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StructuralEng
StructuralEng, Consultant
Category: Structural Engineering
Satisfied Customers: 6689
Experience:  Structural Engineer
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I have a thick (12" deep) concrete retaining wall, built on

Customer Question

I have a thick (12" deep) concrete retaining wall, built on footings pinned to ledge, on the side of a steep hill. My house is set back, up the hill, and very solidly built on footings and a concrete wall foundation. The concrete wall for the house is about 6' from the retaining wall. I want to build a deck that extends from the house as far out over the retaining wall as is safe.
The retaining wall is at least 6' high, from the top of the footing to the top of the wall. (it is probably taller at some places, and may be a little shorter at some places, but since I don't mind an irregularly extended deck, it is sufficient to say 6' high on average.
My question is, if I use a collection of triangular braces (like a giant shelf bracket---I don't know the proper name) whose vertical side is against the concrete wall, and which rests directly on the footing (or rests on a ledger board bolted to the concrete wall), how far out horizontally can the top support extend, to permit the building of a deck on top of it?
The idea is that this brace is a triangle (made of steel if necessary, but I'm assuming pressure treated wood would be sufficient) would allow the forces to go down the back end of the triangle, with little (or minimal) component pushing out from the retaining wall.
If the wall is 6' high, can I use a brace that is a 6x6x6*sqrt(2) to extend the deck 6' beyond the end of the retaining wall?
And, even more, could I build a real room over that bracing?
Submitted: 1 year ago.
Category: Structural Engineering
Expert:  StructuralEng replied 1 year ago.
Hello. I'll be happy to assist you. I'm driving right now. Can I get back to you in a couple hours?
Please post your pictures and sketches here with the paper clip icon
Customer: replied 1 year ago.
Sure. That would be fine.I'll send pictures if you like, but I think the words were sufficient description.Robert S.
Expert:  StructuralEng replied 1 year ago.
The description, but the pictures are very helpful.
Customer: replied 1 year ago.
I cannot find a "paper clip" icon on this page. I do have pictures to make the situation clearer. Please contact me when you have finished driving.
Expert:  StructuralEng replied 1 year ago.
I'm home now. If there is no paper clip icon above the dialogue box, try uploading to mediafire.com and post the URL here for me to access
Customer: replied 1 year ago.
I don't have an account on media fire. I think the words were quite clear, with regard to this question.Can you help me with that?
Customer: replied 1 year ago.
The top of the retaining wall is about 20" below the level of the walk-out lower level. I would build up the deck so that it would be at the same height as the basement floor, with the joists being hung from a ledger attached to the foundation walls of the house. However, I don't want to cantilever the deck out over the retaining wall a crazy distance. I want to support that extended deck with triangular braces (or trusses, again I'm not certain of the correct terminology) so that the supports run from the house foundation (where they are attached), over the retaining wall (possibly with a large sill to support it at the correct height) and then over the triangular truss sticking out from the retaining wall, away from the house.The question is: How far out can that triangular truss extended from the wall (given that the wall is at least 6 feet tall, so that the triangle would run 6 feet along the vertical aspect of the concrete retaining wall, and then out, with a hypotenuse for support from the base of the wall (on a ledger board on the retaining wall, or directly from the footing of the retaining wall).Thank you
Expert:  StructuralEng replied 1 year ago.
I understand the description is sufficient in your mind because you envision what you're describing. The description is not clear to me for a couple reasons. There are many file sharing options (dropbox and mediafire.com are two simple ones) that will allow you to share a sketch. You reference a concrete wall and a retaining wall and it's difficult to tell which you're referring to when or the orientation of the triangular bracket you describe.
Customer: replied 1 year ago.
Okay. Here is the link to several photos and some plans. The "plans" are really just sketches from above, showing the retaining wall and the outline of the house. One shows just the wall and outline. Another shows a possible location for an i-beam to support a deck running from the house, out over the retaining wall, and beyond (supported by the i-beam). Another shows the outline of that extended deck (curved out and cantilevered 25%.HOWEVER, the point of my contacting you is that I think that the i-beam idea is not great, and a better approach would be to support (via a diagonal brace going from the base of the retaining wall (either on a footing under the wall, or from a ledger plate bolted along the wall) to the outer edge of the deck (or the outer edge of the "supported" deck, with a 25% cantilever).The aerial and other photos show the house, at the top of the hill, and the retaining wall about 6' away from it. Again, the top of the retaining wall is about 20" below the floor of the house. You can see that there is presently a sunroom extending from the house, and supported by posts (and a probably-too-long cantilever) on the retaining wall.I trust this will make my question clearer.You can see all this at:
https://www.dropbox.com/sh/pywxhnzp48s6083/AADEC6-NKzGQl26CkN7xPmQTa?dl=0
Expert:  StructuralEng replied 1 year ago.
I think the I beam is a better option and here is why - if you use the diagonal brace that you're talking about, the configuration will impose a constant lateral force on the deck pulling it away from the house toward the retaining wall. Imagine, for a minute, the brace not attached to the to the deck and just standing at the cantilevered tip of the brace. It will want to topple over. It won't topple over, because it will be attached to the deck, but that force to keep it stable will be a continuous lateral force on the deck.
Customer: replied 1 year ago.
I realize that the joists supporting the deck will have to be well-tied to the house. But the brace is attached to the concrete wall, as well as the joists being attached to the house. I realize this isn't the optimal way to design this deck, but putting more footings down the slope, on ledge, may not be practical for our contractor. So, the question becomes, is the lateral force, trying to push the deck away from the house, too strong.Picture the extreme example: Imagine a deck extending far beyond the retaining wall...a way-over-code cantilever of 10' out, with a 6' part attached to the house. (6' on the ground and retaining wall, and 10' sticking out in air.) One way to think about the question is: Will a series of triangulating braces, running from the foot of the retaining wall to the outside edge of the deck, create too much force pulling things out, even as it gives the vertical support needed.I understand that a simple triangulating beam would be bad. But if that beam were tied to form a triangle. Isn't the outward force manageable? How do I calculate THAT.See the same drop box for more images that I've added, to show examples of braces or knee supports
Expert:  StructuralEng replied 1 year ago.
The brace is a little different than I pictured, but the principle is the same. The magnitude of lateral force that the deck can withstand is dependent on how it's designed. I think there is a way to design it to allow no lateral force. Frame a beam below the joists and let the continuous joists cantilever past the dropped beam.
Customer: replied 1 year ago.
"I think there is a way to design it to allow NO lateral force." That is what I want to hear.
But I confess that now I'm having the problem of trouble visualizing. I'm not sure what you mean by "frame a beam below the joists".
Expert:  StructuralEng replied 1 year ago.
Using the lower sketch showing the brace, the two beams you show below the joists....instead of breaking the deck joists over these beams, run them continuous over and cantilever the joists over these beams and lose the brace.
Customer: replied 1 year ago.
I'm sorry, Mr.Customer but I don't follow what you are saying at all.
If I'm looking at the right picture (the one that includes the labels "Verandah" and "Bearer Supported by Knee Brace), I don't see any way to eliminate that brace and keep the support...then it would just be cantilevered...too far.
Expert:  StructuralEng replied 1 year ago.
Why is the cantilever too far?
Customer: replied 1 year ago.
I thought cantilevers are supposed to be no more than 25% of the length. So, if you have a joist attached at a wall (as at the house) and running to a support (in this case the concrete retaining wall) that is 6' away, you can only cantilever out 1 1/2 more feet. I'm trying to cantilever way more than that, which is why I'm looking to support the structure with a brace of some sort.
Expert:  StructuralEng replied 1 year ago.
That is a good rule of thumb, but you can design the cantilever for anything you want as long as the system is designed for it. I don't think the brace option is the way to go.
Customer: replied 1 year ago.
Okay. Thank you for your help. I'm sorry to say that I'm not exactly satisfied. Saying that "You can design the cantilever for anything you want as long as the system is designed for it" doesn't help me very much. I'm the owner of a sunroom where the floor is 8' deep, but cantilevered to 12' (as you can see in the photos), and while I believe it is sturdy enough for a very light sun room wall system (no insulation, just light plastic "windows" that are about 1/8" thick), the shed roof appears to be slowly separating slightly from the ledger board on the house, creating leaks that drip down from the ceiling everywhere; and the cantilevered flooring is measurably bowed down on the outside of the cantilever. Creating a more sturdy support for that outer wall is one of my design issues (though not the main one I was asking about). I'm planning to put braces either from the top of the retaining wall, or from the four footings (under the wall and directly under the posts now supporting that sunroom).I wasn't asking you about that (which I understand is trickier because the loads---snow loads as well as a real, insulated wall with picture windows---would be heavier than a railing on a deck). I was simply asking how far out the brace could go. If your answer is "you can design it to do anything", I guess what would best help me is a pointer or example of such a design.Thank you
Expert:  StructuralEng replied 1 year ago.
Without designing the entire deck for the imposed lateral force, I would not even consider using that detail.
Customer: replied 1 year ago.
Again, I'm not following you. I don't know what "that detail" is you are referring.If there is a way to design the deck that does not impose such lateral forces (other than installing a new set of footings under the end, and thereby eliminating anything the cantilever altogether), I'd love to know about it.I think perhaps I've gone beyond the limits of what this site permits? If you have a handy pointer to designing the kind of deck you appear to have in mind, I'd love to see it.Otherwise, I guess we're done.
Expert:  StructuralEng replied 1 year ago.
Just as I previously said....let the joists cantilever over a beam. Adding the brace imposes a constant lateral force. You'll need to need up the beam and joists in that zone

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