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The trusses are designed theoretically by assuming that joints are pin jointed. But practically the joints in trusses are either welded or bolted. How is it justified??
The key is the arrangement of members in a triangular pattern in trusses and not how the joints are actually created. Since a triangle in two dimensions is a rigid geometric pattern, there is very little bending in members unless a load is directly applied to a member. You can easily verify this using a computer program. Model a truss using beam/frame elements and then using truss elements. You'll see very small bending moments in the frame model.
If the projected centerlines of the web members and their connections coincide with the centerline of the chords, there is no eccentricity. Thus, they will behave as pinned. This assumes the chords are loaded at the panel points/joints, not continuously, or that they are evenly loaded on either side of the joint.
Also, the further apart you place the chords of the truss (the longer the web members), the more the moment in the connections reduce, even if there are eccentricities. At some point moments become negligible (act as pinned).
There are second order effects when the truss deflects that can cause the connection to deviate from purely pinned. Again less pronounced the longer the web members are.
This justifies using pinned connections for the majority of trusses that will not have large deflections or large, unevenly placed loads.
I analyze and design Transmission Towers which are analyzed as trusses. There is a ASCE Standard, ASCE 10, which governs our Industry. It is one of the very few industries that does full scale testing of a new (or old) tower design in order to validate the truss assumptions. The single angle compression equations were the result of testing full scale towers over many years. If a T-Line tower is detailed properly, the axial truss loads from the lacing to the corner legs will induce very little moment to the legs. We do apply distributed wind loads on the members but the software assumes the loads are only at the joints. There is also ASCE 74 that describes the way wind loads are calculated.
Just to add my two bits in
IOver the years I have checked the validity of the pin jointed truss compared to having continuous chords or fixed members. I found that I obtained similar results using pin jointed trusses. Pin jointed truss analysis provided slightly more conservative design but not unreasonable. Alot of what I am going to say has been already said in different ways.
The things I have found is that detailing is critical
Lining up the centroid of the sections at work points is really important. There have been failure of trusses caused by detailers making trusses constructable while missing the additional moment caused by the eccentricity at the joint. Selection of web members and chords profiles may be governed by aligning of the centroid lines. You may wish to consider a truss as a single member when checking the stability of a structure.
------------------------------David Thompson P.E., M.ASCEPrincipalKTA Structural Engineers Ltd.Calgary AB------------------------------