Statics question....
Statics question....
(OP)
Hey folks,
So i am designing a metal structure and i am having a little trouble figuring out the reaction forces in a rigid truss with the shown loading.
i'd appreciate some advice!
thanks folks
So i am designing a metal structure and i am having a little trouble figuring out the reaction forces in a rigid truss with the shown loading.
i'd appreciate some advice!
thanks folks






RE: Statics question....
Mike McCann, PE, SE (WA)
RE: Statics question....
RE: Statics question....
This the restraining forces to horizontal displacement is friction between the bottom horizontal beam and the ground. Rotation is prevented by the bottom member (length of gamma) pressing against the ground.
I don't know what RISA is.
Is there really no way to calculate a solution by hand?
Thanks
RE: Statics question....
RE: Statics question....
Sounds like you may also need to perform a beam on an elastic foundation analysis here too for the bottom portion of the structure if it is sitting on the ground. You will need a spring constants for the soil matrix to solve for that ...
No, this not one for hand anlysis to get a reliable solution.
Mike McCann, PE, SE (WA)
RE: Statics question....
Perhaps not precisely, but probably close enough for practical purposes.
Remove M1 and M2 and calculate R1 and R2. Design top beam for resulting moments.
For bottom beam (length gamma), calculate pressure variation assuming the whole structure is a rigid body. This gives a trapezoidal distribution of soil pressure on the bottom beam from which moments may be calculated from statics, assuming zero moment in the two vertical supports.
If you need to get more precise than that, calculate the angle change in both beams at the vertical supports and conservatively estimate the bending moment in the verticals.
There is no guarantee that the bearing surface will be flat to begin with and you probably don't know the properties of the soil, so a fancy analysis is not warranted in my opinion.
BA
RE: Statics question....
ok, then the reaction would be some "funky" distributed load with a centroid under the load centroid.
at the place where you've sectioned it, sure there could be moments, by a first approximation would be axial load only. This'd show you (I think) that the LH end will have trouble staying in contact with the ground.
after that, if you want moments then it's a doubly cantilevered beam, doubly redundant (so you can't solve it with use equations of static equilibrium); not difficult to hand calc (my preferred solution is unit force method).
another day in paradise, or is paradise one day closer ?
RE: Statics question....
if γ/3 < 35 < 2γ/3 soil pressure is trapezoidal
if γ/3 = 35 OR 2γ/3 = 35 soil pressure is triangular over full length of bottom beam
if 2γ/3 < 35 < γ soil pressure is triangular over partial length of bottom beam
if γ < 35 structure will topple
BA