Lumber span ratings
Lumber span ratings
(OP)
I am planning on designing a stair landing that will be cantilevered off of 24" floor trusses of a loft in a house. The joists I plan on using will be attached to the trusses so that the forces come through the top cords of the trusses and not the webs or the bottom cords. The landing will be around 2.5' or 3' square. I've not had much success finding lumber strength ratings for cantilevers. I find rule-of-thumb statements like "The conventional formula for cantilever of 2x dimensional lumber is that one can overhang the beam 1/2 the span of the anchored portion."
Where should I look for guidance on this? Thanks.
Where should I look for guidance on this? Thanks.
RE: Lumber span ratings
RE: Lumber span ratings
There's a lot to consider here besides the landing framing.
RE: Lumber span ratings
The truss chords are horizontal. With the trusses being 24" in height, there is plenty of room for me to fit long 2x? between the truss webs, crossing as many trusses as I need to. I have plenty of length available to anchor, and can cross up to 6 trusses, each spaced 2' apart, so the load can be pretty well distributed. I was planning on hanging the 2x? from the top chord of the truss with a simpson tie or something similar. I'm open to options.
RE: Lumber span ratings
If new, why not find a way to rotate the framing, even if it is just in that area, in order to have the landing built as part of the floor trusses.
Can you post a plan of the framing?
RE: Lumber span ratings
The stair and landing loads will still be concentrated, as a canti. reaction point loadings, on the edge truss and in particular on the top chord member which was not designed for those concentrated loads. This will be true no matter how many Simpson hangers you use. If you think about the canti. beam actions and reactions, the increased back span tends to reduce the primary canti. reaction value, as long as the back span reaction is actually taken out at the further back span distance. You might want to consider doubling the edge truss with some special attention to added web members and blocking to really get those added loads into the whole truss/fl. system. In addition, you will have handrailing loads, on the truss and fl. system, to deal with in the immediate area of the landing, at the same time the landing might be at max. loading. As mentioned above, a fl. plan and some special detailing would be helpful and might be required.
RE: Lumber span ratings
RE: Lumber span ratings
RE: Lumber span ratings
Most people here are fairly hesitant to touch metal plate connection wood trusses to begin with, much less throwing point loads on ones that haven't been designed appropriately.
What you have proposed COULD work if the truss designer was made aware of the loading at the time of design.
As dhengr alluded to, it really doesn't matter a whole lot how far back you span this, the load will be concentrated on the first truss. Perhaps instead of hanging it off the first truss, you block down tightly to the top of the foundation below so the cantilevered joists are essentially bearing directly on the support below. Then extend the backspan as far as reasonable to limit the uplift reaction on the remaining trusses. If it were my design I may be able to get on board with a methodology along those lines.
RE: Lumber span ratings
Regarding point loads, going back to free body diagrams with a point load on the end of a cantilevered beam with several anchor supports equally spaced, say four, then the reactions to a down load at the two nearest supports would be in the upward direction, while the reaction at the two furthest would be downward again to oppose the torque from the point load. The load will be greatest on the closest support, but the 2nd support would carry part of it as well, not necessarily 1/3 of it, but around that. The sharing should be spread even further if I spanned 6 or more trusses. I have attached a quick sketch showing admittedly simplified reactions to the load, but is it fundamentally wrong?
Any side loading from the stairs could be dealt with by tying into the framing on the side wall (hidden from view).
I understand the hesitation to make any firm recommendations on something like this on a forum where many details are not known/seen. I'm just trying to wrap my mind around the physics as much as possible. It seems that there must be a solution somehow.
RE: Lumber span ratings
Apologies but I really don't see this working out without reinforcing/replacing the floor trusses with ones designed appropriately for the loading.
RE: Lumber span ratings
RE: Lumber span ratings
RE: Lumber span ratings
timsch, since this is new construction, why not pitch this back to the truss designer and see if they will work with you on designing some reinforcing for the truss(es) for these additional loads. Or maybe you can get one additional truss fabricated to be installed at this edge to carry 100% of the landing load as jayrod12 suggests. Even if you use your method of distributing loads to multiple trusses, you are still going to have to analyze the trusses for the additional loads, unless you work with the truss designer to help you do that.
RE: Lumber span ratings
Jayrod, I understand about flexibility with lumber, although I was hoping that if I used something along the lines of 2x10 or even 2x12 that it would be rigid enough to allow for some spread loading. I had also considered using structural steel, but other than finding published span ratings similar to what I'm asking for here with lumber, I've not looked much further into it. If I'm at an impasse with lumber, I will do just that. I could also join a structural steel member to the side of the 2x for additional rigidity.
I had mentioned that I could sister a new truss up against the 1st truss to double the capacity. I had a thought that I could also put structural sheathing on the backside of the 1st truss to add rigidity, as well as taking such a great load off of the metal truss connections.
gte, I will approach the truss makers. I'm trying to get a good idea of what my options are before doing that. My limited experience so far with truss makers is that they are not very willing to stray from the focus on what the trusses are designed specifically for, and that since this landing was not considered initially, that I'd still be on my own to come up with a solution. I hope that I will be proven wrong in this case.
My needs are fairly modest. I'm looking at roughly a landing only big enough for a person to enter and exit a set of 2-1/2' wide alternating-tread stairs that approach parallel to the loft face. The stairs make a 90 degree turn half-way up, so there is no chance of any large, heavy objects such as furniture getting up to that landing.
Thanks again, Tim
RE: Lumber span ratings
RE: Lumber span ratings