shear forces acting on two unconnected timber beams
shear forces acting on two unconnected timber beams
(OP)
Lets say we have 2 timber beams - cross section of a single beam is 180/180 mm
We put one beam on top of the other but we do not connect them at all.
Max shear force above second support is 32,50 kN
What happens to shear stresses in beams? Does shear force distribute based on stiffnes of cross section - in this case half on each beam: 32,50/2 = 16,25 kN?
Or does the bottom beam take over whole force of 32,50 kN?

We put one beam on top of the other but we do not connect them at all.
Max shear force above second support is 32,50 kN
What happens to shear stresses in beams? Does shear force distribute based on stiffnes of cross section - in this case half on each beam: 32,50/2 = 16,25 kN?
Or does the bottom beam take over whole force of 32,50 kN?







RE: shear forces acting on two unconnected timber beams
Personally if it was a new design, I would be designing a single beam for the entire shear force. If I was checking an existing condition, I may check the stiffness distribution if required to make it work.
RE: shear forces acting on two unconnected timber beams
I'm speculating this entirely off the fact that shear is the derivative of moment. So if you're moment isn't composite behavior, then your shear won't be either.
RE: shear forces acting on two unconnected timber beams
But, this is if analyzing existing. If i was doing my own design, i would design the lower one for all the shear. Rarely does shear ever govern in my experience.
Your example of a cantilever seems to be governed by deflection at tip or possibly negative moment.
RE: shear forces acting on two unconnected timber beams
Is this a real world problem, or a homework problem?
Please remember: we're not all guys!
RE: shear forces acting on two unconnected timber beams
RE: shear forces acting on two unconnected timber beams
Jeff
Pipe Stress Analysis
Finite Element Analysis
www.xceed-eng.com
RE: shear forces acting on two unconnected timber beams
why do you guys design beam (at 2nd support)for shear based on force 32,50 kN and not on 32,50 + 30 kN = 62,50 kN (reaction of support). What is the science behind that?
RE: shear forces acting on two unconnected timber beams
RE: shear forces acting on two unconnected timber beams
Dik
RE: shear forces acting on two unconnected timber beams
I got another question about bending stresses.
If we connect both beams properly we have a composite cross section that acts as one element.
But if we dont connect them at all, how do you control bending stresses?
are equations bellow alright?
Im wondering can you add up section modulus (sum W1 + W2) of cross sections when beams are not connected?
RE: shear forces acting on two unconnected timber beams
In practice, I'd be inclined to design each beam for 60% of the load.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: shear forces acting on two unconnected timber beams
I.e. if one beam is twice the depth of the other, it will be eight times as stiff and suck up 8/(8+1) or 88.9% of the moment.
So, yes, you can simply add the section moduli, but only if they are equal. The moments of inertia can be added directly regardless of the relative depths.
RE: shear forces acting on two unconnected timber beams
But for full length beams of equal cross section the answer is simple: both beams have the same curvature along the full length, so they have the same bending moment, so they must have the same shear force at any section, so they both have 50% of the total shear, assuming plane sections remain plane etc.
Of course in the real beams the shear force doesn't transfer to the support at a vertical plane at a point support, but the end shear transfer mechanisms are just the same as for a single simply supported beam, so on that basis taking 50% of the shear on each beam can be justified. On the other hand, it's an unusual situation, so KootK's 60% rule also seems reasonable to me.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/