10
Columns
10.1 Introduction Columns, especially compound or latticed columns, represent one of the more complex categories of steelwork components in a typical building structure. Because of the variety of components that can be connected to columns, for example trusses, girders, beams, sheeting girts, vertical bracing, eaves struts, crane girders, etc, it is usually not possible to achieve a great degree of repetitiveness in column production. For this reason every effort should be made to achieve whatever economies are possible through careful design of the basic column.
10.2 Column sections Typical column cross sections are shown in Fig 10.1. As in the case of beams (see Section 8.2), plain I- or H-sections should be used wherever possible in view of their simplicity. It is preferable to use a heavier plain section than a lighter compound one because of the high labour content of the latter. The plated I-section shown in detail (d) of Fig 10.1 would be cheaper than the welded-plate section in (f), while the box section in (g) would be used only in special cases of very high loading. A square or rectangular SHS as shown in detail (c) is very efficient on a load mass basis, but the much higher cost per ton would have to be provided for.
(a)
(e)
(b)
(c)
(f)
(d)
(g)
Fig 10.1: Column cross sections
10.1
(h)
For comments on compound column sections and a description of composite steelconcrete columns, refer to Section 7.6. This section should also be referred to for details of columns in multi-storey buildings. In such columns the load reduces progressively upwards, enabling smaller sections to be used. It is, however, more economical to reduce the number of site splices to a minimum rather than to reduce the section size every one or two storeys. With column lengths up to 15 m or more, it is possible to locate splices at three to four-storey intervals. The saving in splicing costs will far outweigh the saving in column section material that would be achieved with closer splice spacing.
10.3 Latticed columns Latticed construction is used for columns of great length to limit deflection, or in buildings housing heavy overhead cranes. Examples of the latter are shown in Figs 5.3 and 5.13. For latticed columns consisting of two equal-serial-size I-section legs connected by double-plane angle lacing, the lacing configurations most commonly used are as shown in Fig 10.2.
(a)
(b)
Fig 10.2: Lacing of columns The layout shown in detail (a) is by far the more efficient, for the following reasons:
10.2
•
For a given length of column the total length of lacing bars is about 0,83 times that of the latticed column shown in layout (b).
•
Both the length of each diagonal lattice bar and the force in it are about 0,82 times those in detail (b), so a lighter section can be used.
•
For a given length of column the number of bars required is about 0,86 times those in (b), resulting in far fewer end connection welds to the column legs.
•
The ends of the bars can be more snugly nested into the inner faces of the column flanges, allowing longer welds to be laid.
•
The shear deflection of the column under transverse loading is reduced.
The only disadvantage of layout (a) is that the laterally unsupported lengths of the column legs (i.e. for buckling about their y-axes) are slightly greater, but this is very rarely critical. The lacing bars should be welded to the inner faces of the column legs, as shown in Fig 10.3, rather than to the outer faces. This results in a more compact overall size of column (in plan), a lesser total length of lacing bar and greatly improved appearance. Also, in a column with legs of equal serial size but of unequal mass m, the distance between the inner faces of the flanges on both legs is equal, whereas the distance between the outer faces is not.
Fig 10.3: Welding of lacing bars
10.4 Box columns Box columns, which are only used when large loads have to be carried over a great height or when aesthetic considerations govern, may be of compact cross section, as in detail (a) of Fig 10.4, or be large enough to allow internal access for making joints, painting, etc, as shown in detail (b). In the latter case, because of the large width-to-thickness ratios of the plates, internal stiffeners or diaphragms are required. Sufficient clearance must be available to accommodate a vertical access ladder.
10.3
The flanges of the box should project beyond the webs to allow for the corner welds to be fillets, as shown in detail (c), rather than groove welds as indicated in (d), to avoid the web edges having to be bevelled. If a flush exterior is required the webs can be set back to accommodate a fillet weld, as shown in (e). If groove welds are used, however, they should be partial penetration welds since there is seldom need to develop the full shear strength of the web plates.
(c)
(d)
(e) (a) (b)
Fig 10.4: Box columns
10.5 Column bases Column bases range from simple single plates for columns subject to light to medium axial load to complex stiffened bases when large loads and especially high bending moments have to be transmitted. The latter type of base can have a significant effect on the overall cost of a column so it is well worth spending extra design time to achieve the lowest cost solution. Some economically designed bases are shown in Fig 10.5. Stiffener attachments add considerably to the cost of a base and should thus be avoided where practicable. This can usually be achieved by using a thicker base plate to resist the moment caused by the upward pressure on the underside of the base. A method for assessing the load distribution and calculating the moment is given in Section 4.5 of the Steel Construction Handbook (Ref. 5) and Chapter 12 of Structural Steelwork Connections – Limit States Design (Ref. 7). Where stiffeners are used they should be kept to a minimum. They can usually be arranged in one direction only; it is seldom necessary to use two-direction stiffeners. The end of the column shaft has to be flat to ensure uniform transfer of the force to the base plate. This can normally be achieved by carefully saw-cutting the shaft in the case of a simple section, but will require machining of compound or stiffened column sections. Base plates up to about 50 mm thick have surfaces sufficiently flat and smooth to receive load without any further treatment, but thicker plates will require machining. This
10.4
Grout
T.O.C.
H.D. bolts
Stiffeners
(a)
(b)
Washer plate Gussets Stiffeners
Loose base slab
(c)
(d)
Fig 10.5: Column bases
10.5
should be confined to a strip across the plate slightly wider than the column shaft and extending across the narrower plan width of the base plate. The 50 mm machining thickness limit will tend to act as a guide to base plate thickness, i.e. the designer will try to keep within this limit to avoid having to machine. It is obvious that where machining is required an over-thick plate has to be used, resulting in extra material cost. In designing the base plate outstand of a column subject to moment the use of a rectangular stress block rather than the traditional triangular distribution will in many instances result in a lower moment on the plate and consequently reduce the thickness required. The method is also described in Chapter 12 of Structural Steelwork Connections – Limit States Design. The welding of the shaft and its stiffeners to the base plate should be done with fillet welds – it is very seldom necessary to use groove welds. The usual number of holding-down (HD) bolts to a base is four, but with smaller, pinended columns two will often suffice. Where four bolts are used they should be arranged in a square pattern in plan where this is practicable, as shown in (a) of Fig 10.5. In practice, many cases have occurred where, using a rectangular pattern, the bolts have been incorrectly set in the concrete at 90º in plan to the required orientation. HD bolts for larger column bases may be provided with a pocket in the foundation block at their upper ends as shown in Fig 10.6 to permit the bolts to be bent sideways to allow for inaccuracies in their setting in the concrete. The projection length of the bolts above the concrete should be generous. The setting of HD bolts in the concrete foundation is done by the civil contractor independently of the steelwork fabricator. In practice inaccuracies frequently occur and are difficult to rectify unless allowance has been made for lateral adjustment. One way of ensuring accuracy in the location of HD bolts is for the supplier of the bolts to provide steel templates for the different bolt layouts.
Projection Pocket
Bond length with pocket
Shank dia.
Washer plate
Fig 10.6: Holding-down bolts
10.6
Bond length without pocket
10.6 Moment connections for beams Where a beam with an end moment is connected to a column the column web and or flanges have sometimes to be stiffened to transmit the beam flange forces, generated by the moment, into the column. A typical detail is shown in Fig 10.7. The fitting of stiffeners is costly and can often be avoided by using a column section of the same serial size but slightly greater mass m. However, if stiffeners are omitted, it is necessary to check carefully the strength of the column components transferring the moment. This can be done by following the procedure given in Section 7.6 of the Steel Construction Handbook (Ref. 5). When stiffeners are used, their ends adjacent to the beam should bear against and or be welded to the inner face of the column as required, but at their other ends they may be left clear of the flange, as shown in Fig 10.7. This will avoid the cutting to length of the stiffeners and the sniping of their inner corners; it also makes for much easier fitting and saves on welding.
(b)
(a)
Omit column web stiffeners where possible
Fig 10.7: Beam-column moment connections
10.7
Splices in multi-storey columns
The spacing of splices within the height of a column in a multi-storey building was discussed in Section 10.2 in the context of the overall economy of the column. In this section the more detailed aspects of splicing are dealt with.
10.7
Splices should be located where they can be easily reached on site for the insertion of bolts, which is usually about 1,0 m above a floor level. At these locations the column shaft is near to a point of lateral restraint and is not very prone to buckling. The splice does therefore not need to develop the full axial flexural strength of the column section, as it would have to do were it located at or near to mid-height of the floor-to-floor column segment.
(b)
(a)
(d)
(c)
Note: For splices (a), (b) and (c), ends of column shaft to be faced.
Fig 10.8: Bolted column splices
Some typical bolted splices in I-section columns are shown in Fig 10.8. In detail (a) the upper and lower segments of the column are of the same size, whereas in details (b) and (d) they are of the same serial size but of different masses m. In detail (c) they are of
10.8
different sizes and require side packs and a thick division plate, the latter to spread the load from the flanges of the upper segment to the offset flanges of the lower. In details (a) to (c) the ends of the shafts are machined or accurately saw-cut to transmit the axial load directly, whereas in (d) a full-strength splice capable of transmitting axial load and moment is used. In this case HSFG bolts would be needed to prevent slip under moment. Site-welded splices are rarely necessary and should be avoided where possible because of their much higher cost. Again, if the shaft ends of an axially loaded column are faced for bearing, the welds need not develop the full strength of the section and may be partial penetration groove welds, as shown in details (b) and (c) of Fig 10.9. Where one or both of the flanges are offset, as in details (d) and (e), and a division plate has to be used, advantage should be taken of using fillet instead of groove welds. Note the use of bolted locating plates or cleats in all of the connections to assist erection; these are placed on one side of the column web only.
(a)
(b)
(d)
(c)
(e)
Fig 10.9: Welded column splices
10.8 Summary •
Column shafts should preferably be of plain I- or H-section, but plated I-sections are cheaper than welded-plate sections. Double-section columns may be used for much heavier loads and welded box columns only in special cases.
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Columns in multi-storey buildings should be designed in long lengths (say 3 to 4 storeys) to reduce the number of splices.
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Splices should be located for easy access by erectors; if welded they should have end preparations conducive to easy weld placement.
10.9
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The shafts of heavily loaded columns should be machined at splices to avoid having to develop full strength by welding or bolting.
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Latticed twin I-section columns as used in crane gantries should have double-plane single-angle lacings inclined at 30º to the horizontal, welded to the inner faces of the column legs.
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The corner seams of box columns should be arranged for simple fillet welds without any preparation of the plate edges.
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Column bases should be kept simple, with stiffening kept to a minimum or omitted altogether by employing thicker base plates.
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If base plate thicknesses do not exceed 50 mm machining of the plate can be dispensed with.
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Holding-down bolts should be provided with means for lateral adjustment and should be in a square pattern in plan where practicable.
•
At moment connections of beams to columns, stiffening of the column web can often be avoided by using a section with a thicker web.
10.10
