Lumber span ratings

[timsch]

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.

 

[gte447f]

Off the top of my head, I cannot think of a reference for any span tables for cantilevered wood floor joists. Calculate the bending stress (=M/S), and compare it to the allowable bending stress for your lumber species, size and grade. Also check deflection. There are many references for cantilever beam formulas for bending moments and deflections. Make sure the floor truss that will support the cantilever is up for the job.

 

[jayrod12]

There's no hard and fast rule. There's also a lot more to consider in this then just the cantilevered joist strength. How do you know the floor trusses can handle the additional load? At the floor truss chords in a vertical orientation or a horizontal orientation. If horizontal, how do you expect to make that connection work?

There's a lot to consider here besides the landing framing.

 

[timsch]

Thanks for the responses. I understand that there is alot to consider. I am having the truss specs sent to me this morning and I will review them ASAP.

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.

 

[jayrod12]

Ugh, This is off the side wall even.. Is this a new build, or an existing build?

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?

 

[dhengr]

Timsch:
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.

 

[timsch]

The trusses are already in place, and the rest of the house is framed in. Nothing further than that. There was a set of spiral stairs in the plans, but those were decided against after construction had already proceeded to this point. I have uploaded a couple of files showing the truss design. Again, I have a half dozen of these trusses that I can spread this load out over, with no side loads on these trusses. I can also build a sister truss or two against the outer trusses if needed.

 

[timsch]

2nd file

 

[jayrod12]

Unfortunately I don't think you're really going to get the answer you're looking for from the forum in this case.

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.

 

[timsch]

The whole reason for the cantilever is because support from below is not an option. Support from above is, but it'd be preferable not to go that route.

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.

 

[jayrod12]

Not with timber. It is not stiff enough to load share as you are describing that 2/3, 1/3 type idea works assuming infinitely stiff members such that they'll rotate as a rigid body. Timber is not that stiff. In order for the second truss in to take any load, you need to fail the connection at the first truss. Then the cantilever length gets longer, the load at the second truss increases and the connection fails etc. You should be designing the connection to the first truss, and the first truss itself to take 100% of the gravity loading and I would be including a fairly hefty safety factor.

Apologies but I really don't see this working out without reinforcing/replacing the floor trusses with ones designed appropriately for the loading.

 

[gte447f]

The reactions to the trusses will be dependent on the relative stiffness of the trusses and the cantilevered member. If the cantilevered member is significantly stiffer than the trusses, then you can distribute the reactions to the trusses based on rigid body rotation the way that timsch has shown in his sketch. The steel joist institute has a decent presentation of this concept in some of its material addressing reinforcing open web steel joists. There, the concept is designing a stiff spreader beam to distribute hanging loads to multiple bar joists, but I think the same principle would apply to this situation. You would need an estimate of the stiffness of the trusses in order to use this method.

 

[jayrod12]

It also has to do with the stiffness of the truss chord in bending between webs, as that will also have some flex when subjected to a point load. With all those uncertainties, I feel a prudent engineer would be designing the first connection for the entire load.

 

[gte447f]

jayrod12, the limited stiffness of a flatwise 2x4 top chord in bending between panel points would further support the idea of distributing loads to additional interior trusses. On the other hand, of course it would load the top chord in bending, which it was not designed for. In general, I don't disagree with you that it is prudent to approach this situation with caution and conservatism.

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.

 

[timsch]

Thank you all for your your responses and guidance. It is greatly appreciated.

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

 

[gte447f]

Tim, you are right that truss designers can be hesitant to go outside the box of their simple designs for uniform floor loads, yet they are the ones with complete knowledge of their design parameters and material capacities (especially their truss plates), so you can see why other engineers without their complete knowledge would be even more hesitant to mess about with their trusses. However, adding a point load to a design is not too troublesome for the designer, and they are more likely to have proven past experience for reinforcing designs than other engineers, so I would not hesitate to ask for their help. Since this is new construction, you won't have to deal with one of the biggest hurdles, which is their ability or willingness to dig up records of old designs.

 

[boo1]

add new beam to take the entire point load?