Moment Capacity of pinned vs fixed beams
Moment Capacity of pinned vs fixed beams
(OP)
Hi, I'm doing some calculations for girders on that open-air storage building from my other thread. The girders are going to be rigidly attached to the columns, so in my computer model I modeled them as fixed. I looked at the report it gave me and it said the beam was failing. So I tried modeling it as pinned just for the heck of it.
In the report it shows the moment capacity for the girder, a W21x44, is 357.75 k-ft when pinned, and 134.34 k-ft when fixed.
Does that make any sense? Is there a part in the code that says a beam can support more moment when pinned at both ends than when it's fixed at both ends? I don't remember anything about that. Might my software be screwed up?
In the report it shows the moment capacity for the girder, a W21x44, is 357.75 k-ft when pinned, and 134.34 k-ft when fixed.
Does that make any sense? Is there a part in the code that says a beam can support more moment when pinned at both ends than when it's fixed at both ends? I don't remember anything about that. Might my software be screwed up?






RE: Moment Capacity of pinned vs fixed beams
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Moment Capacity of pinned vs fixed beams
RE: Moment Capacity of pinned vs fixed beams
I think this discussion will then drift into what unbraced length to use in the bottom flange to calculate moment capacity
RE: Moment Capacity of pinned vs fixed beams
RE: Moment Capacity of pinned vs fixed beams
The beam, when fixed to the columns will have a compression due to the moment in the columns, the moment capacity of the beam might be the residue after it takes account of the axial compression.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Moment Capacity of pinned vs fixed beams
BTW I am ignoring braced lengths in my assumptions
RE: Moment Capacity of pinned vs fixed beams
RE: Moment Capacity of pinned vs fixed beams
Thank you all, that was a silly oversight.
RE: Moment Capacity of pinned vs fixed beams
RE: Moment Capacity of pinned vs fixed beams
RE: Moment Capacity of pinned vs fixed beams
The moment capacity of the beam is dependent on the bracing of the compression flange. For a pin-ended beam, the top flange is always in compression for gravity loading whereas a fixed-end beam has the bottom flange in compression at the supported ends. If the top flange is braced and the bottom flange is not, this will result in different moment capacities for the different support conditions.
Adam Vakiener, P.E.
RE: Moment Capacity of pinned vs fixed beams
Yes.. that is a very common detail.
RE: Moment Capacity of pinned vs fixed beams
RE: Moment Capacity of pinned vs fixed beams
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Moment Capacity of pinned vs fixed beams
If you extend only one side, won't you still be designing the joist as a fixed on one end, pinned on the other? Am I missing something? Could you clarify?
The kicker must be able to laterally brace the bottom flange as a nodal brace, right?
RE: Moment Capacity of pinned vs fixed beams
RE: Moment Capacity of pinned vs fixed beams
You use a kicker on one side to prevent the bottom flange of the supporting beam from rotating sideways. But there is very little torsional stiffness in the supporting beam so the joist that has the kicker doesn't transfer much (if any) moment out of its end.
I suppose that as you approach the supported end of the beam, the beam at that point may have a lot more torsional stiffness so there might be an increase in negative moment in the joist, but not much.
RE: Moment Capacity of pinned vs fixed beams
Why are you modeling the beams as fixed? They are rigidly attached to columns, but the columns will flex. Shouldn't you be doing a frame analysis?
Alternatively, you could consider using cantilevered beams supporting simple spans between ends of cantilevers (the Gerber system) thereby controlling the location of the inflection points.
Something seems to be wrong with your software or your understanding of it. What happens when you brace the bottom flange at the same spacing as the top? Do you still find that a simple beam carries more than one fixed at each end? If so, there is a software problem.
And finally, why are you spacing your joists as close as 3.42'?
BA
RE: Moment Capacity of pinned vs fixed beams
BAretired, I should do a frame analysis, but this is just for estimating purposes for now, so I just needed some rough sizes.
When I model it with the same bottom chord bracing, pinned and fixed have the same capacity.
I guess maybe 3.42 was kinda close, but I was shooting for 10 even spaces. Maybe that isn't so important.
RE: Moment Capacity of pinned vs fixed beams
Just one more point. Ten spaces at 3.42 means your beams are spanning 34.2' and in your previous thread you indicated the columns were spaced at 30'-9" which would also be the joist length.
In my experience, the best economy in a rectangular bay is to allow joists to span the longer distance and beams to span the shorter distance. Joists would usually be spaced at about 6' centers.
BA