Cantilever Beam Question
Cantilever Beam Question
(OP)
I'm working on an existing steel building where the owner wants to add a new generator on the roof and I'm checking the existing girders. There are 6 bays and the steel girders cantilever past the columns and are spliced with intermediate girders. I can't find any information in text books about this type of design. I'm having trouble with the negative moment at the columns and the lateral torsional buckling. I'm assuming an unbraced length for the bottom flange of the entire distance between the columns. Does anyone know if I can take instead the length from the support column to the inflection point in the moment diagram? Are there any publications out there that deal with this situation? Thanks!






RE: Cantilever Beam Question
RE: Cantilever Beam Question
RE: Cantilever Beam Question
RE: Cantilever Beam Question
As Structural EIT said, you need to brace the bottom flange at column else capacity significantly affected.
Is there an intersecting beam to brace bottom flange at columns? Do record drawings show? Or have you made a field visit?
If not, can add diagonal kicker brace to brace bottom flange at column.
Be careful when you say 'spliced' intermediate girders - these are typically just shear connections, so the intermediate girder 'spans' between cantilevered girder ends and the cantilevered girder must be designed to support this concentrated load (as well as other design loads)
RE: Cantilever Beam Question
RE: Cantilever Beam Question
This document was published by the Canadian Institute of Steel Construction.
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RE: Cantilever Beam Question
RE: Cantilever Beam Question
I only posted the document because it goes over the system and practices for someone who is not familiar with it. It looks like you already might be.
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RE: Cantilever Beam Question
"The column braces the bottom of the girder...."
I question how that can be possible? Is the column a cantilever? K= 2.1? I suspect the bottom of the girder is actually unbraced at the column unless you have an extended joist bottom chord tied into it. Then it is the joist that is bracing the girder and column.
Just remember, you cannot have it both ways.....the column bracing the girder and the girder bracing the column.
RE: Cantilever Beam Question
I believe that if you have full depth stiffeners in the beam (over the column, like you're supposed to), and the beam is adequately attached to a column cap plate, then I think they do brace each other.
RE: Cantilever Beam Question
RE: Cantilever Beam Question
The column is usually braced by the roof system, i.e. the joists. It is braced at roof level, at the top flange of the beam. It is not braced at the bottom flange of the beam. The bottom flange of the beam is braced by the column if the column is made continuous through the beam and can resist, say 2% of the compression in the beam flange without excessive deflection as stipulated in most codes.
The unbraced length of the beam is the distance from the end of the cantilever to the point of inflection in the span. That is the length of bottom flange in compression. I have presented this argument previously and I am aware that not everyone agrees, but I have not yet heard any reasonable arguments in opposition.
To me, the matter is clear, but I am perfectly prepared to hear arguments to the contrary.
BA
RE: Cantilever Beam Question
However, I prefer to assume the bottom flange of the beam is unbraced over its full length (column to column), and include the Cb factor.
DaveAtkins
RE: Cantilever Beam Question
RE: Cantilever Beam Question
Yes, it is because the beam is not in compression beyond that point.
Consider a beam of length L supported at its midpoint, i.e. a double cantilever. Neither end is laterally braced, but the effective length for buckling of the bottom flange is L.
BA
RE: Cantilever Beam Question
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RE: Cantilever Beam Question
BA
RE: Cantilever Beam Question
I would use this analogy - at the end of a simply supported beam there is zero compression (similar to the inflection point of a beam in reverse curvature), but AISC still requires the ends to be brace against LTB, because that is what the equations in AISC are based on. If they are not braced at the ends (points of zero compression), then the equations are not valid. I don't think that the point of zero moment somewhere other than the end of the beam changes that logic.
RE: Cantilever Beam Question
RE: Cantilever Beam Question
I find myself agreeing with you on most issues, but we have locked horns on this one before. If designing a new structure, I would go along with your way of thinking, i.e. I would provide lateral bracing at the end of the cantilever, at the column and at the point of inflection, simply in order to conform to the opinion of the majority of engineers. Then I would use the distance between braces as the unbraced length of the compression flange.
You stated:
I almost agree with your analogy, i.e. a simple beam with a point load needs to be braced at the ends if it is not otherwise braced. If it is braced at the point of load application, the ends need not be braced. A simple lifting beam with central support and a point load each end is a clear example of that.
In your next post, you state:
I cannot speak for the rest of the engineering community, but that is precisely what I would assume. Why would you believe otherwise?
So far, we have not talked about the height of load above or below the neutral axis. If a point load on a simple beam is applied above the n.a., there is a magnifying effect on lateral buckling. If it is below, there is a stabilizing effect. For the sake of this discussion, let us assume that all loads and reactions are acting at the centroid of the section.
BA
RE: Cantilever Beam Question
I have emailed AISC for their opinion on this. Let's see.
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RE: Cantilever Beam Question
http://www.modernsteel.com/
If the beam in this thread is prevented from rotating about its horizontal axis at the column, how is it different than a doubly cantilevered beam, each laterally unsupported? One from column to tip, the other from column to inflection point.
What am I missing here?
BA
RE: Cantilever Beam Question
ht
BA
RE: Cantilever Beam Question
RE: Cantilever Beam Question
If the "unbraced length" is considered to be the length between "braced points" and the inflection point is NOT a "braced point" then the unbraced length is the length between the actual physical points where bracing intercepts the beam. It seems to me that the inflection point is not part of the argument really at all for beams in double curvature.
A spreader beam or double cantilever is not subjected to double curvature bending.
RE: Cantilever Beam Question
If the cantilever length is C and the inflection point is 2C away from the column, the buckling length of the compression flange is C + 2C = 3C. The length of beam beyond the inflection point is irrelevant.
BA
RE: Cantilever Beam Question
shotthought before taking off on holiday, if you substitute the inflection point with an actual pin, then you have a double cantilever, n'est ce pas?BA
RE: Cantilever Beam Question
RE: Cantilever Beam Question
RE: Cantilever Beam Question
I agree with 100%.
RE: Cantilever Beam Question
RE: Cantilever Beam Question
Braces present = BRACED
No Braces present = UNBRACED
RE: Cantilever Beam Question
Most of us now accept that this is not correct for current design methods.
RE: Cantilever Beam Question
Condition A - Top and bottom flanges are laterally braced at a, b and c.
If C = L, the beam buckles in an "S" shape of wavelength L (or C).
If C << L, the beam approaches fixity at point b and the buckling length is less that L, maybe about 0.75*L.
Condition B - Top and bottom flanges are laterally braced at a and b but not at c. The unbraced length of the span is L, but the unbraced length of the cantilever is undefined.
When P is gradually increased until buckling, the compression flange of the beam buckles in a continuous curve from a to b to c. The buckling length of the beam in Condition B is greater than span L. I believe it should be taken as L + C. To assume the buckling length is the braced length L is to err on the unsafe side.
BA
RE: Cantilever Beam Question
A mezzanine floor was built without permit which came to light during a random inspection. The mezzanine is used for light storage (75 psf) which is supported by metal grating.
The framing system consists of pipe columns that support W6 beams, which in turn support C-shaped joists and the metal grating.
My question is as follows:
1. AISC flexure equations require that the beam be prevented from rotation at the supports. It appears that a welded connection between the top flange and the channel and its subsequent connection to the metal grating might prevent such a rotation.
I haven't visited the job site yet, but it appears that the metal grating is connected to building columns that extend to support the roof and may provide lateral stability. I am however worried about sway type behavior if the grating is not connected to a lateral brace.
2. If it agreed that the beam is rotationally braced and since there are no stiffeners at the top of the column , will the column be designed as a column with an effective length of 2? Pinned-free to rotate?
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RE: Cantilever Beam Question
I don't think I would count on the channel bottom flange to beam top flange connection only to brace against twist. Again, if the grating is a diaphragm and is attached to the channels, then I think it's ok, but not the bottom flange connection by itself.
RE: Cantilever Beam Question
But that still leaves the question regarding the column. For the column to be pinned-pinned, I think that the W6 beam's web will have to be able to resist 2% of the compression in the column without significant lateral displacement. Right?
The columns are 3.5" O.D. pipes, they are pinned at their base.
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RE: Cantilever Beam Question
RE: Cantilever Beam Question
Then consider the column as a member of variable EI, pinned at the base and the underside of channels. If the web of the beam is too flexible, add stiffeners on one side of each beam.
BA
RE: Cantilever Beam Question
Do you have any comment on
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RE: Cantilever Beam Question
Sounds reasonable to me. It is difficult to say how much of the beam web participates in the column extension through the beam. Maybe 4" is about right. If in doubt, I would add stiffeners.
BA
RE: Cantilever Beam Question
RE: Cantilever Beam Question
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RE: Cantilever Beam Question
Must read past that.
Sorry Slick.