diaphragm action question 1

[LOKSTR]

I am new to Structural field and looking for the principle involved in my following question?

1. As far as I know, Steel deck is supposed to transfer the lateral forces to perimeter walls thru diaphragm action. I am talking about 22 ga. 1-1/2" deep 3 span cont. deck in a steel frame warehouse type structure.

But some Engineers show horizontal roof bracing along the perimeter in addition to roof deck. What is the purpose of this bracing?

2. Deck supplier( Canam) catalogue show tables in which deck can be spanned upto 8' c/c, but practically nobody is using joist c/c span > 6' or 6-6" for design of deck( for 22ga. 3-span continuous deck). Why is it so? Why can't I use 8' span for deck?

 

[SperlingPE]

1. Buildings that you see horizontal bracing in the roof do not have roof deck acting as the diaphragm. The lateral load is being collected by the bracing which distributes it to the walls and whatever system is being used there for lateral support.

2. Check the vertical capacity of the deck. 8'-0" is a stretch. Also check your joist depth. Spacing joists at 8'-0" o.c. will increase the depth of the joist. It is a balancing act between structural requirements, economy, and building requirements of the architect, mechanical engineer, electrical engineer, and owner.

 

[JedClampett]

Good questions. As far as the local lateral bracing, it's possible they are defining subdiaphragms as required in ASCE 7-02. Maybe someone else can add to that. The reason the spans are limited to 6'-0" or 6'-6" has a couple of possibilities. For one, Factory Mutual only covers 22 gage 1 1/2 deck spanning less than 6'-1". I've never had that explained to me very well, but I hear it's a good idea not to exceed it. Also, just because the deck capacity is acceptable for LL + DL doesn't mean that some beefy construction worker carry a tool belt and a roll of roofing material isn't going to exceed the capacity.

 

[jike]

If bracing is used in the plane of the roof, it may be that the diaphragm is being ignored or perhaps the load or deflection of the diaphragm exceeds the capacity. It seems to be more cost effective to use more fasteners closer to the perimeter. Don't forget that alot of large roof openings take away from the diaphragm's ability to transfer load.

The 6'-0" and 6'-6" limit of the deck span comes from FM (Factory Mutual underwriter) requirements. I understand that some studies were done for a workman on the roof with a fully loaded wheel barrow. This is the maximum deck span at which built up roofing would not tear when subjected to that load.

I do not recommend exceeding that span limit even if it is a building not underwriten by FM. I believe FM allows you to go 6" beyond those limits to obtain equal joist spacings in a bay. I thought 6'-8" was the limit but it depends upon the deck manufacturer. It is just good practice.