I'm working on a 5 unit pre-fab multi-residential set of building permit plans for a client and I have a detail that I'm a bit stumped on. It's been quite a while since I did such details so I'm having some trouble figuring out the nitty gritty. We will have an engineer review, but client expects me to come up with my best stab at engineering the building. The detail in question is a cantilevered bit of property over an underground garage ramp. See my sketch here (and attached).
Questions:
I'm not sure if the steel beams that cantilever out in order to support the loads above should sit on top of the concrete foundation wall or should be nestled into it? If on top, how would I separate the exterior from the interior floor? Either way, how do I create a separation between the two?
The client said that the floor for the exterior bit must be steel and the interior sections can be wood. He wants to use Hambro top bearing joists for the exterior bits on other sections of the garage, but I'm not sure if Hambro is specifically needed for this cantilevered bit. As you can see with my sketch, I'm not sure how I would resolve the transition from the steel joists to the wood joists (as the wood joists would have to sit on top of the beam, as far as I know). Is there a better was to resolve this detail than what I'm showing? Can the wood joists be top flush with the steel beam? If so, how would I detail this?
i don't even know where to start. the concrete connection can be achieved about 10 different ways - consult a structural engineer, or even better figure it out and tell them what you want. The problem is going to be the closure pieces out of plane - nested or not, that sucks. The transition from interior to exterior space could be greatly informed if you actually figured out how to drain that balcony. Assuming there is going to be a door threshold there also.
Yes amateur hour indeed. It's been some years since I worked on this level of detail.
To be clear, It's not a balcony above, but living space. Also, is it possible to have the wood joists supported at the sides of the beams rather than the top? Top flush?
1. If the beam is actually bearing on the foundation wall, you'll probably use some type of masonry-to-steel saddle. Whether it is fastened/ welded/ or just catches the beam is up to structural.
2. Even the steel joists that are outside the foundation wall should be "inside" - you don't want your thermal break happening at the floor of that unit. You want your entire floor assembly to be inside the insulation and water barrier.
3. The easiest thing to do is make your steel beam "inside" also, so that your building enclosure wraps down, encloses the steel joists and the steel beam, then down the (hopefully insulated) foundation wall.
4. I don't understand how your steel beams work. Where is the cantilever anchored?
@curtkram Part of one of the issues I have to figure out. I could use some guidance.
@archanonymous Thank you for your thoughtful and helpful response!
Thanks. Since the beam is bearing on the wall and supporting the cantilevered portion, Im guessing it doesnt need a saddle?
Yes, the entire space above is an interior residential space (living room in this case) and it must be insulated outside all the way around. I hadn't sketched the insulation and exterior cladding as I wanted to focus first on the structural elements, but I do see now that it's quite a large opening to the outside. Would I insulated between the joists and between the beams in this case? Not sure.
Yes Im thinking it's going to wrap down the face of the building, under the steel beam, and continue down the face of the foundation wall. But Im a bit concerned that such a design will reduce the headroom for the car ramp. Do you think a saddle anchoring the joists to the side of the beam is the way to go (rather than having them sit on top)?
The beam extends back into the garage and is anchored to another perpendicularly running beam.
My client, an architect, said that anywhere where there is an exterior face (whether above or below, as in this case with the cantilever), it needs to use metal framing. He didn't explain if this was a code issue or what, so Im not sure to be honest. I'm just going with the assumption that he has a good reason.
ah, that's your problem. Never work for architects, they are the worst clients.
If the beam is bearing on the wall it will most definitely need a saddle or an embed of some type to distribute the load evenly to the masonry... I guess maybe you could get by with a grout setting bed. Talk to structure.
I don't really see what they problem is with determining inside/outside - seems like you've got it figured out!
if you are trying to reduce the assembly height, flush the top of joists with the top of beam... with the steel, this is really easy to do... welded, bolted, whatever.
With the wood, you can specify an off-the shelf joist hangar that welds or bolts to steel members.
How deep is the cantilever??? Is there a way for you to rotate your floor trusses to achieve the cantilever? Or, potentially look at the beam pockets illustration here.
@archanonymous hahaha. ;) Thanks. I've updated the design to have the joists sit flush with the beam, and the beam flush and protruding thru the concrete foundation with a saddle of sorts. See my sketch here. Since this is to be a pre-fabricated modular construction, the whole piece must essentially just drop into place so I have to figure out the cladding and how exactly to weather proof and thermally break the beam protrusion (I insulated in between the joists and rigid insulated the exterior, which may be overkill?). And I realize now that the q-deck section over the protruding/cantilevered bit is strange and possibly not necessary. Your thoughts?
@wood guy: Thank you.
@senjohnblutarsky The cantilever is about 6ft and the beam extends inside past the foundation wall about 14ft (meeting the rough 1/3 cantilever to 2/3 ratio). I thought about rotating the trusses and I think it would make sense, but client seems to think we need steel for the cantilevered section, and Im not sure why. I understand given it's a modular build it needs some extra reinforcing, but it seems a bit overkill. I do like your beam pockets suggestion. So many options/directions to go with. What do you think of my current solution? https://drive.google.com/open?id=0B_p6c-RB9BE4OURQV3VMeFVTSTQ
I think if you use rigid insulation on the exterior soffit and fascia you won't need any insulation between joists. Also don't see any reason to thermally break the beam... the whole thing is inside the insulation.
You might want to drop the top of the masonry wall down a bit below bottom of finish floor/ subfloor so there is room for some hat channel or something... you want some wiggle room there so you can frame it out to be perfectly flush. I don't know what your experience with concrete guys is, but usually +/- 1-2" is doin good.
I still don't understand why you need steel joists instead of wood beyond the cantilever, so i'm not going to comment on whether or not you need q-deck.
From your sketch it appears that the steel and wood joists have the same span and carry the same load. There shouldn't be any structural reason you'd need metal joists instead of wood in the cantilevered section. I've done structural work on a few projects similar to this one, and every time the same wood joists were used in cantilevered sections like that.
Ya I'm thinknig the steel joists for this end bit with the q-deck is unnecessary. I'm just going to present an all wood option here.
I spoke to the client and he did clarify that since these are modular off-site pre-fab units, they will be all wood floor/joists (for weight/transport reasons) and the floors for exterior walkways above will be built on site as hambro w q-deck. The pre-fab units will literally be dropped into place. Now that makes sense to me. I wish I would have asked for more clarification early on.
@ Schoon I've decided Im most likely going with wood. And for that matter, do you think it's necessary to have the metal beams in place to take the loads from above? And for strength during transport of the mod pre-fab from the factory?
For the first part, that would depend on the loads in that area. I've worked on some multi-residential that use an entirely wood structure, and others that have required steel beams in certain areas of higher loading.
I don't have experience with prefab, so I can't answer the second part of your question. However, an issue that comes up a lot with wood structure is moisture penetration during storage of materials on-site. You should look into how this issue changes for prefab units.
Like others have said, consult a structural engineer. It's hard to give you a clear answer without knowing your loads or dimensions.
In my view, steel beam on foundation wall is not a good idea due to waterproofing issue. Try to cast the floor in RC and separate the Exterior & Interior Balcony with an upstand RC beam or an RC kerb.
Rotate the floor joist 90 deg at the cantilever section. FJ should always run in the shortest direction. If it's going to carry the weight of 5 stories can you propose columns there?
Although it's doable, Im guessing the cantilever stl beams are going to be much deeper than what you think. The remaining details are peanuts but separation between podium and residential tends to be 3-hr rated so extra drywall type C or X will be req'd as well as blocking as a closure between FJ openings. Wood may not be be allowed to penetrate both construction types.
Thank you for your help everyone. @cadomestique No, I can't do columns there... the cantelever is over a sloped drive leading to underground garage.
I like the simple idea of rotating the joists and cantilevering out, but from my talks with the client, he's confirming that the cantilever section must be non-combustible materials (hence the steel joists and exterior walls). I'll have to meet with his engineer to get a definitive answer, but this is a fire rating issue it seems, so not sure.
What do you mean "Wood may not be be allowed to penetrate both construction types. "?
Jun 22, 16 7:39 pm ·
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Cantilever section detail for multi-residential. Your thoughts?
I'm working on a 5 unit pre-fab multi-residential set of building permit plans for a client and I have a detail that I'm a bit stumped on. It's been quite a while since I did such details so I'm having some trouble figuring out the nitty gritty. We will have an engineer review, but client expects me to come up with my best stab at engineering the building. The detail in question is a cantilevered bit of property over an underground garage ramp. See my sketch here (and attached).
Questions:
Thank you.
amateur hour.
i don't even know where to start. the concrete connection can be achieved about 10 different ways - consult a structural engineer, or even better figure it out and tell them what you want. The problem is going to be the closure pieces out of plane - nested or not, that sucks. The transition from interior to exterior space could be greatly informed if you actually figured out how to drain that balcony. Assuming there is going to be a door threshold there also.
Yes amateur hour indeed. It's been some years since I worked on this level of detail.
To be clear, It's not a balcony above, but living space. Also, is it possible to have the wood joists supported at the sides of the beams rather than the top? Top flush?
is there a thermal break in your steel beam?
1. If the beam is actually bearing on the foundation wall, you'll probably use some type of masonry-to-steel saddle. Whether it is fastened/ welded/ or just catches the beam is up to structural.
2. Even the steel joists that are outside the foundation wall should be "inside" - you don't want your thermal break happening at the floor of that unit. You want your entire floor assembly to be inside the insulation and water barrier.
3. The easiest thing to do is make your steel beam "inside" also, so that your building enclosure wraps down, encloses the steel joists and the steel beam, then down the (hopefully insulated) foundation wall.
4. I don't understand how your steel beams work. Where is the cantilever anchored?
5. Why are the joists steel? I don't get it.
@curtkram Part of one of the issues I have to figure out. I could use some guidance.
@archanonymous Thank you for your thoughtful and helpful response!
ah, that's your problem. Never work for architects, they are the worst clients.
If the beam is bearing on the wall it will most definitely need a saddle or an embed of some type to distribute the load evenly to the masonry... I guess maybe you could get by with a grout setting bed. Talk to structure.
I don't really see what they problem is with determining inside/outside - seems like you've got it figured out!
if you are trying to reduce the assembly height, flush the top of joists with the top of beam... with the steel, this is really easy to do... welded, bolted, whatever.
With the wood, you can specify an off-the shelf joist hangar that welds or bolts to steel members.
These folks make some nice thermally-broken structural supports that may work for you: http://www.schock-us.com/en_us/solutions/isokorb--189.
How deep is the cantilever??? Is there a way for you to rotate your floor trusses to achieve the cantilever? Or, potentially look at the beam pockets illustration here.
Talk to structural, sooner rather than later.
http://www.mitek-us.com/floortrusses/mitekfloortrussadvantages.aspx
@archanonymous hahaha. ;) Thanks. I've updated the design to have the joists sit flush with the beam, and the beam flush and protruding thru the concrete foundation with a saddle of sorts. See my sketch here. Since this is to be a pre-fabricated modular construction, the whole piece must essentially just drop into place so I have to figure out the cladding and how exactly to weather proof and thermally break the beam protrusion (I insulated in between the joists and rigid insulated the exterior, which may be overkill?). And I realize now that the q-deck section over the protruding/cantilevered bit is strange and possibly not necessary. Your thoughts?
@wood guy: Thank you.
@senjohnblutarsky The cantilever is about 6ft and the beam extends inside past the foundation wall about 14ft (meeting the rough 1/3 cantilever to 2/3 ratio). I thought about rotating the trusses and I think it would make sense, but client seems to think we need steel for the cantilevered section, and Im not sure why. I understand given it's a modular build it needs some extra reinforcing, but it seems a bit overkill. I do like your beam pockets suggestion. So many options/directions to go with. What do you think of my current solution? https://drive.google.com/open?id=0B_p6c-RB9BE4OURQV3VMeFVTSTQ
I think if you use rigid insulation on the exterior soffit and fascia you won't need any insulation between joists. Also don't see any reason to thermally break the beam... the whole thing is inside the insulation.
You might want to drop the top of the masonry wall down a bit below bottom of finish floor/ subfloor so there is room for some hat channel or something... you want some wiggle room there so you can frame it out to be perfectly flush. I don't know what your experience with concrete guys is, but usually +/- 1-2" is doin good.
I still don't understand why you need steel joists instead of wood beyond the cantilever, so i'm not going to comment on whether or not you need q-deck.
From your sketch it appears that the steel and wood joists have the same span and carry the same load. There shouldn't be any structural reason you'd need metal joists instead of wood in the cantilevered section. I've done structural work on a few projects similar to this one, and every time the same wood joists were used in cantilevered sections like that.
archanonymous Thank you. Yes I wasnt sure but makes sense now that it's insulated all the way around.
I"m thinking there's going to be a channel hat on top of the beam so that should elevate things a bit. Works? http://www.finehomebuilding.com/2006/07/01/anchoring-wood-to-a-steel-i-beam
Ya I'm thinknig the steel joists for this end bit with the q-deck is unnecessary. I'm just going to present an all wood option here.
I spoke to the client and he did clarify that since these are modular off-site pre-fab units, they will be all wood floor/joists (for weight/transport reasons) and the floors for exterior walkways above will be built on site as hambro w q-deck. The pre-fab units will literally be dropped into place. Now that makes sense to me. I wish I would have asked for more clarification early on.
@ Schoon I've decided Im most likely going with wood. And for that matter, do you think it's necessary to have the metal beams in place to take the loads from above? And for strength during transport of the mod pre-fab from the factory?
PB80,
For the first part, that would depend on the loads in that area. I've worked on some multi-residential that use an entirely wood structure, and others that have required steel beams in certain areas of higher loading.
I don't have experience with prefab, so I can't answer the second part of your question. However, an issue that comes up a lot with wood structure is moisture penetration during storage of materials on-site. You should look into how this issue changes for prefab units.
Like others have said, consult a structural engineer. It's hard to give you a clear answer without knowing your loads or dimensions.
In my view, steel beam on foundation wall is not a good idea due to waterproofing issue. Try to cast the floor in RC and separate the Exterior & Interior Balcony with an upstand RC beam or an RC kerb.
Rotate the floor joist 90 deg at the cantilever section. FJ should always run in the shortest direction. If it's going to carry the weight of 5 stories can you propose columns there?
Although it's doable, Im guessing the cantilever stl beams are going to be much deeper than what you think. The remaining details are peanuts but separation between podium and residential tends to be 3-hr rated so extra drywall type C or X will be req'd as well as blocking as a closure between FJ openings. Wood may not be be allowed to penetrate both construction types.
Thank you for your help everyone. @cadomestique No, I can't do columns there... the cantelever is over a sloped drive leading to underground garage.
I like the simple idea of rotating the joists and cantilevering out, but from my talks with the client, he's confirming that the cantilever section must be non-combustible materials (hence the steel joists and exterior walls). I'll have to meet with his engineer to get a definitive answer, but this is a fire rating issue it seems, so not sure.
What do you mean "Wood may not be be allowed to penetrate both construction types. "?
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