Moment Capacity of pinned vs fixed beams 2

[jay156]

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?

 

[paddingtongreen]

Without knowing more....perhaps...is the bottom flange laterally restrained in your model? the fixed end reverses the moment.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[larsacious]

Something is wrong. Check your flange bracing. It sounds like your top flange is continuously braced and your bottom flange is unbraced. When you induce negative moment into the beam, the unbraced bottom flange length comes into play and reduces your moment capacity.

 

[slickdeals]

I think paddington and larsacious are both right.

I think this discussion will then drift into what unbraced length to use in the bottom flange to calculate moment capacity (-:

 

[archeng59]

Capacity will vary with the unbraced length of the beam. If Lb is the same in both models, the allowable capacity should be the same regardless of end fixity.

 

[paddingtongreen]

Another factor that may play in to this:
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.

 

[jtx]

Just out of curiosity...it that your max POSITIVE moment at the CENTER of your beam? If it is, then that may make sense because the moment at the center of a fixed beam is w*l^2/24 and the moment at the center of a simple beam is w*l^2/8. The simple beam would have 3 times the moment at the center than the fixed beam. Be sure you are reading your max moment which may be your negative moment.

BTW I am ignoring braced lengths in my assumptions

 

[slickdeals]

No, I think he is reporting Moment capacities (phi Mn)

 

[jay156]

Thanks, yes, that was the problem, the bottom flange was unbraced. The top flange has joists framing in every 3.42 feet, so I called that braced.

Thank you all, that was a silly oversight.

 

[jay156]

Now my next question is, how could I brace that bottom flange? Is extending the bottom chord of the joist and tying it to the bottom sufficient? I don't usually see that in buildings I look at, but I suppose it's done, right?

 

[rb1957]

i don't understand why a fixed end beam (with an unbraced flange) has a lower "capacity" than a pin ended beam (also with an unbraced flange). i understand lateral buckling. i'd've thought that the moment capacity of the two beams is the same, if capacity is a measure of the moment that can be applied.

 

[AVak]

rb1957,

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.

 

[frv]

jay-

Yes.. that is a very common detail.

 

[frv]

Let me clarify.. the bottom chord of the joist isn't what braces the bottom flange.. you add a kicker from the bottom flange of the beam to the joist bottom chord.

 

[paddingtongreen]

Also, the brace from the bottom chord of the joist to the bottom flange must be on only one side of the beam or else the joist and the beam (extra load) must be designed for the fixed end moment on the joists.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[slickdeals]

@Paddington:
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?

 

[archeng59]

Add an angle diagonally from the beam bottom flange to one or more of the joist top chord panel points.

 

[JAE]

slickdeals,

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.

 

[BAretired]

jay156

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

 

[jay156]

Thank you everyone for your help.

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.