Extending Slab on Deck

[bookowski]

I have an existing building with a 1.5" deck with 3.25" lw topping (4.75" total thickness). This is a roof over a restaurant and they are expanding the space. I have a new steel frame that will extend this slab by about 12ft (x 30ft in length). The new slab is independently supported vertically and laterally. I have my own new edge beam which will be parallel to and a couple of feet from the existing edge beam.

Question is about the existing slab on deck to new slab on deck joint. There will be an existing pour stop and then my new slab, likely with its own pour stop/closure. The architect is worried that this joint would allow water infiltration into the space if the waterproofing ever fails. This seems like excessive worrying but it is a very high end space and she is really on me about it.

I considered drilling and doweling from existing to new - but is this reasonable in 4.75" of thickness? This would at least keep the joint closed - but she's not very satisfied with that and I wonder if this is enough thickness. Technically it appears so by hilti, albeit with huge reduction factors.

Stitch weld a new closure plate to the existing pourstop and then cast in an expanding waterstop?

Feel like I'm trying to solve a problem that doesn't exist.

 

[Archie264]

Precast parking garages have expansion joint waterstops; would that be an option? This is the first one that came up on the search engine; I'm sure there are others.

http://www.cetco.com/en-us/Products/Building-Materials/Waterstops-Expansion-Joints

With something like that underneath the roofing membrane it would seem that two thing would have to fail for water intrusion, but never say never.

 

[KootK]

Would both the new and existing pour stops be light gauge? Or both hot rolled angle? Or one of each?

If the existing pour stop is a hot rolled angle, then it's easy. Use a smaller angle for the new pour stop and stitch it to the existing.

I don't think that dowels are out of the question as long as your confident that the differential deflection between adjacent edge beams will be manageable.

It would probably cost a few nickels but another strategy could be tying the adjacent edge beams together somehow to prevent differential movement.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Nothing you do with the wood and concrete structure will make it watertight. Watertightness must depend entirely on the flashing and membrane system. Architects these days are not very good at detailing systems to make buildings shed water, thus your architect's concern.

 

[KootK]

Is it just gap formation that we're worried about or differential vertical movement as well?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

Because most (all) of my experience is in high seismic areas, I would calculate the expected drift and specify a building separation or structurally connect the diaphragms and analyze it as one building, If separated, the architect specifies and/or details the joint covers.

 

[bookowski]

I am not generally too worried about differential vertical movement, both beams have very little live load deflection and are relatively close. What kootk sketched is accurate except that I believe the existing pourstop is light gauge. Arch's concern is a bit vague but in general she is worried that the slab, which should be the last line of defense in the envelope, has a gap that she believes won't stay completely closed.

Kootk - I thought about tying them together in this manner but it seems that you'd need a fairly deep member that is moment connected to get any vertical compatibility. Not much space to get in there to work either.

I like Archie's waterstop idea. Maybe dowels + waterstop will calm her down. Thanks

 

[KootK]

High seismic or no, I think that one of Wannabe's approaches is the way to go.

Kootk - I thought about tying them together in this manner but it seems that you'd need a fairly deep member that is moment connected to get any vertical compatibility.

I'd actually intended the left side connection to be a moment connection. A little weld or a 4-bolt group and you've got a 10-15 kN-m moment connection. I think that you only would need a nominal moment connection to smooth out the vertical. If gap control is the only issue, no moment connection would be required.

Arch's concern is a bit vague but in general she is worried that the slab, which should be the last line of defense in the envelope, has a gap that she believes won't stay completely closed.

Sounds as though she could use some gentle re-education. The slab shouldn't be any meaningful part of the envelope.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I imagine that you'll be re-roofing a bit of the existing deck. If that's the case, could you throw down some steel strapping fastened to the top side of the respective decks?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

I agree this architect needs either educating or re-educating, not sure which. The membrane cannot include the slab's contribution in any way. If the membrane leaks, water starts to do its job on the structure. Maybe it shows up quickly in the "very high end space", but if not, it will be insidiously doing its thing.

 

[Teguci]

The new slab is independently supported vertically and laterally

If this is your intent, then this joint is an expansion joint and needs to be treated that way when waterproofing it. I'd think it'd make more sense to join the elements together, as you seem to be suggesting, and then treat this area as just additional roof that needs to be waterproofed. However, to join the elements together, you will need to "touch" the existing structure to verify it's capability to support this additional mass at the roof.

In either case, this decision is your responsibility and not the Architect's. Once you've made your decision, detail the connection correctly and then tell the Architect to waterproof it correctly (moving expansion joint or just more roof). I rarely say this, but kudos to the architect.