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!
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Cantilever Beam Question
- Thread starter mjordao
- Start date
SEIT,
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
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
slickdeals
Structural
What have we concluded? It looks like we are still debating......
I have emailed AISC for their opinion on this. Let's see.
We are Virginia Tech
Go HOKIES
I have emailed AISC for their opinion on this. Let's see.
We are Virginia Tech
Go HOKIES
For a cantilevered beam laterally unsupported at the free end, MSC has published the following article:
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
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
Oops! I gave the wrong reference in the last post...should have been:
BA
BA
AISC 360-05 6.3 pg 193 "...In members subject to double curvature bending, the inflection point shall not be considered a brace point...".
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.
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.
The portion of beam extending from inflection point to the tip of cantilever is in a similar state of stress as a double cantilever with ends laterally unbraced.
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
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
I just come back to the idea of the unbraced length being the length between physical brace points. You can modify the moment capacity for a given unbraced length with Cb, but the unbraced length is what it is - the distance between physical brace points for the given flange. If that flange happens to switch from compression to tension, it doesn't matter, the unbraced length is still the length between physical brace points.
Consider Beam a-b-c with span a-b of length L and cantilever b-c of length C. The beam is free to rotate about a vertical axis at points a, b and c. The only load acting on the beam is a concentrated load P applied at the neutral axis at point c. Dead load of the beam is neglected.
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
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
slickdeals
Structural
This is a related question regarding beam design. Please see attachment.
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?
We are Virginia Tech
Go HOKIES
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?
We are Virginia Tech
Go HOKIES
I think you need stiffeners in the beam over column and then in the channel over beam to have the columns be pinned-pinned - assuming the grating can act as a diaphragm and is ultimately braced to something.
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.
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.
slickdeals
Structural
I believe that the metal grating is acting diaphragm (will be certain after going there). It is a convoluted load path, but I am inclined to think that the beam bearing on the column is somehow braced against rotation.
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.
We are Virginia Tech
Go HOKIES
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.
We are Virginia Tech
Go HOKIES
I would add blocking between the grating and top of beam between every third or fourth joist to prevent the joists from racking.
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
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
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