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The loading for roofs not accessible except for normal maintenance and repair is given in the table on the right. It should be noted that imposed loads on roofs should not be combined with either snow or wind. Snow loads may sometimes be the dominant gravity loading. Any drift condition must be allowed for not only in the design of the frame itself, but also in the design of the purlins that support the roof cladding. The intensity of loading at the position of maximum drift often exceeds the basic minimum uniform snow load.

The calculation of drift loading and associated purlin design has been made easier by the major purlin manufacturers, most of whom offer free software to facilitate rapid design. This Eurocode gives much scope for national adjustment and therefore its annex is a substantial document. Wind actions are inherently complex and likely to influence the final design of most buildings. The designer needs to make a careful choice between a fully rigorous, complex assessment of wind actions and the use of simplifications which ease the design process but make the loads more conservative.

Free software for establishing wind pressures is available from purlin manufacturers. Wind loading calculator. The most common form of craneage is the overhead type running on beams supported by the columns. The beams are carried on cantilever brackets or, in heavier cases, by providing dual columns.

In addition to the self weight of the cranes and their loads, the effects of acceleration and deceleration have to be considered. For simple cranes, this is by a quasi-static approach with amplified loads.

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For heavy, high-speed or multiple cranes the allowances should be specially calculated with reference to the manufacturer. Each project should be individually assessed whether any other accidental actions are likely to act on the structure. Robustness requirements are designed to ensure that any structural collapse is not disproportionate to the cause. BS EN [2] sets the requirement to design and construct robust buildings in order to avoid disproportionate collapse under accidental design situations.

BS EN [9] gives details of how this requirement should be met. For many portal frame structures no special provisions are needed to satisfy robustness requirements set by the Eurocode. In the United Kingdom, structural steel in single storey buildings does not normally require fire resistance. The most common situation in which it is required to fire protect the structural steelwork is where prevention of fire spread to adjacent buildings, known as a boundary condition , is required.

There are a small number of other, rare, instances, for example when demanded by an insurance provider, where structural fire protection may be required.

When a portal frame is close to the boundary, there are several requirements aimed at stopping fire spread by keeping the boundary intact:. Comprehensive advice is available in SCI P BS EN [2] gives rules for establishing combinations of actions, with the values of relevant factors given in the UK National Annex [10]. All combinations of actions that can occur together should be considered, however if certain actions cannot be applied simultaneously, they should not be combined.

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At the ultimate limit state ULS , the methods of frame analysis fall broadly into two types: elastic analysis and plastic analysis. The term plastic analysis is used to cover both rigid-plastic and elastic-plastic analysis. Plastic analysis commonly results in a more economical frame because it allows relatively large redistribution of bending moments throughout the frame, due to plastic hinge rotations. These plastic hinge rotations occur at sections where the bending moment reaches the plastic moment or resistance of the cross-section at loads below the full ULS loading.

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For this reason members where plastic hinges may occur need to be Class 1 sections , which are capable of accommodating rotations. The figure shows typical positions where plastic hinges form in a portal frame.

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Two hinges lead to a collapse, but in the illustrated example, due to symmetry, designers need to consider all possible hinge locations. A typical bending moment diagram resulting from an elastic analysis of a frame with pinned bases is shown the figure below. In this case, the maximum moment at the eaves is higher than that calculated from a plastic analysis. Both the column and haunch have to be designed for these large bending moments. If stiffer sections are selected in order to control deflections, it is quite possible that no plastic hinges form and the frame remains elastic at ULS.

Bending moment diagram resulting from the elastic analysis of a symmetrical portal frame under symmetrical loading. Portal frame analysis software Fastrak model courtesy of Trimble. When any frame is loaded, it deflects and its shape under load is different from the un-deformed shape. The deflection has a number of effects:.

Taken together, these effects mean that a frame is less stable nearer collapse than a first-order analysis suggests. The objective of assessing frame stability is to determine if the difference is significant. The geometrical effects described above are second-order effects and should not be confused with non linear behaviour of materials. As shown in the figure there are two categories of second-order effects:.

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F cr is the elastic critical buckling load for global instability mode, based on initial elastic stiffnesses. Rules are given in the Eurocode to identify when the axial force is significant. When the frame falls outside the specified limits, as is the case for very many orthodox frames, the simplified expression cannot be used. Further details are given in SCI P For plastic analysis :.

Once the analysis has been completed, allowing for second-order effects if necessary, the frame members must be verified. Both the cross-sectional resistance and the buckling resistance of the members must be verified. In-plane buckling of members using expression 6. SCI P identifies the likely critical zones for member verification. SCI P contains numerical examples of member verifications.

Member bending, axial and shear resistances must be verified.

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If the shear or axial force is high, the bending resistance is reduced so combined shear force and bending and axial force and bending resistances need to be verified. In typical portal frames neither the shear force nor the axial load is sufficiently high to reduce the bending resistance. When the portal frame forms the chord of the bracing system, the axial load in the rafter may be significant, and this combination of actions should be verified.

Although all cross-sections need to be verified, the likely key points are at the positions of maximum bending moment:. The figure shows a diagrammatic representation of the issues that need to be addressed when considering the stability of a member within a portal frame, in this example a rafter between the eaves and apex. The following points should be noted:. In-plane, no member buckling checks are required, as the global analysis has accounted for all significant in-plane effects. The analysis has accounted for any significant second-order effects, and frame imperfections are usually accounted for by including the equivalent horizontal force in the analysis.

The effects of in-plane member imperfections are small enough to be ignored. Because there are no minor axis moments in a portal frame rafter, Expression 6. Compression is introduced in the rafters due to actions applied to the frame. The rafters are not subject to any minor axis moments. Optimum design of portal frame rafters is generally achieved by use of:. Purlins attached to the top flange of the rafter provide stability to the member in a number of ways:. Initially, the out-of-plane checks are completed to ensure that the restraints are located at appropriate positions and spacing.

The figure shows a typical moment distribution for the gravity combination of actions, typical purlin and restraint positions as well as stability zones, which are referred to further. Purlins are generally placed at up to 1. In Zone A, the bottom flange of the haunch is in compression. The stability checks are complicated by the variation in geometry along the haunch.