Flange Check for Point Load
Flange Check for Point Load
(OP)
I want to check the top flange of a floor beam for local bending due to the point load from a floor joist. The beam is in the center of a bay so that there are joists attaching to the beam on both sides of the web. The joists are in line with each other so they do not extend across the web but stop so that there is a ¼" gap between the ends of the joists. See the attached sketch. I started to use '05 AISC equation J10-1 (even though the loading is in compression) but I think it would only be applicable if the load is continuous across the beam web rather than point loads on each side of the web. The flange capacity is easy to calculate if I consider that ½ the beam flange as fixed at the web with three sides free, but I think that may be too conservative. I could easily oversize the flange of this one beam but I want to make this check a part of my standard procedures for floor beams. Can anyone point me towards something that might be more accurate?
Thanks.
Thanks.






RE: Flange Check for Point Load
RE: Flange Check for Point Load
BA
RE: Flange Check for Point Load
RE: Flange Check for Point Load
BA
RE: Flange Check for Point Load
RE: Flange Check for Point Load
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Flange Check for Point Load
Its a very simple check and like Boot said, easy to make a spreadsheet.
RE: Flange Check for Point Load
RE: Flange Check for Point Load
Run your checks for flange bending.
RE: Flange Check for Point Load
I'm not sure that other folks are really looking at it those connections so closely. At least, not for each and every project. After all, if this were a common failure mechanism, I'd think the joist references would talk about it. And, I don't recall seeing a discussion of this anywhere.
Personally, I'd start out using section J10. The joist seats are going to spread that load out over the flanges a decent amount. So, it really is an over simplification to treat that load as a point load.
RE: Flange Check for Point Load
Adam Vakiener, P.E.
RE: Flange Check for Point Load
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Flange Check for Point Load
The beam flange is usually more than capable of supporting a joist reaction. This can easily be checked by the Yield Line method in a couple of minutes, so why not just do it?
BA
RE: Flange Check for Point Load
RE: Flange Check for Point Load
RE: Flange Check for Point Load
It is true that the yield line method provides an upper bound solution and it is important to find the correct yield line pattern. I would agree that your yield line pattern is possible if the load is considered a point load at the middle of the bearing area of the joist shoe.
I have added to your sketch as shown attached. The red lines represent negative yield line...the green represent positive yield lines. The width of joist shoe is dimension 'c'.
If there are any other potential yield lines, they too should be considered.
BA
RE: Flange Check for Point Load
You are correct that flexural stress in the flange plays a role in this, so some conservatism is warranted. I think that the load is best considered as uniform over the bearing area of the shoe, but that could be argued.
You are absolutely right about the potential for a few Ph.D. theses. Funny thing is...I don't think too many engineers calculate this effect because they assume that the flange is more than adequate and most of the time, they are right.
BA
RE: Flange Check for Point Load
you are correct in your critical yield line pattern as shown in the attachment. In hindsight, it should have been apparent to me.
These calculations are based on a plate having a unit moment resistance, m. The value 'm' is the factored moment resistance per inch which, for a stress relieved plate is the same in every direction, namely φ*t^2/4 where φ is 0.9 and t is plate thickness. R in the attachment is the factored joist reaction which is considered uniformly distributed in the crossed rectangle. The arrows indicate the slope of sections between yield lines with unit deflection assumed along side 'c'. R deflects 0.5 units.
If the joist reaction is located at a point of high flexural stress in the flange, m(parallel) remains the same but m(normal) should be reduced an amount reflecting the axial stress in the flange.
Finally, to allow for potential corner effects, m required should be increased by 10%.
BA