5 5.1
Industrial Buildings
Introduction
The design of industrial buildings is governed mainly by functional requirements and the need for economy of construction. In cross section these buildings will range from single or multi-bay structures of large span when intended for use as warehouses or aircraft hangars to smaller span buildings as required for factories, assembly plants, maintenance facilities, packing plants, etc. The main dimensions will nearly always be dictated by the particular operational activities involved, but the structural designer's input on optimum spans and the selection of suitable cross-section profiles can have an important bearing on achieving overall economy. Discussions between the owner or his architect and the engineer at an early stage of planning can help to secure a good balance between operational and structural considerations. An aspect where the structural designer can make a more direct contribution is in the lengthwise dimensions, i.e. the bay lengths of the building. Here a balance must be struck between large bays involving fewer, heavier main components such as columns, trusses, purlins, crane beams, etc, and smaller bays with a larger number of these items at lower unit mass. An important consideration in this regard is the cost of foundations, since a reduction in the number of columns will always result in lower foundation costs. In large industrial buildings with heavy overhead cranes, an economical dimension for the centres of the main columns (i.e. the span of the crane girders) is 15,0 m. Another rule of thumb is to make this dimension equal to the height of the rails above floor level. In the simpler type of building an optimum purlin span can have a bearing on bay length. Where crane girders are not required and the structure comprises mainly of columns, trusses (or rafters), purlins and girts, the spacing of roof principals at large intervals will nearly always be more economical. The use of continuous cold-formed purlins – sleeved or lapped at the joints – will permit truss spacings of 9,0 m or more, greatly reducing the total steelwork mass. The subject of purlins is dealt with in greater detail in Chapter 13 and the detailed design of continuous cold-formed channel purlins is given in the South African Steel Construction Handbook (Ref. 5).
5.2
Cross sections
The choice of cross section for a single storey industrial building is very wide, but experience has shown that a limited number of shapes are the most practical and economical. Some of these are shown in Fig 5.1.
5.1
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(j)
(k)
Fig. 5.1: Industrial building cross sections
5.2
In detail (a) of Fig 5.1 the traditional Fink truss, which has been popular for many years, is employed. The column bases are fixed and the truss has nominally pinned connections to the columns. If knee-braces do not cause an obstruction they can be used (as shown dotted). In this case the column bases can be pinned, resulting in a saving in foundation costs. With the current trend towards flatter roof slopes – mainly for architectural reasons – the Pratt truss shown in detail (b) has become popular. When considering gravity loads it has a structurally favourable shape and can be designed very efficiently. Lateral stiffness is provided by the depth of the truss at the columns and the column bases can be made pinned if desired. A comparison of chord forces between this truss and the type shown in detail (a), is shown in Fig 5.2. The maximum forces indicated are based on a uniformly distributed gravity load of 100 kN over the span. Because the Pratt truss conforms more closely to the parabolic distribution of the bending moment, the member forces are much more favourable. The truss has a smaller depth and can thus be transported more easily, while the configuration of the members makes for easier shop assembly. The single or multi-span I-section portal frames shown in detail (c) of Fig 5.1 are among Span of trusses 16.0 m Load / truss = 100 kN Slope 1:13
Slope 1:10
104 50
50
50
Fink truss
50 Pratt truss
Fig.5.2: Fink and Pratt trusses - maximum chord forces. the most popular building frames because of their simplicity and attractive appearance and the better overhead clearance provided. Their mass per square metre of floor area is higher than for the Fink and Pratt truss construction, but the labour input per ton is much less. Where fabricators are properly equipped, portal frames can be more economically produced than latticed trusses for spans up to about 40 m. Special care is required to cut the rafter ends accurately at the haunches and apex and to weld on the end plates without excessive distortion. Column bases are usually pinned on portals of low height, but may be fixed for greater height requirements. The most efficient roof slope for a portal frame is around 15° to 18°. A lower pitch is often favoured, but a heavier rafter is then required to accommodate the higher bending moments. Portal frames are discussed in greater detail in Chapter 6. The cross sections shown in details (a) to (c) of Fig 5.1 (but without the kneebraces in (a)) account for the great majority of single-storey industrial buildings. The mass per square metre of Fink trusses is lowest for spans from 15,0 m upwards, of Pratt trusses from 10,0 m to 25,0 m and of portal frames from 10,0 m to 20,0 m. For all spans the portal frame has a higher unit mass than the two truss types, with the difference becoming greater from 20,0 m span upwards. A useful comparison of the relative masses and costs per square metre of area covered in the above three types of cross section, taking into account the variation in the spans
5.3
and spacings of the roof principals, is given in Ref. 11. Comparative curves showing the mass per square metre are given for roof trusses, lattice girders, portal frames and space frames. The information is given for buildings 90,0 m in length and has been derived from buildings constructed in the United Kingdom. The design loading used is of course higher than would be applied in South Africa, the gravity loading being about 1,0 kPa (including live load, but excluding the selfweight of purlins and roof principals) and the basic wind speed 46 m s. For comparative purposes, however, the curves would be valid for the lower loading used in this country. The tied rafter shown in detail (d) of Fig 5.1 is also economical in material usage. The rafters can be designed plastically and generally have a bolted haunched connection at the apex. The tie prevents outward spread at the eaves and the moment distribution in the rafter is therefore more favourable than in a conventional portal, the plastic moment being about one-half of that in a conventional portal for a 3:10 (16,7°) roof slope. The control of eaves spread also makes this type of construction suitable for multi-span buildings and buildings with overhead cranes. The column section can be lighter because of the lower bending moments and the total steel content in the roof and columns can be as much as 20 to 25 per cent lower than that of a conventional portal. The roof shape shown in detail (e) can be used instead of an I-section portal (detail (c)) and is suitable for all building sizes from farm sheds to large storage or warehouse buildings. It is stiffer than the portal and thus has a much smaller eaves spread. Detail (f) shows a type of pre-engineered portal developed especially for wide spans and low pitches. The rafters and columns are welded plate I-sections using thin webs and are tapered to match the bending moment distribution. This type of portal frame is discussed in greater detail in Section 6.11 of Chapter 6. In Detail (g) and (h) the roof profiles are similar to those shown in details (b) and (d), but the columns are designed to carry gantry girders for overhead cranes. In either case the columns may be latticed or have projecting brackets to support the girders. For reasons of rigidity (including maintenance of the crane span), column bases must be fixed and the column tops kept at the correct distance by the roof tie/strut. The multi-span double-slope cross section shown in detail (j) is suitable for wide buildings and is very economical in material usage and labour content. In the case, of an odd number of bays (as shown) there will be no central column and the rafters will need to be site-spliced for full continuity at the apex. The rafters are designed either as fully continuous over the whole width of the building or as semi-hinged, with hinges (i.e. flexible splices) near to but not at the internal columns. Lateral stability is provided by the outer columns, which have adequate stiffness because of their short height and moment resisting connections. The internal columns may have pinned bases and caps and thus be very simple to fabricate. If they are of H-section they will possibly not need lateral restraint within their height. The absence of valley gutters and downpipes also contributes significantly to the economy of this cross section. For large single-span buildings the three-pinned arch shown in detail (k) is often used. The deep haunches correspond with the locations of high bending moments. In even larger span buildings such as aircraft hangars a two-pinned arch may be used, with parallel-chord rafters and full continuity at the apex.
5.4
General comments applicable to the various profiles shown in Fig 5.1 are as follows: • Deep trusses, as in details (b), (e) and (g), are very effective in providing transverse
stiffness to the building; column bases may thus be pinned. • The panel lengths of the lattice trusses should be arranged to coincide with the purlin
positions, which in turn are governed by the maximum allowable span of the sheeting (see Chapter 13 ). • When the building height is great the columns should have fixed bases to reduce
side sway. • Where deep latticed trusses are used in multi-span buildings they need not necessarily
be made continuous over the internal columns since they have sufficient depth to act efficiently as single spans. In this case a sliding joint should be provided at bottom tie level of one or both of the trusses where they meet at the column. Fig 5.3 shows two efficiently framed cross sections for large industrial buildings with heavy overhead cranes. Both may have latticed columns, which are necessary to resist crane lateral loadings because of the height of the buildings – or may have welded Isection columns where the height is less. The tied portal is suitable in the smaller span building, but would not be practical over a long span because of the increased roof height. The main tie would act as a strut over the full span length under certain combinations of load and would have to be stabilised horizontally by longitudinal ties running down the length of the building. A suitable section for the main tie/strut would be a circular hollow section.
5.3
Roof lighting
For the most efficient daylighting of building interiors, roof lighting monitors and glazing planes should be arranged to face south only or north and south, but not west, because of glare and heat from the afternoon sun. In buildings where the length is in the east-west direction, the monitors would run parallel to the length for optimum lighting, as shown in detail (a) of Fig 5.4, whereas in buildings with the long axis on a north-south line the monitors would have to run transverse to the building length, as shown in detail (b). The shape shown in detail (c) is also suitable for a building with an east-west long axis. In this arrangement the glazing faces south only, an efficient design that avoids direct light penetration. The longitudinal orientation, as in details (a) and (c) is much simpler and cheaper than the transverse alignment in (b).
5.5
Glazing
Glazing
1.8
3 0.0
1.8
(a)
Glazing
Glazing
3 6.0 (b)
Fig 5.3: Cross sections for large industrial buildings with cranes
5.6
Fig 5.5 shows enlarged sections through the monitors, corresponding with those in details (a) and (b) of Fig 5.4, and indicates how the monitor frames can be constructed quite simply. Note that in detail (a) a longitudinal ridge strut (shown as a circular hollow section) is usually necessary at the apex of the roof because of the absence of the two ridge purlins that are normally present; the strut is needed to stabilise the rafter laterally against compression at this point and must be tied into an adequate bracing system. N
S
E
W
(a)
N
S
(b)
(c)
Fig 5.4: Orientation of roof lighting As shown in detail (b) of Fig 5.5 the slope-mounted monitor consists of a series of frames mounted on top of the purlins. Because of the absence of roof sheeting over the purlins within the width of the frame the purlins need to be checked for lateral instability over this length and, if necessary, a down-slope tie or strut must be inserted. In arrangements (a) and (b) the lighting plane facing north must be glazed with material having a reduced translucency compared with that on the south (or extended overhangs).
N
N
Sloping or vertical
OG
CG
OG
CG
Ridge strut
Purlin (a)
OG = Opaque Glazing CG = Clear Glazing
(b)
Fig 5.5: Roof lighting monitor frames As an alternative to the monitors described, a very simple and cheap arrangement is to place translucent sheets in the sloping planes of the roof, say one translucent sheet to every ten or so metal sheets, running the full slope-length of the roof from ridge to eaves. The drawback of this system is that direct sunlight passes through the roof and tends to cause glare inside the building, but this can be offset to some extent by using sheets with reduced translucency.
5.7
5.4
Double-slope versus multi-slope roof
Multi-bay buildings are often designed as shown in detail (a) of Fig 5.6, where each bay has a double-slope roof. This arrangement is suitable where steep roof slopes are required, but is not as economical as the profiles shown in details (b) and (c), where the whole roof has only two slopes. The reduction in ridging, valley gutters and the number of purlins required is obvious, while the roof principals are much easier to construct. A further benefit is a substantial reduction in eaves spread due to gravity loadings. A two-slope roof in long buildings has the additional advantage of eliminating internal rainwater pipes and underground drains. Special care must be taken to ensure that the roof cladding is watertight on low-pitched roofs that have long roof slopes. With modern roof sheeting profiles such as the standing seam this is not unduly difficult to achieve.
(a)
Underfloor drain
(b)
(c)
Fig 5.6: Multi-slope and double-slope roofs
5.5
Gable framing
Where provision has to be made for the lengthwise extension of a building in the future it is necessary for a roof principal to be located at the gable end. The gable framing should in this instance be placed just clear of the truss or rafters and the tops of the gable posts would be connected laterally to the roof structure.
5.8
Where no future extension is envisaged the need for a roof principal at the end falls away and the gable framing would include a light rafter, spanning across the tops of the gable posts to support the purlins. This arrangement is much simpler and therefore more cost-effective. If the rafter in each slope can be made continuous between the posts a very light section can be used. The gable frame should be braced within its plane to resist wind loading on the sides of the building and the corner columns can thus be of smaller section than the side columns typically used down the length of the building. An example of a light gable frame as used in a portal frame building is shown in Fig 6.7.
A
0.46h
0.5h
In the case of tall single-storey buildings the gable posts will have to be of substantial size to resist the wind loading on the gable end. However, if the bases of the posts are made fixed and they are designed plastically, a much lighter section can be used. Where the gable is very high, but the width moderate, a horizontal wind girder at about half the height, spanning across the gable width, will be effective in reducing the gable-post size and in particular, its deflection. Fig 5.7 compares the plastic bending moments in a fixed-base post without a horizontal wind girder (b), a pinned-base post with a horizontal girder (c), and a fixed-base post with a horizontal girder (d). In the latter case not only is a minimum moment achieved, but the loading applied to the wind girder and at roof level is a minimum.
A
0.5h
0.54h
h
Wind girder
Mp = 0.086Wh Mp = 0.021Wh (a)
(b)
(c)
Mp = 0.0182Wh (d)
W = total wind load on centre gable post Section A - A
Fig 5.7: Wind loading on gable posts
5.6
Three-dimensional framing
Where the length-to-span and span-to-height ratios of a building are within reasonable limits, the three-dimensional bracing system shown in Fig 5.8 can be used to good effect. Internal clearance is good and the structure is simple to fabricate since there are no latticed trusses or fixed-ended beams or rafters. The bracing members can be light
5.9
because of the small span-to-depth ratios of the bracing systems, but care must be taken during fabrication and erection to ensure accurate assembly of the frame. In the design of the bracing provision must be made for proper resolution of forces at all bracing node points. Eccentricities at connections should be limited and adequately provided for.
Ridge strut
Optional gable wind girder
Pin connections
Isometric view
Cross section
Fig 5.8: Three-dimensional framing If pinned-base columns are used the frame analysis is simple. It involves the resolution of all the applied loading into the planes of the roof and the side and gable bracing systems. When the column bases are fixed the structure is no longer statically determinate and a three-dimensional computer analysis is necessary. A fairly steep roof slope is necessary in this case to keep the resolved gravity loading forces in the rafter bracing planes within acceptable limits. All bolts in the roof bracing should be well tightened since any slip under load would result in outward spread at the eaves. Three-dimensional framing is ideal for tall pallet storage buildings where the racking is independent of the main structure and for buildings housing self-supporting storage bins. It can also be adapted to mine winder houses if fixed-base latticed crane columns are incorporated.
5.10
5.7
Multi-storey industrial buildings
Multi-storey industrial buildings cover a very wide range of uses and include warehouses, process plant buildings, transfer houses in conveyor lines, crusher buildings, boiler houses, steel mill buildings and bin houses. Warehouses may have uniform storey heights and transverse bay widths, but the bay heights and widths of most other industrial buildings vary considerably in accordance with the particular process or function pertaining to each bay or level. In many of these buildings allowance must be made for the location of conveyors, plant items and storage bins. The feature common to all multi-storey industrial buildings is their large height-to-width ratio in cross section, which often militates against an economical structural configuration as far as the provision of resistance to lateral wind or possibly plant loading is concerned. A tall building without transverse bracing will inevitably require a heavy structure because of the need to incorporate a rigid moment-resisting frame. It is often possible, however, to incorporate vertical bracing without interfering with the required operating clearances. This is invariably the best way of providing lateral resistance and stiffness to the structure.
(a) No cross bracing
(b) Tension – Compression bracing
(c) Tension only bracing
Fig 5.9: Two - storey buildings
5.11
Rigid frames without triangulated bracing are heavy and expensive because of the moments induced in the beams and columns and the stiff beam-to-column connections required. Transverse bracing should be used wherever possible, but should be arranged for minimum plant interference, maximum effectiveness and least material usage. As the wind overturning moment on a building is greatest at ground level the width of the bracing system should be greatest at the base. Cross sections of various buildings are shown in Figs 5.9 to 5.11 and the features of each are discussed in general terms in what follows. Detail (a) of Fig 5.9 shows a simple two-storey building with a low height-to-width ratio. The provision of fixed bases to the centre and outer columns contributes significantly to the lateral stiffness of the frame. At the expense of a heavier centre column of lesser height, savings are achieved in the two much taller outer columns. The top of the centre column and the outer ends of the beam are pinned. A similar building is shown in detail (b) of Fig 5.9, but here it is assumed that cross bracing can be incorporated in at least one of the bays. In this case the bracing provides all the lateral resistance and all three column bases and beam ends can be pinned, with substantial savings in column and foundation sizes. The side columns act as cantilevers from first floor level upwards and can easily resist the relatively small amount of wind loading applied over this height. The beams and their connections to the outer columns are designed to resist the axial forces induced in the beams in addition to the normal moments within the span and shear forces at the ends of the beams. Note that the brace is a single tension-compression member. With the single brace all of the lateral wind load is resisted at ground level by only one foundation. This is an economical solution when the brace length is not excessive. In larger buildings the bracing to be employed should be the chevron-type (tension-compression) or X-type (tension only or tension-compression), in which case lateral wind load resistance is shared by two foundations. If both bays were available for bracing, as in detail (c) of the figure, an even cheaper solution might result because of the absence of wind-induced axial force in the outer columns. Fig 5.10 shows a much taller building, where plant items, conveyors or other obstructions on the upper floors prevent the use of full-width bracing in certain bays. When the overall width of the building is not excessive, the centre column may be omitted in the top storey as shown and a full-span truss be used instead. In this way an unobstructed top floor is achieved. The example illustrates an economical bracing arrangement where the amount of bracing used in each storey progressively increases from zero in the top storey to a maximum at ground level. A cross section through a bin or bunker bay is given in Fig 5.11. Here the bulkhead of the bunker is a stiff panel that is effective in providing lateral bracing. In the storey below this cross bracing is used; if chevron bracing were used it would be heavily loaded at its apex by the gravity loading from the bunker. The chevron bracing in the lowermost panel provides maximum headroom below it.
5.12
If column allowed, use rafter.
Item of plant
Conveyors
Fig 5.10: Plant building
5.8
Bracing of industrial structures
Vertical bracing in the general context of its function, viz. to provide lateral stability to a structure has been referred to in the previous section. The more detailed aspects of bracing systems, including efficient configurations, and economical member types, are dealt with in Chapter 11.
5.9
Crane buildings
In buildings containing gantries for overhead travelling cranes, the span of the gantry beams has a significant effect on the overall economy of the structure. Increased column spacing results in fewer columns and roof principals, with large cost savings, especially when latticed columns are used. Although the size of the crane beam is increased, and provided this does not require a change from a rolled section beam to a plate girder, the cost increase is relatively small since the cost of additional material is largely offset by the proportionately smaller increase in workmanship. Balanced against this is the increase in purlin and girt sizes, but here again fewer, heavier members may not be any more costly than a larger number of smaller ones. A further consideration is whether intermediate
5.13
columns to support the side sheeting become necessary. Generally, larger bay lengths tend to be more economical than smaller ones.
Fig 5.11: Bunker buildings
In certain circumstances crane beams can be made semi-continuous, i.e. continuous over two spans. This results in a much lighter section and reduced deflection, without requiring an increase in labour input. In the case of rolled beams, however, this would entail a maximum span of about 7,5 m, since an overall (two-span) length exceeding 15,0 m would tend to be uneconomical as regards purchasing, handling and transportation. A 7,5 m span is rather short, except when used in small buildings with light cranes. On the other hand, welded girders as used for heavier cranes could be made up to, say, 2 x 9,0 m spans. A point to be remembered, though, is that with continuous beams or girders greater care must be taken in erecting the columns to ensure a uniform level of the crane caps so as to avoid introducing of secondary bending stresses in the beams. Further considerations regarding crane beams, some of them relating to economical design and detailing, are given in the South African Steel Construction Handbook (Ref. 5). Sections through typical crane beams and girders are shown in Fig 5.12, ranging from the simple I-beam as used for light cranes and short girder spans to the stiffened plate girder with lateral plate as used in heavy crane buildings. The provision of the lateral plate adds considerably to the cost, but is essential for severe-duty cranes of large capacity. The plate serves both to support the top flange of the girder against lateral buckling and to resist the lateral forces from the crane wheels caused by crab braking and acceleration and crane skewing. The lateral plate should be friction-grip bolted to the flange; welding
5.14
could cause distortion of the girder and result in fatigue cracking. Intermittent welds may accelerate fatigue cracking.
(a) (b) (c) (d)
Fig 5.12: Sections through crane gantry beams and girders An alternative method of providing stability to the top lateral flange of a crane girder is to use a latticed lateral girder with bolted or welded single-angle lacing. However, if bolting is used, great care needs to be taken to achieve non-slip connections, especially in the case of severe-duty cranes. The bolts in such connections have a tendency to work loose under the dynamic loading. Intermediate stiffeners should be continuously welded to the main girder web, but should stop short of the bottom flange. They should also be wide enough to stabilise the top flange against the twisting effects produced by the lateral wheel loading at rail-head level whilst still allowing for coping. Gantry girders are an example of structural components that should be designed and built with great care. No short cuts must be taken as ill-considered cost savings at the construction stage could result in very expensive repairs, out of all proportion to the initial savings, during the service life of the gantry. Gantry girders may be supported on the columns in various ways, the most common ones being shown in Fig 5.13. The details are arranged in ascending order of crane size and duty rating. The bracket support shown in detail (a) is only suitable for light loading because of the moment induced in the column. Detail (b) applies to welded-plate columns and details (c) and (d) to latticed columns. In (d) the girder has been moved towards the back leg of the column so as to distribute the load between the two legs. In latticed columns both legs are made of sections of the same nominal width (but different mass per metre) to facilitate the connection of the lacings. In detail (c) the back leg would probably be unnecessarily wide and consequently understressed. In detail (d) more load would be transferred to this leg, while the main leg would be relieved of load and could then be of smaller section. The economy gained, especially on tall columns, could be appreciable. A further significant advantage is that if the girder were placed at or near the
5.15
centroid of the two legs, the horizontal deflection or sway tendency of the column under vertical load would be eliminated. This would have the dual effect of relieving the roof trusses of compressive force in the main tie and, more importantly, of reducing the side wear on the crane wheels that is caused by the rail gauge being reduced. It is clear that this arrangement results in real economies, both in initial cost and in subsequent maintenance costs. The use of detail (b) and (d) may be restricted by crane clearance requirements.
(a) (b)
(d)
(c) Crane rails not shown
Fig 5.13: Support of crane gantry girders
5.16
5.10
Summary
• In smaller buildings an increase in truss or rafter spacing (with an increase in purlin
size) is usually beneficial because of the reduction in the number of roof principals and columns. • Economical cross sections are provided by Fink and Pratt trusses and by portal
frames, but the choice would be governed by the span. • For larger spans and steeper roof slopes the tied portal frame is economical. It is also
suitable for multi-span buildings. • For low-pitch roofs the parallel-chord latticed truss is efficient and has the advantage of
providing fixity at the column cap; the column bases may therefore be pinned if desired. • Multi-span double-slope portals of low pitch are suitable for wide buildings and are
economical in material usage and labour input. Internal valley gutters and downpipes are eliminated. • Buildings incorporating crane gantries should have stiff columns with fixed bases, as
well as transverse ties (or latticed trusses) at eaves level to maintain the column caps and the gantry girders at the correct centres. • Roof lighting monitors should allow daylighting from the south only or from the south
and north, but not from the west. Monitors running along the ridge(s) of a building are structurally more economical than those running at right angles, i.e. up and down the roof slopes. • Within the width of monitors the roof sheeting is omitted. For transverse monitors the
purlins are laterally unrestrained and for longitudinal monitors the rafters are laterally unrestrained and, if necessary, arrangements must be made to prevent buckling. • Where buildings are to be extended lengthwise in the future, temporary gable framing
must be provided adjacent to the end frames of the building; the gable framing should be designed as light as possible. Where no lengthwise future extension is envisaged the end-roof principals can be replaced by permanent gable frames. • In tall buildings the gable posts can be reduced in size by providing fixed bases and by
designing plastically. • In tall but not unduly wide buildings a horizontal gable wind girder may be provided at
mid-height to reduce the gable-post size and control deflection. • Where the proportions of a building are compact, i.e. where the length is not
excessive, three-dimensional framing may be used in order to arrive at a low-mass structure. Care must be taken to ensure proper triangulation of all bracing systems. • In multi-storey industrial buildings it is often possible to find acceptable locations for
transverse stability bracing. This is much cheaper than providing fixed beam-to-column moment connections.
5.17
• In buildings containing crane gantries, longer girder spans, i.e. a wider spacing of
columns and trusses, usually results in economy. The gantry girders may be designed in two-span semi-continuous lengths, but care must be taken during erection to ensure uniform column cap heights. • The detail and connections of welded crane gantry girders and lateral or surge girders
should be designed carefully to prevent fatigue cracking and a working loose of bolts. No short cuts to save money should be taken. • Latticed crane columns may have the crane girders located between the tops of the
legs to share the vertical load between the legs, thus reducing the combined mass of the two legs, as well as the horizontal deflection of the columns under vertical loading.
5.18
