Beams - Design for restricted depth - Steel Structures.

Frequently beam design will be constrained by a need to keep the beam depth to a minimum. This restriction is easy to understand in the context of floor beams in a multi-storey building for which savings in overall floor depth will be multiplied several times over, thereby permitting the inclusion of extra floors within the same overall building height or effecting savings on expensive cladding materials by reducing building height for the same number of floors. Within the floor zone of buildings with large volumes of cabling, ducting and other heavy services, only a fraction of the depth is available for structural purposes.
Such restrictions lead to a number of possible solutions which appear to run contrary to the basic principles of beam design. However, structural designers should remember that the main framing of a typical multi-storey commercial building  typically represents less than 10% of the building cost and that factors such as the efficient incorporation of the services and enabling site work to proceed rapidly  and easily are likely to be of greater overall economic significance than trimming steel weight.

An obvious solution is the use of universal columns as beams.While not as structurally efficient for carrying loads in simple vertical bending as UB sections, as illustrated by the example of Table 16.8, their design is straightforward. Problems of web bearing and buckling at supports are less likely due to the reduced web d/t ratios.



Comparison of use of UB and UC for simple beam design

Lateral–torsional buckling considerations are less likely to control the design of laterally unbraced lengths because the wider flanges will provide greater lateral  stiffness (L/ry values are likely to be low).Wider flanges are also advantageous for supporting floor units, particularly the metal decking used frequently as part  of a composite floor system.

Difficulties can occur, because of the reduced depth, with deflections, although dead load deflections may be taken out by precambering the beams. This will not assist in limiting deflections in service due to imposed loading, although composite action will provide a much stiffer composite section.Excessive deflection of the floor beams under the weight of wet concrete can significantly increase slab depths at mid-span, leading to a substantially higher dead load. None of these problems need cause undue difficulty provided they are recognized and the proper checks made at a sufficiently early stage in the design.

Another possible source of difficulty arises in making connections between shallow beams and columns or between primary and secondary beams.The reduced web depth can lead to problems in physically accommodating sufficient bolts to carry the necessary end shears.Welding cleats to beams removes some of the dimensional tolerances that assist with erection on site as well as  interfering with the smooth flow of work in a fabricator’s shop that is equipped with a dedicated saw and drill line for beams. Extending the connection beyond the beam depth by using seating cleats is one solution, although a requirement to contain the connection within the beam depth may prevent their use.

Beam depths may also be reduced by using moment-resisting beam-to-column connections which provide end fixity to the beams; a fixed end beam carrying a central point load will develop 50% of the peak moment and only 20% of the central deflection of a similar simply-supported beam. Full end fixity is unlikely to be a realistic proposition in normal frames but the replacement of the notionally pinned beam-to-column connection provided by an arrangement such as web cleats, with a substantial end plate that functions more or less as a rigid connection, permits the development of some degree of continuity between beams and columns. These arrangements will need more careful treatment when analysing the pattern of internal moments and forces in the frame since the principles of simple construction will no longer apply.

An effect similar to the use of UC sections may be achieved if  the flanges of a UB of a size that is incapable of carrying the required moment are reinforced by welding plates over part of its length.Additional moment capacity can be provided where it is needed as illustrated in Fig. 16.11; the resulting non-uniform section is stiffer and deflects less. Plating of the flanges will not improve the beam’s shear capacity since this is essentially provided by the web and the possibility of shear  or indeed local web capacity governing the design must be considered. A further development of this idea is the use of tapered sections fabricated from plate. To be economic, tapered sections are likely to contain plate elements that lie outside the limits for compact sections.

Because of the interest in developing longer spans for floors and the need to improve the performance of floor beams, a number of ingenious arrangements have developed in recent years.

Selective increase of moment capacity by use of a plated UB
Fig. 16.11 Selective increase of moment capacity by use of a plated UB

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