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RE: 125' long steel truss, connection question

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> Once connection strength has been changed, all the structural properties,
> i.e., stiffness, mass, damping, may change. Undoubtedly,
> structure must have a different response. 
Depends. The stiffness changes a lot between pinned and fixed ends, but 
not as much for different connection styles. The rule of thumb for nuke 
plants is that damping about doubles between welded and bolted 
conections, (from 2% to 4% at working stress levels) but the difference 
for different connection designs is usually ignored.  Connection mass is 
very small compared to the structure. I haven't found much effect 
whenever I've had a chance to do any testing--seems like the structure 
stiffness and mass really drives things. It's not easy changing dynamic 
response, once you have the basic shape, I guess because it's tricky 
increasing the stiffness without increasing the mass. 

>never check if the perfect-rigidity assumption
>is OK at the final design. It is a strange and incomplete procedure.
I think that depends on the type of connections. Straight 
member-to-member welds without a lot of bracketing won't stand much 
relative deformation. if the deformations aren't continuous the relative 
deformation will break the welds. As an example, the displacement (angle 
in radians times radius) at the outer fiber of a 12 inch deep member is 
0.011 in (=0.00175x6) for 0.1 degree of relative rotation. The _average_ 
strain in a 1/2 in fillet weld would be 2.5%; the local strain (at the 
fillet heel) is probably 10 times that, which is more than enough to 
crack it.

And bolted connections, in machinery, anyway, need to be secure enough to 
prevent relative movement. Otherwise the bolt loading cycles and fatigue 
failure results. If the bolt doesn't loosen and drop out first. 

Brackets and such can handle relative movement because the connecting 
elements allow for it. And if the connection doesn't see much movement in 
service, from temperature changes or gust loading, a few big stretches 
won't hurt much if the weld is properly made.

Christopher Wright P.E.    |"They couldn't hit an elephant from
chrisw(--nospam--at)        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)