fb > Fcr ??? (Girder Stability)
fb > Fcr ??? (Girder Stability)
(OP)
I am planning on erecting a girder over a steam. The problem I am running into is that when the first girder is at rest (simple span,) fb is greater than Fcr, besides supporting the girder in-between the span is there anything I can do to increase Fcr or reduce fb. (Notes Fcr and fb taken from AISC 9th edition)
The Properties of the girder are as follows
(all properties are in inches unless noted)
TF -0.75x16
WEB -44x0.5
BF -1x20
Depth - 45.75
Length of Beam - 111'
Span - 110'
Weight - 20,900 lbs
W = .188 k/ft
NA (from bottom) - 19.639
Ix = 19074 in^4
Stop - 730.502 in^3
Mmax = wL^2/8 = (0.188*111^2/8)*12 = 3474.52 k*in
fb = Mmax/Stop = 4.76 ksi
Fcr = 4.62 ksi (taken at 110' span)
thus where fb (should be) < Fcr it is not.
Thank you for your time.
The Properties of the girder are as follows
(all properties are in inches unless noted)
TF -0.75x16
WEB -44x0.5
BF -1x20
Depth - 45.75
Length of Beam - 111'
Span - 110'
Weight - 20,900 lbs
W = .188 k/ft
NA (from bottom) - 19.639
Ix = 19074 in^4
Stop - 730.502 in^3
Mmax = wL^2/8 = (0.188*111^2/8)*12 = 3474.52 k*in
fb = Mmax/Stop = 4.76 ksi
Fcr = 4.62 ksi (taken at 110' span)
thus where fb (should be) < Fcr it is not.
Thank you for your time.






RE: fb > Fcr ??? (Girder Stability)
RE: fb > Fcr ??? (Girder Stability)
RE: fb > Fcr ??? (Girder Stability)
RE: fb > Fcr ??? (Girder Stability)
If you can't get the stresses to work, you'll have to support it, with a shoring tower below or a holding crane above.
Also read Fundamentals of Beam Bracing by Yura. http:
RE: fb > Fcr ??? (Girder Stability)
RE: fb > Fcr ??? (Girder Stability)
RE: fb > Fcr ??? (Girder Stability)
My suggestion was more of a thought experiment. I don't have time to run the calc myself.
Honestly, the whole setup sounds dangerous. That long a span unbraced?! The existing beam could already be slightly bent out of shape, etc. What about lateral load from wind or accidental impact?
RE: fb > Fcr ??? (Girder Stability)
...sounds like a homework problem.
RE: fb > Fcr ??? (Girder Stability)
RE: fb > Fcr ??? (Girder Stability)
Also Nutte i have checked it with the 13th edition as well it actually creates a larger difference between the 2.
RE: fb > Fcr ??? (Girder Stability)
RE: fb > Fcr ??? (Girder Stability)
RE: fb > Fcr ??? (Girder Stability)
RE: fb > Fcr ??? (Girder Stability)
RE: fb > Fcr ??? (Girder Stability)
Here's another thought: Can you put a temporary support 1 foot in from the centerline of bearings? This way your fb will equal your Fcr.
RE: fb > Fcr ??? (Girder Stability)
Yes bridge buster placing a temporary support slightly in will allow my fb = Fcr. Was checking if there was a solution i may have been over looking
The only other way I have thought of to come up with is to clamp w/ heavy duty C-clamps, a plate over the middle where the max moment happens or clamp a bar joist to top flange but that bar joist ends up being about 70', which equals a lot of clamps.
RE: fb > Fcr ??? (Girder Stability)
Often people assume an "X" ton crane can lift "X" tons no matter what....obviously not the case.
RE: fb > Fcr ??? (Girder Stability)
Other than that you could look at moving the supports in, as others have suggested, or providing a temporary king post if you have enough clearance.
RE: fb > Fcr ??? (Girder Stability)