Building exhibit torsional rotation under seismic loading due distance between the center of mass and center of rigidity. According to building codes, does this distance or torsional rotation have any limitations? And if not, how could I check the capacity of building for large torsional rotation?
Various codes provides procedure to calculate the torsion induced and resulting effect on frame structures in the form of additional moments.
Therefore only way is to provided symmetry in mass and rigidity.
ASCE 7-16 requires that an 'accidental torsion' be added to the theoretically calculated torsion. The 'accidental torsion' is 5% of the building dimension perpendicular to the direction of shaking.
Thank you Timothy Huff . But does ASCE 7-16 have any limits to the spacing between the center of rigidity and mass? Could I design a building with spacing approximating half width of it?
Dear Khaled
ASCE 7 like other Seismic codes allows you to design structures with severe irregularities in planes and heights which produce a huge amount of torsional moment in different stories of them. The limitations just vary on the accuracy of different approaches you are choosing to consider seismic actions and analyzing method of the structure. for example, for the moderate amount of eccentricity between the center of mass and rigidity, you can use the equivalent lateral force system of seismic loading and elastic method of analysis. in other cases, depends on the amount of irregularity, model analysis, time history or even nonlinear analysis of structure become necessary. You can check the limitations in ASCE 7. It is worth noticing that considering the effects of the torsional moment, especially for tall buildings which sometimes their primary period of vibration is their torsional one, is really important.
Good luck
All torsional rotations are about the shear center, dead load forces act through the centre of mass. A computer analysis automatically proportions the load to the stiffness of the vertical elements. If the core is offset say at the back face the distance between the shear centre and the centre of mass can be large, this induces large torsional forces. For a significant building under seismic load it is necessary to have the core centrally placed so that the centre of mass and the shear centre coincide this will eliminate tortional forrces. If not the structure must be designed to resist the forces that are generated from torsion about the shear centre.
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