Rigid Diaphragm Analysis
Rigid Diaphragm Analysis
(OP)
Hello everyone!
I am currently designing a 35 foot high warehouse with LFRS system consisting of precast wall panels and with roofing made up of precast/prestress double T joists which would be interconnected along joint lines as per MFR design guidelines. I have assumed that the diaphragm is going to be rigid but the catch is that there is no topping slab over it. Now my question is how to ensure that this double T joist slab would behave as a rigid diaphragm. These joists run from wall to wall (almost 88’-0” long) with joints along parallel line between them. I have never encountered this situation before. Am I supposed to give MFR connection forces (it may be either due to drag, shear or bending) to connect these joist along panels joint lines to make sure that the rigidity behavior holds true. In general these connections are by MFR but not sure what is the industry standard to approach this type of problem. I am not sure how to check deflection of such diaphragm to see whether this would be rigid or flexible.
The reason for this concern is due to the fact that I have varying rigidity along shearlines. There is lot of load getting dragged into stiffer walls and if this rigidity concept won’t hold good I would end with lot of shear in other walls which are relatively less stiffer but have a very large trib area.
Any reference material or theory guidelines would do. I did talk to my principal about it but was not convinced with his response so wanted to check opinions of rest of the engineers.
Thanks in advnace!
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I am currently designing a 35 foot high warehouse with LFRS system consisting of precast wall panels and with roofing made up of precast/prestress double T joists which would be interconnected along joint lines as per MFR design guidelines. I have assumed that the diaphragm is going to be rigid but the catch is that there is no topping slab over it. Now my question is how to ensure that this double T joist slab would behave as a rigid diaphragm. These joists run from wall to wall (almost 88’-0” long) with joints along parallel line between them. I have never encountered this situation before. Am I supposed to give MFR connection forces (it may be either due to drag, shear or bending) to connect these joist along panels joint lines to make sure that the rigidity behavior holds true. In general these connections are by MFR but not sure what is the industry standard to approach this type of problem. I am not sure how to check deflection of such diaphragm to see whether this would be rigid or flexible.
The reason for this concern is due to the fact that I have varying rigidity along shearlines. There is lot of load getting dragged into stiffer walls and if this rigidity concept won’t hold good I would end with lot of shear in other walls which are relatively less stiffer but have a very large trib area.
Any reference material or theory guidelines would do. I did talk to my principal about it but was not convinced with his response so wanted to check opinions of rest of the engineers.
Thanks in advnace!
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RE: Rigid Diaphragm Analysis
LonnieP
RE: Rigid Diaphragm Analysis
You could indvidually design them, but most of the time they're designed for the transfer load at the walls and called good. You would give that load to the precaster and he would supply the number and type of details required.
RE: Rigid Diaphragm Analysis
TT flange slabs are obviously pretty rigid internally. It's the connections between TT's that introduces flexibility. I reconciled myself to the rigid diaphragm assumption by remembering that deflection is, mathematically, the integration of M/EI along the span of your diaphragm (similar for shear deformation). Consequently, local soft spots don't add as much to overall deflection as you might think.
You could specify an effective diaphragm stiffness or a maximum deflection in your drawings to ensure that you get a diaphragm that's rigid enough to suit your design assumptions.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Rigid Diaphragm Analysis
No specific codal requirements like chapter 21 of aci318 cover this situation directly which i think should be done in future editions to clear the air. Thanks a lot Jed for that reference.
RE: Rigid Diaphragm Analysis
Also, when I have a code permitted "flexible" diaphragm, I will still design my lateral system for the envelope between flexible and rigid. Some programs will allow you to design for this using a "semi-rigid" diaphragm.
RE: Rigid Diaphragm Analysis
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