-
sections 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

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 rafter
b 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. 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

9. 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

10. 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

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