Mezzanine Joist Space Directions?

[mfleming]

Hello.

I'm just working on a complex project right now with a weird shape for the building. I was wanting to span one section of jsts one way and another span perp to them. My concern is we are using 1-1/2" steel jst deck, and I don't know exactly what happens to connect the different directional roof decks together.

Is this even possible and if so are there any problems associated with this type of construction?

Thanks

Matt

 

[JAE]

Yes you can change the deck direction. Just provide a continuous edge support at the intersection so that both decks can attach along that edge. If you have concrete over the deck, consider a control joint or expansion joint in line with the change in direction.

 

[nutte]

The joist catalogs have details for this exact condition.

 

[haynewp]

And mention to the architects to make sure the any leakage potential at the interface support angle is addressed.

 

[Ron]

mfleming...since you are changing the deck direction, the roof membrane system will be pulled in two different directions from structural movement. This has to be accommodated in the roof membrane system design.

If the deck change is perpendicular to the slope of the roof, then you can't use an isolation joint. In that case you must use a concealed "slip" joint that will attenuate the strains in both directions. This is for a built-up roof system or a fully adhered single ply roof system. For mechanically fastened single ply systems, the direction change is not a big issue.

 

[haynewp]

Ron, when you are talking about movement for a built up roof deck, can you describe the structural movement that would be an issue? I see if there is concrete over the deck there should be a joint because of the different shrinkage directions but I don't quite follow the problem with the movement in the plain roof case. Are you speaking in the vertical plane since a continuous edge is supported in one case and the deck end is supported in the other direction?

 

[Ron]

haynewp....no, it's primarily thermal movement of the steel. It happens worse when you install rigid insulation onto the steel deck (as compared to a topping slab or lightweight insulating concrete), then apply a built-up roof membrane (hot mopped) to the insulation.

You would think that theoretically there wouldn't be much differential movement since both edges are fully supported and it isn't a structural expansion joint. But there's enough movement to split a roof membrane and if the insulation is installed across this joint, enough movement to break the insulation. I've seen lots of them here in Florida.

We had to design around a similar occurrence on a General Mail Facility in Atlanta.

 

[Lutfi]

I normally add a cont. Tube 2.5X2.5 on top of the joist bearing beam. This way the perpendicular deck is supported and diaphragm is transferred properly.

Regards,
Lutfi

 

[dik]

Lutfi... usually use a downturned angle welded to the joist seat to pick up the right angle deck and, for floors, a sawcut...

Dik

 

[Lutfi]

Dik,

I use that detail as well. It depends on the situation.



Regards,
Lutfi

 

[mfleming]

Thanks for everyones input. If we do end up doing this design I will mention using a downturned angle to transfer the lateral loads. As we use a built-up roof system design and we don't need any leaking problems.

Matt