3 posts in this topic
I want to comment on some modelling issues in ETABS. Though some of these things are discussed elsewhere in the forum, I hope to extract some more useful conclusions.
First thing is related to modelling the bending stiffness of flexural members, for strength level loads, that is representative of their condition near failure.
The ACI code specifies the modifier of 0.35 on gross moment of inertia to represent its condition at yielding.
Some people say that the factor should be multiplied by 2 to represent the stiffness of T-beam. This approach would be justified if you are not taking into the account the out of plan bending stiffness of slab.
But, ETABS does include the out of plane bending stiffness if you have modelled the slab by using shell elements. So, a factor of 0.7 would overestimate the stiffness of your structure in this case, and will lead to under-design.
If one has used the modifier of 0.35 in ETABS for beams in beam-slab floor system, then what value should be adopted for slab? It should not be 0.25, as this value has been specified for flat plates and flat sab floor system.
If one is using some value of modifier for out of plane bending stiffness on shells, then the share of the bending moment in beams will be reduced accordingly. This approach is correct if one will be providing the reinforcement in column strips of slab. But, if you are providing reinforcement in slab in the direction perpendicular to supports only, i.e. beams, as is the general practice in Pakistan, then you are under-estimating the flexural demand in beams.
Now, there is also a question of factors to be used while deciding the amount of reinforcement required in beams, columns and shear walls.
If you are using factors 0.35 for beams and shear walls, and 0.7 for columns, then you are finding out the demand in members at the point of yielding, and this conforms to the code. But, this also means that the structure might experience unacceptable cracks widths. So, if you are using 0.35 for calculating the demand at strength-level forces, then you should also perform crack-control-check at service-level loads by using the factor of 1.
If you are calculating the strength-level demand with a modifier of 1 for all structural members, after you have decided the location and the number of shear walls with modifier of 0.35, then you are overestimating seismic forces, as you are underestimating the time-period. But, the structural performance will improve.
By Waqas Haider
Assalam o alaikum,
I am having a building with a semi circle shape. None of the footing is either parallel to global X-axis nor to the global Y-axis. The plan of footings is attached. The problem I am facing is, ETABS reports joint reactions along global X-axis and global Y-axis instead of along the local axes of column. Since my columns are at a degree to global axis, the joint reactions also becomes at a degree from column and footing. Since to use simple combined stress formula of
stress = P/A + Mx*Y/Ix + My*X/Iy
I need forces along axis of footing. Or i will have to resolve either moment of inertia of footing along direction of forces or vice versa, which is quite tough and time taking. The other option I can go for is to design footing in SAFE. But again here, there i dont find any option to rotate footing at a certain degree to match orientation of columns. If i rotate local axis of footing, It only rotates its local axes and meshing but not the footing physically itself. The orientation of footing remains same. How can i solve this problem? Either having reactions in etabs along local axes of columns can solve my issue so that i can design it manually or rotating footing in safe to match local axes of columns can solve my issue. So can any one guide me how to do either option? Thanks.
By kHURRAM ALI
aslamwalekum , when we design reinforced concrete elements , for beam we take 0.35 , for column and wall we take 0.7 and for slabs we take 0.25 as modifiers , but i read in aci that for servicibilty analysis these modifiers has to increase by 43% which means for beam it become 0.5 , for column and shear walls it become 1.0 and for slab it become 0.35 , by doing this model drift and deflection both reduce to almost half of its original value.
one thing more when we do manual check we dont take crack sections , for e.g for simply supported beam the deflection is 5\384 WL^4\EI , here the I (inertia ) is not the for the crack section ,its for the uncracked section , so what modifiers should be use 0.35 or 0.5 or 1.0