DESIGN OF CONCRETE BRIDGES
When should torsional stiffness of members be taken into account in the analysis of a structure?
If the torsional stiffnes of the member has been used in analysing the distribution effects using a computer model, such as a grillage, then the torsional resistance of the member will need to be checked against the analysis results.
It is generally satisfactory to assume the torsional stiffness of the beam to be zero for a computer analysis, this will increase the bending moments in the beam but avoid the complication of providing reinforcement to resist the torsion effects.
If the type of beam, such as a box beam, is chosen beacuse of its torsional strength then the torsional stifness and resistance both need to be considered.
When torsional stiffness of a member is used in a computer model then the position of the centriods of adjoining members needs to be modelled correctly. This is particularly significant in a beam and slab construction where the deck slab can act as a torsional restraint to the beam (see the Tutorial Grillage Models for more information).
What Class of Member does the allowable tensile stress refer in paragraphs (1) and (2)?
Refer to BD 24 and BD 37.
BD 24 Clause 2.7 says that all prestressed concrete members shall be checked as being in Class 2 in load combinations 2 to 5.
BD 37 Clause 220.127.116.11 says that temporary erection loads are considered in Combinations 2 and 3.
Thus for a prestressed beam during construction, at transfer the beam is considered as a Class 2 member and tension is allowed under the loading for either condition (1) or (2) in BS 5400 Pt 4 Clause 18.104.22.168(b).
The beam has to be checked as a Class 1 member under Combination 1 loading and as a Class 2 member under Combination 2 to 5 loading.