Idealizing connections
Idealizing connections
(OP)
I have been doing some thinking on idealized vs. real connections and I'm looking for a good reference, or explanation that helps me understand when it is really safe to assume pin connection and full moment connection.
My train of thought started today thinking of two trusses spaced 15' apart with purlins running between them and the purlins nailed to the top of the truss. Now I picture a load in the center of the purlins and think surely that would create a moment on the trusses and a great enough load would pull the tops of the trusses in and collapse them. To me it seems like the nail through the top of the purlins create some moment resistance and makes the trusses unstable with only a nail into a top plate supporting them. Is this correct and the only way the trusses stay upright is due to sheathing, Or is the connection with the purlin truly a pinned connection and flexes enough to not allow lateral deflection of the truss?
Again any reference material on the idealization of connections and how we can make these assumptions would be appreciated, so I can feel safe that one day I will not have trusses that topple :)
My train of thought started today thinking of two trusses spaced 15' apart with purlins running between them and the purlins nailed to the top of the truss. Now I picture a load in the center of the purlins and think surely that would create a moment on the trusses and a great enough load would pull the tops of the trusses in and collapse them. To me it seems like the nail through the top of the purlins create some moment resistance and makes the trusses unstable with only a nail into a top plate supporting them. Is this correct and the only way the trusses stay upright is due to sheathing, Or is the connection with the purlin truly a pinned connection and flexes enough to not allow lateral deflection of the truss?
Again any reference material on the idealization of connections and how we can make these assumptions would be appreciated, so I can feel safe that one day I will not have trusses that topple :)






RE: Idealizing connections
I don't have any documentation that would give you guidance.
Unless you are providing significant connection of both purlin flanges to the supporting material then the connection can usually be idealized as pinned.
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RE: Idealizing connections
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To state that the connection is not a perfect pin is absolutely correct. Some moment will be transferred, but not nearly enough to cause the trusses to collapse.
BA
RE: Idealizing connections
Do you know of any references that covers this and maybe similar theory? I'm very interested in knowing "why" for many of our assumptions and theories
RE: Idealizing connections
RE: Idealizing connections
You'll find it often doesn't take much.
RE: Idealizing connections
Think of it this way...., you have a structurally sufficient purlin spanning 15' and supported by two trusses. If the purlin meets ultimate strength requirements and serviceability criterial, BA’s L/360 or L/240 deflection criteria, etc., it will deflect a bit, but not change the chord length btwn. the two trusses/supports very much, but that is also why we usually say one end of a beam/purlin is pinned and the other is pinned and on rollers, so you aren’t inducing an axial load in the beam, or a thrust on the supports, due to the beam’s curvature during deflection. Nails or screws have some lateral cap’y. so they will impart some small lateral load to the tops of the trusses, but the truss lateral movement is limited to the chord length change, and limited by some axial strength in the purlin, or by adjacent purlin spans and roof sheathing. The nails have almost no withdrawal cap’y. so they won’t impart a fixed end moment to the top of the truss, due to the slight end rotation/curvature of the purlin. In fact the purlin might pull the nails out a bit over several loadings. Screws have a bit more withdrawal cap’y. than nails, so to that extent, whatever rotation in the end of the purlin, times a lever arm from the edge of the truss top chord to the screw, that might impart some small fixed end moment to the truss. But, as already mentioned crushing/compression perpendicular to the grain, and some small rotation/rolling of the top truss chord will cause all that activity to be compatible with a small (negligible) end moment. On the other hand, if you had a cable in place of the purlin, the only way it can support the load is by imparting no moment, but a significant deflection and a very large lateral load to the tops of the trusses. Undoubtedly, we could design an end connection which would impart an end moment to the top chord of the truss, but that would probably just roll the top chord a bit.