out of plane seismic forces in diaphragm or subdiaphragms

[tkhartle]

How do I figure out if a flexible metal deck can handle the compressive out of plane forces needed seismic design, parallel to the direction of the deck span.

I have a building assigned to design category C. The force is calculated from 12.11.2.1 ASCE 7-05. I would like to use the deck as the continuous ties parallel to the deck span. 12.11.2.2.4 does not apply, I will be using my joist chords in that direction. I am concerned that the deck may buckle under the compressive force. Joists are spaced at 6.25 feet. I'm using 1.5 type B 22 ga deck.

Any ideas?

 

[kslee1000]

I wouldn't utilize metal deck in this manner, why not utilize the reinforced concrete topping as strut/tie?

 

[Prestressed Guy]

If this is a roof deck, you should provide a structural steel member to carry the chord and collector forces. The deck is only intended to carry vertical/out-of-plane forces and diaphragm/shear forces.

 

[tkhartle]

It is a roof deck, no concrete topping. It has tilt-up shear walls. I am using the edge angle for the chord members for the main diaphragm and the building is a box so no collector members are needed. I am interested in the out-of-plane forces in the deck. I understand that these forces could be tension or compression and the crossties for these forces (which I would like to use the deck for this in the direction parallel to the deck span)need to be continuous or I would need to use a subdiaphragm. How do I calculate what the deck can take in compression?

 

[kslee1000]

The deck can take in plane shear. You need to find something else to take on tha in plane compressive force. How is the deck supported?

 

[tkhartle]

it is supported on joists and joist girders

 

[haynewp]

This question has come up before. Call the deck manufacturer and ask for research on the specific deck profile and span length you are looking at to put axial load into. You can have combined uplift/downward plus compression on the deck so it is not an easy calc. I have seen angles placed in a horizontal truss type fashion along the edges of a building where the forces were "high" to act as the subdiaphragm instead of counting on the perp. metal deck.

 

[COEngineeer]

It will be taken to whatever supporting members or shea walls in the same direction you look at your seismic. Since the deck is fastened to these members only every very short distance, then this member wont buckle. Just make sure the fastener can take the seismic load. Am I missing something here?

Never, but never question engineer's judgement

 

[haynewp]

There is normally a 5ft or more space between the last joist and the wall, this is where the concern is usually the highest about the buckling of the deck parallel to its span happening...until the force spreads out into the rest of the diaphragm.

 

[kslee1000]

haynewp:

What you have suggested is very good for the 5' spacing. However, I am surprised to see it (5' spacing) is the normal practice in these days. I used to place the first joist as colose to wall as possible, 1'-2' clear from wall face.

 

[haynewp]

The first joist placed at the typical joist spacing (5 to 6 ft) away from the wall is pretty standard for load bearing walls in my area.

 

[kslee1000]

Wow!

 

[COEngineeer]

So you only do percentage of the trib width of the last joist then right? I dont think it would be very much because it would only be a fraction of the total sq ft.

Never, but never question engineer's judgement

 

[kslee1000]

CO: Assume you were asking me.

The end Joist is same as the others. It is there to stiffen the edge.

 

[haynewp]

There is a deflection difference between the joist at the very edge and the load bearing wall, that could tear the deck from the joist at this location. This may be the reason most drawings I have seen use the wider spacing from the bearing wall. Vulcraft's standard schematic details have joists appearing about a standard space away from a wall, not directly beside the wall.

 

[kslee1000]

Never had such problem. We also provide kickers from the joist to wall align with bridging.

 

[kslee1000]

When spacing is greater than 2'.

 

[haynewp]

kslee,

I think every big retail box store I have ever been in with load bearing walls has a typical joist space between the first joist and the wall. You are saying the standard where you are is to provide a joist right up against the bearing wall?

 

[kslee1000]

haynewp:

You made me feel anxitious on my old designs. Now I feel the urge to go back have a peek to make sure the roofs still there.

Yes, that's the way been done through my hand. Reasons include, but not limited to, to better handle snow drift, provide strong edges, thus improve stability at the cost of one extra joist, maybe.

 

[JAE]

kslee1000, having a joist that close to the wall is not a good detail. The joist wants to deflect, and does deflect, under wind loads (eave - zone 2 higher loads for that matter) and this flexes the deck a lot if the joist is only a foot or so away from an "infinitely" rigid wall.

This might put stress on the deck to wall connection screws or welds - you could check it by looking at the midspan joist deflection of a typical joist, and seeing what sort of rotation results in the deck. From this deck bending rotation, you would be able to see what sort of bending stresses are in the deck - also what sort of prying action on the fastener might be induced.

It may not be a serious problem, but most designers I know don't put it close - and certainly don't use X-bridging or diagonal kickers between the joist and wall.

I think SJI even discusses this in their spec or somewhere - not sure.