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Economic Design vs Compromise

Premjit Vasudevan
24/09/2016 0 0

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 want the building to be robust and stable. Compromising should not be resorted to achieve economy,However improvisation can be.
At times,there are misunderstandings or no understanding that makes the design uneconomic.Torsion(Twisting of structural elements) in the structure is such an entity.
TORSION : Basically torsion can be divided in to 2.Equilibrium torsion and compatibility torsion.
Primary torsion/Equilibrium torsion/statically determinate torsion 
This 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.
Secondary torsion/Compatibility torsion/statically non determinate torsion 
This 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. Cantelevers with back anchorages/continuous slabs behind the cantelever etc are examples.
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
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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