torsion in main girder (1)

8/13/2019 Torsion in Main Girder (1)

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TORSION IN MAIN GIRDER

Referring the discussions on thecompatibility torsion mypoint of

views are:

Er. Raghul has posted the content of page 245 of the book ReinforcedConcrete Design by S.Unnikrishna Pillai & Devadas Menon. Clear and

good explanation with reference to IS code Provision of the clause Cl.40.1

is given.

As a structural engineer we should be aware that the code provisions are

only a guide and need to exercise our engineering knowledge and

experience.

It is to be understood that in any software structures are formed asmathematical model which should resemble at least 99% true to the site

condition and as furnished by the architect or engineer. We all agree to

this and no one can deny this important aspect while modeling in any

software.

Going back to the compatibility torsion I would like to draw the attention that

practically the main girder and secondary girders with the slab will be cast

monolithically by in situ concrete. How many of us can offer the guarantee

that:

1. there will be a hinge formed as we do not provide any hinge as

modeled in the software and this is similar to the modeling of a

footing as either fixed or hinged and the real condition at site is

different. ( It is best to use spring support to footings)

2. there will not be any torsion developed since it is a monolithically

cast.

Under the above situation it is better to do as do analysis with no releaseof moment for torsion. We do not blindly follow that code says we can

model with a release since there is a provision in the software.


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Find out the torsional moment value and check whether this is below or

equal to the value prescribed in ACI-318 code clause 11. 5.1as there is

no such provision given in IS code 456:2000.

It shall be permitted to neglect torsion effects if the factored torsionalmoment Tu is less than:

(a) For nonprestressed members

(c) For nonprestressed members subjected to anaxial tensile or compressive force

Minimum torsion reinforcement:

In the pages as attached by Er. Raghul the authors explained as:In order to control the subsequent cracking and to impact ductility to the

member, it is desirable to provide a minimum torsion reinforcement, equalto that required to resist the cracking torque.In fact, one of intention of the minimum stirrups reinforcement specified bythe code (cl.25.5.1.6) is to ensure some degree of control of torsionalcracking of bems due to COMPATIBILITY TORSION.Please note that the minimum specified in the code does ot spell whether itis for shear due to gravity load or inclusive for COMPATIBILITY torsion.

Also in ACI code specifies a minimum area of steel for the torsion is as

11.5.5 Minimum torsion reinforcement11.5.5.1 A minimum area of torsional reinforcement shall be provided inall regions where Tu exceeds the threshold torsion given in 11.5.1.

11.5.5.2 Where torsional reinforcement is required by 11.5.5.1, theminimum area of transverse closed stirrups shall be computed by


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This is in addition to the minimum area for the regular shear provisions.Vide Clause 11.4.6 (ACI 318).

Also more guidance is given in IS 456 as well as CI-318 on detailing ofreinforcement i.e. stiruups for the shear as well as for torsion.

The above are pointed out as we structural engineers are responsible forsafety and not on modeling a secondary beam with hinged condition foreasiness in analyzing the structures.

For argument sake and as furnished in code and explained in a goodreference does not relive our responsibility.

Hope that the above makes it clear how to deal with the torsional momentin main girder due to the interception of secondary beams.

For equilibrium torsion which is a clear one and no ambiguity to model withrelease ends.

Comments and views from experts and seniors are welcome.

T.RangaRajan.

torsion in main girder (1)

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