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Auto meshing doesn't ensure adequate connectivity between member to member and is therefore recommended for horizontal area elements enclosed by line elements only (floors), where there is no structural connection between floor and any element in between the panel. In case of vertical elements, connectivity between vertical and horizontal elements is of due importance and is better to be achieved through manual meshing. In case of auto meshing as you indicated, change in size of auto mesh could solve the problem as its possible that connections inadequate (nodes not coinciding) on 1m element size can be adequate for 1.2m size (nodes start coinciding) and so on. For p-delta,a possibility exists that due to any meshing error some connection is modelled with inadequate lateral stiffness i.e when program try to impose lateral deflection due to seismic loads,modes start yielding frequency below shift. If you are interested in studying the problem,then easy approach is to check each mode shape and investigate the member that is going in unrealistically large displacement. Solving this member's connectivity inadequacy will solve your problem.

Aci 318-08 section 13.2.5 and 13.2.6 address this issue and may be useful.

Dear for this particular error, I think u r the best person who can search for this as u know the problem in details.......... But kindly tell me how do u know that by converting to shell element torsion will be reduced and your reinforcement will be reduced. How did u concluded that. Thanks Muneeb

Dear All, Just a brief introduction about the Diaphragm Design for Lateral Forces in case of Major Earthquake Areas: Please correct me where I am wrong: There are two types of forces in a member 1- Out of Plane Forces (out of plane behavior) 2- Inplane Forces (inplane behavior) Followings are the major components for design purpose in any structure a- Foundation b- Shear wall c- Column d- Beam e- Slab Foundation : We design it mostly for out of plane forces Shear wall : We design it for purely inplane forces because we neglect its out of plane stiffness Slab : We design it for out of plane as well as for inplane bending. Slab: Out of Plane In slabs, we normally provide the flexural reinforcement and check the thickness of slab which is out of plane behavior. This design should be conducted on gravity load basis even the building is located in severe earthquake areas. In case of Slab supported on beams we need to design for out of plane forces based on gravity loading. In case of Flat Slab we also need to design it on gravity loading but we just need to satisfy one requirement of ACI code 21.13.6 (b ). Actually code asks this condition to be satisfied due to the rotation limit of slab due to punching at these joints. One option is to satisfy this requirement (ACI 21.13.6 (b ) or second option is we can check the actual D/C ratio of these junctions by using PEER/ATC 72 guidelines. PEER/ATC 72 guideline is attached here. Both are equally reliable just the later is a guideline not a codal provision. Inplane The second design for a slab which is MUST in severe earthquake areas and normally nobody perform is inplane design. As we know, earthquake acts on a structure laterally, and diaphragm is used to transfer lateral forces to vertical members. We need to assign the proper diaphragm to the slab. Proper means the diaphragm which can distribute the forces to vertical members as well as it transfer the forces through slab. So we have two options. a- Rigid Diaphragm and b- Semi Rigid Diaphragm. So we ll assign semi rigid diaphragm. There is one question why, we ll put this question to some other topic. So when we assign semi rigid diaphragm it will transfer the forces through slab member and in ETABS we can see the forces in the slab. Followings are the reinforcements which we need to design for inplane forces 1- Shear Reinforcement at basement Slab Level and Ground Floor Slab Level. 2- Shear Reinforcement at Podium Levels 3- Tension Reinforcement 4- Chord Reinforcement 5- Shear Friction Reinforcement All of these reinforcements are used to guide the inplane forces from retaining wall to shear wall at basement levels and from shearwall to slab at upper levels. for example tension or collector reinforcement collects axial inplane force and transfer to shearwall. Slab shear reinforcement is used to avoid lateral cracking of slab in case of earthquake. So we must design the slab for these two forces and both have different design practices. The attached NEHRP file is very use full guideline to understand diaphragm design. In ETABS, we can check the inplane forces from F11, F12 and F22 and then making the section cuts from slab. From these section cuts we can obtain shear forces and axial force and flexural force. In addition to slabs, we need to check the retaining wall shear reinforcement (distribution reinforcement) and flexural reinforcement (vertical reinforcement) for these inplane forces. We also need to provide the reinforcement at the junction of basement slab and retaining wall in the form of U bars. This is also based on inplane forces Thanks Muneeb PEER-ATC-72.pdf NEHRP Guideline for Diaphragm Design.pdf