wind loads per ASCE 7-05

[AlpineEngineer]

I do not find any direction in ASCE 7-05 as to how to apply the given wind loads to a building. And in the real world I have seen it done a couple ways, is there a correct way?

In particular: If I have a simple stickbuilt 2 story building (8' walls & 1' of 2nd floor system)I would assume that 9' of windward wall would be the contributing area on the 2nd level floor Diaphragm and then the roof Diaphragm would recieve the top 4' of contributing wall area (as well as windward roof area) and the first level floor Diaphragm would recieve the bottom 4' of contributing wall area. Is this the proper way to distribute these windward wall loads? I would assume that in a 1 story structure half the windward wall load would go into the roof Diaphragm and half into the floor Diaphragm? I assume its the same for leeward wall force distribution? If you get into really tall walls is it still the same; say a 20' tall wall with no intermediate floor system?
Thanks guys,

 

[msquared48]

In a two-story stick-built structure, any wind load, windward or leeward, that is above the top plate line of the second story, in addition to half of the story height of the second story, goes to the roof diaphragm.

The second floor diaphragm sees half of the second story plus half of the first story.

The bottom half of the first story goes direct to the first floor diaphragm.

So on and So on.

Mike McCann
McCann Engineering

 

[UcfSE]

Depending on your detailing, the bottom half of the first story may go straight into the foundation.

 

[AlpineEngineer]

Without going into to much detail could you explain the concept that would allow the bottom half of the first story to go straight into the foundation? I am guessing the wall would be bolted straight to the foundation and the joists hang on a ledger of sort? I am assuming the first floor system is stick built (not slab on grade).
Thanks.

 

[UcfSE]

I was thinking of what's usual in my area: masonry walls to strip footings. Typically we have an expansion joint at the slab/wall interface. No load will then transfer to the slab but instead continue to travel down the wall to the top of the footing and into the earth. There are few first floors in Florida that aren't SOG.

 

[msquared48]

AlpineEngineer:

To answer your question...

The wall studs of the first floor span vertically between the first and second floors.

Half the shear or lateral load is transmitted by the wall studs to the first floor diaphragm, half to the second floor by beam action.

The load to the first floor diaphragm is transmitted through the diaphragm to the concrete foundation endwalls parallel to the line of the force, hence into the foundation.

No worries.

Mike McCann
McCann Engineering

 

[OldPaperMaker]

Alpine Engr,
In the Pacific NW most first floor diaphragms are plywood or OSB on joists with a rim board on the perpendicular joist. The plate on the bottom of the first floor shearwall (SW) is attached to the rim with specified plate to plate nailing. The rim bd is attached to the P.T. plate with Simpson A35 angle clips to get the shear from the sheathing to the PT plate. The plate is attached to the concrete fdn with anchor bolts spaced to handle the shear load. The posts at the end of the shearwalls (vertical chords) are equipped with Simpson holdowns, above the top of the diaphragm, and the anchor bolts are attached to the concrete fdn with all-thread that is either cast or epoxyed into the concrete.
Chapter 15 of "Design of Wood Structures" by Donald E. Breyer, et al has a very detailed explaination of these connections.

 

[AlpineEngineer]

Oldpapermaker,
I have seen the detail you are describing; it is used on the sides (end walls parallel to line of force) of the diaphragm as Mike McCann is describing; the rim joist becomes a drag member. I think the situation that UCFSE was describing (which made me curious) was a situation where the windward loading doesn't even go into the diaphragm but straight into the foundation directly under the windward wall; It sounds like that situation is not typical with stickbuilt construction.

Thanks for all the input guys!!