Posted by Paul Malo on January 10, 2002 at 06:13:57:In Reply to: Re: SurePlate steel building connections WTC?? posted by d on January 09, 2002 at 21:45:53:
It's been a long time since I did any truss design, but my recollection (or hunch) is that the design of the truss would be indeterminant if both the top and bottom chords transmitted a vertical load. You see the vertical force has to be calculated as a vector of the resisting force in the first diagonal.
Furthermore, the lower chord would be incapable of transmitting and significant vertical load because it has no diagonal component, so that the entire load would induce bending in the lower chord and have to be resisted by a far larger section than provided by this tension member. Envision this extension of the lower chord as a cantilever. How much load could it carry?
I think if fair to surmise that no vertical load was intended to be transmitted by the lower chord. The only function of the the connection I can envision entails horizontal stress--but if it is a slip connection, as you have sketched, it couldn't do that either. For this reason, I suspect that such a connection might simply serve to impede rotation of the truss about its own axis under loading--or possibly there is some dampering effect, as seems suggested by other posts, where frictin in the slip joint does not prevent movement but simply applies resistance to slow it down.
Are we getting close?
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