Click on the Clause No. for the commentary.
|188.8.131.52||Analysing Beams Without Using Torsional Resistance.|
|184.108.40.206||Possitive and negative values of Mg and Mq when calculating εm.|
|220.127.116.11||Class of Member for Temporary Loads During Erection.|
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 stiffness 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).
Do the possitive and negative values of Mg and Mq need to be considered when allowing for stiffening effects to determine εm?
You need to consider which is the tension face of the concrete under live load.
If the dead load moment is greater than the live load moment (ignoring signs) then the dead load moment will be reduced by the live load and is therefore not a critical condition; dead load will produce a greater crack width.
If the live load moment is greater than the dead load moment (ignoring signs) then you will be considering tension on the opposite face to that produced by the dead load. If you need to consider crack widths on the opposite face then you can consider the net effect as the live load and put the dead load to zero (or a very small value such as 0.001 if you are using spreadsheets, dividing by zero produces an error).
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 18.104.22.168 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 22.214.171.124(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.