ECONOMIC DESIGN vs COMPROMISE

Updated: Nov 26, 2021



Today,the structural design world is pressurized for economy and more economy! There is a limit to economize if one want to abide by the codes of practice and also if one wants the building to be robust and stable. Compromising should not be resorted to achieve economy,However improvisation can be. These ethical points need to be taught from younger days to civil engineering students from colleges. Atleast when any structural engineering institute offers a staad pro training or any structural engineering courses for that matter, should cover ethical consulting practices that shapes up the future of civil engineering industry. At times,there are misunderstandings or no understanding that makes the design uneconomic. Torsion(Twisting of structural elements, especially beams) in the structure is such a structural topic.

TORSION

Torsion means twisting. A beam when fixed or connected monolithically to columns at either ends,can experience torsion in some situations. To understand this, first we need to know that basically,torsion can be divided in to 2 types.

Equilibrium torsion and compatibility torsion.


Let me first discss what Equilibrium torsion is all about. A thoorough understanding of the concepts of torsion will allow you to appreciate the release of torsion.

Primary torsion/Equilibrium torsion/statically determinate torsion

Primary torsion exists when the external load has no alternative load path but must be supported by torsion. For such cases, the torsion required to maintain static equilibrium can be uniquely determined. This cannot be released/ignored since the structure will not be stable if released. To understand it beter,consider a free cantelever slab from a beam.There is no back anchorage for the slab-Just a projection to 1 side from a beam.This is a case of equilibrium torsion.The slab can be in equilibrium or stable only if the beam absorbs this torsion and thereby supports the slab.This torsion from slab has only 1 load path and that is through beams.If the building has such type of structural element then it needs to be designed for torsion.

It is very important to note that, if the cantilever slab has a back anchorage or a continuity then the beam supporting the cantelever will have stability even if the torsion is releases as there is an alternate load path available for the torsion.


If the cantilever slab and the back slab had a level difference (as in the case of a balacony in an apartment) and if the slab rebars are bent and anchored in the the beam from which the slab is cantelevering, then it is a case of equilibrium torsion and hence the torsion cannot be released.

Secondary torsion/Compatibility torsion/statically non determinate torsion


Compatibility torsion arises from the requirements of continuity/compatibility of deformation between adjacent parts of a structure. An internal readjustment of forces is possible and an alternative equilibrium of forces can be found.That is torsion has more than 1 load path. Cantilevers with back anchorages/continuous slabs behind the cantelever etc are examples. IS 456 Code provisions: IS-456 clause 41.1 says that compatibility torsion can be ignored in design if the torsion stiffness of member is completely ignored/released in the analysis model. Code further adds that nominal shear reinforcement provided as per clause 40, will be adequate to control any torsion cracking. Releasing compatible torsion in model has 2 parts. 1) Release of torsion in members when slabs are modeled as plates/shells and slabs designed as per the model forces. 2) Release of torsion in members when slabs are not modeled as plates/shells. • In case 1 there will be complete compatibility of deformation between adjacent parts and there is no need of designing for torsion.

• In case 2 there will not be complete compatibility of deformation if the adjacent slab panels has different spans or loading.(Prudent if the span and/or load differences are considerable) In case 2 two design solutions are possible. A. Either the beam needs to be designed for that differential moment (as torsion) or B. Design the slabs as per clause 24.4.1 If we adopt Solution A ie; designing for differential moment, we will have to provide additional steel for resisting torsion. Plain concrete will have a nominal capacity to resist torsion and if the design torsion increases the nominal value, the concrete needs to crack to transfer forces to additional steel provided. In that case, torsion stiffness shall be half the value calculated for plain concrete section. This means that torsional stiffness in analysis model shall be modified by using a factor of 0.5

A practical case of compatibility torsion can be seen as a question in our structural discussion forum

In the structural forum one of the student is discussing about the need of accounting wall eccentricity in beams.There is also a video there that explains why etabs may not be able to model this site situation and why it may not be really needed.


Will Simply supported beam develop Torsion?


This is a general question I asked a student in a civil engineering interview. I thought will include this in the blog as it is closely related to the topic of this blog on torsion. When many students focus on designing for torsion in a beam, they don’t focus on the very point what is torsion. Torsion is different from over turning moment. When Torsion twists the cross section of the beam, over turning is instability or rolling over. It cannot twist the cross section and hence you can’t design for it. You need to overcome it by ensuring the beam is stable.

Deciding torsion reinforcement in beams is useful if and only if you know what torsion in beams are. If not you will be merely over designing the beam and still risking your structure as overturning is not prevented by additional reinforcement for torsion.

I have made this point clear in the video. Please take a look.


SUMMARY

Equilibrium torsion cannot be released. Compatibility torsion can be released. If we choose to design for compatibility torsion,the torsional stiffness needs to be modified by 0.5 With out realising these points,certain designers considers torsion for all member design and results in uneconomic designs.Some do not consider even equilibrium torsion and this results in unsafe designs. It is of high importance to understand these points for safe and economic designs. Improvisation helps,compromise kills!


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