I am calculating the stamp stresses in a house and have come across an issue that I need help with.
Imagine a truss that has support on an edge beam of a certain height. Below this beam is a load-bearing wall with a sill plate, vertical studs, and a top plate. See image below.
Calculating the stamp stress that the truss applies on the top edge of the edge beam is not a problem.
However, I am wondering if one can assume that the load spreads at a certain angle downward through the edge beam, thus allowing me to calculate a broader contact area against the top edge of the top plate? Is there any basis for this in Eurocodes or other documentation?
I don't work with wood but have done some load calculations. Why would you want to calculate that way? Isn't the buckling case of the standing beam determining?
I will of course calculate the buckling of the wall rule, but have the option to use double or triple rules, so it is still the stamping pressure that will be the hardest to manage.
One sometimes considers a dispersion that follows approximately a 60-degree line towards the horizontal plane. I believe this is quite a cautious method.
Okay, but is there anything in the construction regulations that verifies this? All common sense says that a certain dispersion occurs, but I would like it to be anchored in some investigation, handbook, or regulations.
Eurocode has two cases:
The structural component is fully supported - only the contact pressure on top of the framework is checked.
The structural component rests on supports - contact pressure is checked for the support (the standing stud) and contact pressure on top of the structural component.
Uncertain if the wall is stiff enough to consider the edge stud as fully supported. Are you really having trouble calculating it if you let the force go directly down into the standing wooden stud? What contact surfaces and forces do you have?
The contact area at the top hammer beam (C24) is what I'm having trouble with.
There are double edge beams in Kerto and the wall is 95 mm thick. Thus, I have a contact area of 45x90 mm between the edge beam and the hammer beam. Since the Eurocode allows me to increase the contact length by a maximum of 30 mm on each side, the effective contact area is (30+45+30)x(90+5) = 105x95 mm².
Other calculation assumptions:
(National choices for Norway)
kmod=0.9
Gamma-m=1.25
fc90k=2.5 MPa
kc90=1.0 (Here I am unsure if I can use 1.5 instead?)
The design bearing pressure becomes:
0.9x2.5x1.0x105x95/1.25 = 18 kN
If I can use the higher value of kc90, the result is 27 kN, but since I have a load from the truss of about 30 kN, I'm not quite satisfied...
If you follow the standard exactly, I would clearly use kc90=1.0 unless I can show that the wall is stiff enough for the wall plate to be considered fully supported. Unfortunately, the Eurocode doesn't offer a middle ground, but if a deeper analysis were done, I'm convinced that one could use a value closer to 1.5 (somewhat depending on the type of sheathing material in the wall, plywood=better than gypsum).
I don't know how it is in Norway, but can you count on safety class 2 like in Sweden (Some risk of serious personal injury)? In that case, you can reduce the load by 0.91 and you're almost there...
I also don't know what they say in Norway about kmod, but if it's snow load, you'll probably need to reduce it somewhat (0.8?).
I'm not that well-versed in Eurocode yet, but that's my interpretation.
In Norway, small houses are classified as safety class 1, which according to Norwegian adaptations allows for a reduction of the load by 0.9. I have already done this. In Norway, snow load is considered a short-term load, which gives kmod of 0.9.
However, if I assume that the wall stud is not there, the load goes via the edge beam (which holds!) to the wall studs (c/c 600) beside it. Then the load per wall stud is halved, and I can also use kc90=1.5.
The worst that can theoretically happen is that you get a break in your wall stud before the forces transfer to adjacent studs. Can you manage without it? In practice, it might at most mean that the frame will creak a little under high load. You could let 18kN go down into the wall stud and see what deflection the edge stud gets if you design the rest of the force to be absorbed by adjacent wall studs. If the edge stud only bends down a couple of mm, it might be okay?
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