Trusses - Detailed design considerations for elements.

1 Design loads
The current British Standards for steel structures in buildings and bridges are both limit-state codes.The magnitude of the partial load factors to be applied is dependent on the load type, the load combination and the limit state (ultimate or serviceability) under consideration.

The following approach may be adopted in deriving the critical  load combinations for each truss member:

(1) The member forces and moments are calculated for each, unfactored, load type (dead, superimposed dead, imposed, wind, etc.) using an appropriate method of analysis.
(2) Load combinations are identified and the appropriate load factors for each combination applied for both serviceability and ultimate limit states.
(3) The critical loads in each element and joint are extracted for both limit states.

The above process is long-winded but with experience the designer can often take short cuts in determining the critical load combinations for each element.

In the analysis the member forces and moments due to joint fixity should be calculated and superimposed on the global member forces. For trusses in buildings the secondary effects due to joint fixity may normally be ignored provided the  slenderness, in the plane of the truss, is greater than 50 for the chord elements and 100 for most of the web members. If this condition is  satisfied the members are assumed to be pin jointed in the analysis. Secondary effects due to axial deformations are usually ignored in building trusses. Local effects due to joint eccentricities and where loads are not applied at nodes should  be taken into account.

For bridge trusses to BS 5400: Part 3: 2000, the effects of joint rigidity are required to be taken into account. Secondary stresses due to axial deformations may be ignored at the ultimate limit state but should be considered at the serviceability limit state and for fatigue checks. As for building trusses, the local effects due to joint eccentricities and cases where loads are not applied at nodes must be considered in bridge trusses.

2 Effective length of compression members
For building trusses the fixity of the joints and the rigidity of adjacent members  may be taken into account for the purpose of calculating the effective length of  compression members. The designer should be careful to ensure that the critical slenderness is identified. For chords, out-of-plane unrestrained lengths do not necessarily relate to the truss nodes, and effective length factors are usually unity; in-plane effective length factors may be demonstrated to be less than unity if the restraining actions of tension members and non-critical compression members are mobilized at the ends of the member. Single angle elements, for both the webs and chord, have minimum radii of gyration that do not lie either in, or normal to, the plane of the truss.

For compression members in bridge trusses the effective lengths may either be obtained from Table 11 of BS 5400: Part 3: 1982 or be determined  by an elastic  critical buckling analysis of the truss.

In the case of simply supported underslung trusses the top compression chord will be effectively restrained laterally throughout its length provided the connection between the chord and the deck is capable of resisting a uniformly distributed lateral force of 2.5% of the maximum force in the chord. The effective length in such a case is taken as zero where friction provides the restraint, or as equal to the spacing of discrete connections where these are provided.

The economic advantages of underslung trusses over through or semi-through trusses is obvious in this respect, due to the dual function of the deck structure.

In the case of unbraced compression chords, that is, chords with  no lateral restraints, the provision of U-frames is necessary. The effective length of the com- pression chord is a function of the stiffness of the chord and  the spacing and stiffness of the U-frame members. Clause 12.5 of BS 5400: Part 3: 1982 gives guidance on the calculation of the effective length of compression chords restrained by U-frames. 19.6.3 Detailed design

For building trusses to BS 5950 the members need only be designed at the ultimate limit state for strength, stability and fatigue where applicable, and at the serviceability limit state for deflection and durability.
Compression members in bridge trusses to BS 5400 are designed at both the ulti- mate and serviceability limit states. Certain compression members, however, are exempt from the serviceability limit state check as defined in clause 12.2.3 of BS 5400: Part 3. Tension members need only be designed at the ultimate limit state.

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