Wind loads on Diaphragms 1

[SemiPE]

Hi all, We usually use earthquake loads to design buildings against lateral loads because it yields a greater force compared to wind. I usually apply the earthquake load per storey on the floor diaphragm's center of mass + accidental eccentricity. Now going to wind loads, I have read that wind loads are usually applied on the perimeter walls of the building so I assume if the walls are non structural (ie brick) the loads would be applied on the exterior columns of the moment frames as a Uniformly distributed load (based on tributary area of each column). Please correct me if I am wrong about this.

My main question on this is can i treat the wind load as point load acting on the floor diaphragm on every storey (similar to wind loads) if yes, where would the load act on the diaphragm? on the center of rigidity? the geometric center on the face where the wind is acting (average of center of the leeward wall and windward wall)? center of mass?

 

[SemiPE]

Sorry that should be "My main question on this is can i treat the wind load as point load acting on the floor diaphragm on every storey (similar to EARTHQUAKE loads)...."

 

[JAE]

No - wind loads would be applied to the exterior walls. Then you follow the load path of the wind force from there.

For example - you might have exterior walls comprised of vertical studs. The studs span from floor to floor.
So as the load is applied to the wall, the 1/2 story of wind load is delivered to the diaphragm along the diaphragm width....both windward and leeward sides.

The codes distinguish between windward and leeward applied winds so these would be applied on either side (near side and far side) of the floor....1/2 wind from each adjacent half story.

The wind then essentially becomes a uniform load along the diaphragm which will "span" from brace line to brace line.

This would follow for other similar exterior wall systems that span vertically.

Other exterior walls (such as for industrial type buildings) may have horizontal girts that span from column to column. Just follow the load path as the wind is applied to the exterior envelope.

 

[msquared48]

Wind loads wind down to the foundation through the wall studs, plate connections, diaphragms, drag struts (if any), shear walls, frames, etc..., and are applied as uniform pressures that translate to a line load to the floor and roof diaphraqgms as JAE described, not point loads.

Mike McCann
MMC Engineering

 

[WillisV]

I'll give an alternate opinion from Mike and JAE: for a certain class of typical steel or concrete commercial building with standard rigid diaphragms and an architectural facade that is generally designed by others, it is perfectly acceptable and quite normal in my experience to apply the entire wind load at each level to a point on a rigid diaphragm in order to design the lateral force resisting system for wind load effects - this point should coincide with the center of pressure of the load pattern you are looking at. You should also have a concentrated torsion or move the load off of the center of pressure the correct eccentricity to allow for the code prescribed torsional/partial loading cases.

 

[hokie66]

It depends on what element you are designing. As JAE and M^2 said, the wind is applied as pressures which vary with location on the building, and individual external members are designed using those pressures. As WillisV said, the vertical lateral force resisting systems (such as core walls) are usually designed using loadings which are the resultants of these pressures.

 

[JAE]

Nothing I stated above is in contradiction to what WillisV indicates. A point load for wind based on the geometric center of the wind total story load can be used for the overall lateral force design of the MWFRS. But for large buildings with longer, perhaps flexible diaphragms, a point load is not appropriate.

But for the commercial office building - rigid and small diaphragm - the point load application would be fine for the overall lateral force system.

 

[SemiPE]

Thanks for all your replies. The projects that I usually handle has a small floor area such as residential condo's, using core walls to resist lateral loads. the external cladding is to be designed by another engineer who specializes on facades.

The conclusion I've got from the comments here is that the point load method is only applicable for rigid and small diaphragms. My follow up question to this is how can you determine if you have a rigid or flexible diaphragm? I have always thought that using RC slab equates to a rigid diaphragm.

 

[Jerehmy]

SemiPE, rigid and flexible diaphragms are defined in the IBC.