Open Front Structure - Rigid Diaphragm 1

[Bala0404]

Hi Friends and Experts,

We have an open front structure. Three side masonry shear walls and one side open with roof joists. Inorder for me to transfer the wind and seismic loads I have to make the roof deck rigid. Flexible diaphragm will not transfer the torsional loads. I cannot add any beams as cross bracing because of aesthetics.

I took the option of metal deck but after few calculations it came out to be flexible. at the most semi rigid.

Last option for me is to make the deck composite with concrete topping. Like floor deck.

Are there any other types of roof decks? that are rigid? or any suggestions? Or how can we make metal deck rigid?

 

[KootK]

I took the option of metal deck but after few calculations it came out to be flexible. at the most semi rigid.

The trick here is to recognize that diaphragm rigidity is not required in order for you to classify this as a rigid diaphragm. This is because a three sided building is statically determinate with regard to the distribution of the lateral loads to the various shear wall lines.

Where diaphragm stiffness will come into play is in your assessment of diaphragm drift and deflection. And that should be checked rigorously in a three sided structure.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Bala0404]

So you say even the flexible diaphragm will transfer forces to side walls ? I was referring to other materials " irregular shaped buildings " they only used rigid diaphragm. But they did mention that deflection is the main governing factor.

I have to find a way to manually calculate deflection. or model it.
Still little confused as how flexible diaphragm will transfer torsional loads?

thanks for your response.

 

[RFreund]

I think Kootk's point is that diaphragm rigidity is a relative definition. In that it depends on how stiff the walls are. You would have to find the deflection at the corner of the open end due to walls and diaphragm and determine if that is acceptable. Easier said than done obviously. There is a good example in Terry Malone's book on irregular diaphragms. The example is for wood but the general concept could apply to metal deck.

EIT

http://www.HowToEngineer.com

 

[Bala0404]

@ RFreund. : That is the same book I have an referred. It only used rigid diaphragm. It gave formula to verify deflection but not a hand calculation method. So I might have to model it.

I got your and KootK's Point. Will review the deflection.

Thanks

 

[KootK]

I think Kootk's point is that diaphragm rigidity is a relative definition. In that it depends on how stiff the walls are.

My point is a bit different really. I contend that the distribution of load to the various walls can be made completely independently from the absolute -- or relative -- stiffnesses of the various elements in the system. Lots of folks disdain the three sided structure but, analytically, it's quite simple. You could say that it's the simple span beam of the diaphragm world.

So you say even the flexible diaphragm will transfer forces to side walls ? Still little confused as how flexible diaphragm will transfer torsional loads?

I do say that. As RFreund pointed out, the classification of a diaphragm as flexible, rigid, or semi-rigid is about the relative stiffness of the diaphragm and vertical lateral force resisting system. In this case that relative stiffness simply doesn't matter as it isn't required to distribute the lateral loads to the walls. And, in reality, all diaphragms posses some degree of in plane stiffness so the ability to transfer forces to the side walls isn't in question.

It gave formula to verify deflection but not a hand calculation method. So I might have to model it.

I would be hesitant to resort to modelling a situation that you don't know how to handle, at least conceptually, by hand. In reality, I think that this situation is pretty straight forward to deal with manually. In your typical three sided masonry building, the masonry wall are so stiff that all of the sources of drift become insignificant except for diaphragm shear deflection. And I've proposed a simple method of evaluating that below for your consideration.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.