Rigid diaphragm?

[aapl2k7]

I'm having trouble determining which of the two options represent a rigid diaphragm (pdf file attached), it is for a two storey house, using confined CMU walls, to me figure #2 is correct but i've seen both types of construction so i'm confused. For a brief explanation on Confined Masonry check

http://www.world-housing.net,

i now this system is mostly common in third world countries.

 

[drile007]

Vote for figure #2.

 

[COEngineeer]

both are rigid. They are both concrete floor.

Never, but never question engineer's judgement

 

[structuresguy]

I would say neither are rigid diaphragms, as each will be somewhat flexible. Though option #2 would seem to me to be more rigid.

Why do you need rigid diaphragm? Why not analyse as semi-rigid?

 

[aapl2k7]

Thank you all for the help, i realize they are both a little flexible, but what i really wanted to know is if they are rigid enough to transmit the lateral loads to the vertical elements, construction wise i believe figure #1 is easier to build, but i like option #2 because i thought it was more rigid.

 

[structuresguy]

I would say either would be stiff enough to transfer lateral loads to the shear walls. The problem with option #1 is that you lateral load would transfer through the slab, then down through the joists into the wall below. This will cause a roll-over load on the light gage joists. With a box section (front-to-front C channels), you might be ok, but you will need to check the roll-over capacity of both the box section and the connection. You may find you need to provide shear blocking between the joists.

With option #2, you can provide a pour stop angle just for the slab depth which is directly bolted to teh wall (or welded to embed plates). This then directly transfers the shear without putting any roll-over load into the joists.

 

[UcfSE]

Check your building code to see if it has any definitions for what will be considered rigid or flexible, or in between.

 

[msquared48]

# 1 is semi rigid, mainly due to the questionable connection over the CMU wall.

#2 I can live with as rigid.

Mike McCann
MMC Engineering

 

[frv]

In a situation like this, it may be prudent to analyze as both and design worst case.

 

[tngolfer]

My vote is for #2 and I would consider it rigid.

Do you not have cmu walls above in #1 as you do in #2? It seems #2 would be easier to continue up.

 

[aapl2k7]

Structureguy, thanks for the observation, i had thought about it before.

UcfSE, the building code used in Nicaragua is practically a copy of the Mexican code, the rigid diaphragm definition that appears on the Nicaraguan code, i'm almost 100% sure it's taken from the IBC.

tngolfer, there are CMU above in #1, i will try to take pictures and post them.

 

[gyroman10]

NONE
as per diagram, not enough lateral fixity between the concrete deck and the cold rolled box beam.

 

[csd72]

provide another beams along the wall in option#1 to prevent rollover or provide a downturn edge. Check fire resistance.

 

[Lexatus]

How can you judge rigidity from a schematic figure?

Check your code, you will find the correct dimensions and detailing to define a diaphragm as rigid.

For example, if you give a look at Eurocode 4 - Design of composite steel and concrete structures, you’ll find minimum requirements in order to have a rigid diaphragm.
So, if you have the right dimensions for slab elements, both solutions can be considered rigid. But this isn’t the only problem, because even if they are rigid, you have to check if the walls can support the actions received; looking at the figure 2 solution, seems you can have roll-over problems. You have to check it.

 

[scubthebub]

I would check it as a flexible and rigid diaphragm and use the worst case loading for the resisting elements. I had a mezzanine level concrete floor at a movie theater that was between 20-50 wide and about 200 feet long. I checked it as rigid and flexible and found that some walls were controlled under the flexible condition, and some the rigid. I designed for the worst case and did not have to worry how rigid or how flexible the diaphragm really was.