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laterally bracing compression flange (short question) 4
- Thread starter weron4u
- Start date
weron4u,
To resist lateral torsional buckling, look at where the neutral axis is located and determine the location of the compression and tension zones. Theoretically, you would only need to extend bracing at a depth equal to the depth of the compression zone. The lateral support is only required to resist compression, not tension. However, keep in mind that the shapes of the compression zones are different for elastic and plastic analysis. The elastic is triangular while the plastic is rectangular since it assumes that each fiber is equally stressed.
Unless the cost would become a significant factor, I usually try to provide lateral support members with the same depth as the main member. Also for ease of bolting or welding, the deeper the member the better.
Good luck
To resist lateral torsional buckling, look at where the neutral axis is located and determine the location of the compression and tension zones. Theoretically, you would only need to extend bracing at a depth equal to the depth of the compression zone. The lateral support is only required to resist compression, not tension. However, keep in mind that the shapes of the compression zones are different for elastic and plastic analysis. The elastic is triangular while the plastic is rectangular since it assumes that each fiber is equally stressed.
Unless the cost would become a significant factor, I usually try to provide lateral support members with the same depth as the main member. Also for ease of bolting or welding, the deeper the member the better.
Good luck
- Thread starter
- #3
Sorry MotorCity but that made a whole lot of no sense to me. I have an S10x25.4 that spans 21.5ft and is lifting 5000lbs. It needs a lateral support at midspan to be capable of this while having a safety factor of 1.5 based off of Fb from AISC ASD. I will most likely use a double angle to provide this lateral support.
My question is, how far below the top(compression) flange of the S10x25.4 can I weld those double angles and still have them provide lateral support for the compression flange. Is there a rule of thumb? or should I try to weld the lat support directly to the compression flange?
My question is, how far below the top(compression) flange of the S10x25.4 can I weld those double angles and still have them provide lateral support for the compression flange. Is there a rule of thumb? or should I try to weld the lat support directly to the compression flange?
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- #4
DaveAtkins
Structural
Yura stated that a lateral support near the neutral axis of a section will not brace that section properly. I would weld a plate to the underside of the top flange, and to the web, and then bolt the double angles to this plate.
DaveAtkins
DaveAtkins
J1D
Structural
- Feb 22, 2004
- 259
- 0
- 0
weron4u,
Is this a mono-rail? Welding stiffener plates under the top flange (and to the web) is the way I'd use. The plate can be half height of the beam.
Lateral support at neutral axis location is not enough. The buckled shape of top flange is still whole length sway. That's why in some analysis softwares (such as STAAD), unsupported length of top flange needs to be specified in addition to lateral unsupported length.
Is this a mono-rail? Welding stiffener plates under the top flange (and to the web) is the way I'd use. The plate can be half height of the beam.
Lateral support at neutral axis location is not enough. The buckled shape of top flange is still whole length sway. That's why in some analysis softwares (such as STAAD), unsupported length of top flange needs to be specified in addition to lateral unsupported length.
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- #8
DaveAtkins - Yura also says that if you brace at the neutral axis AND resist torsional movement, you are almost as good as if you braced the compression flange translationally.
So dropping your brace down to the center of the member, and ensuring that you have adequate torsional restraint (stiffness) AND strength, you should be OK to count this as a brace.
If the brace is dropped down to the neutral axis, but has NO torsional restraint, I wouldn't count on it at all.
So dropping your brace down to the center of the member, and ensuring that you have adequate torsional restraint (stiffness) AND strength, you should be OK to count this as a brace.
If the brace is dropped down to the neutral axis, but has NO torsional restraint, I wouldn't count on it at all.
- Thread starter
- #9
Thank you all.
The most common practice I see for providing lateral support during erection is using a seated connection from the end of the lateral support to the web of the beam in question, with the depth of the connection being approximately 3/4 the web depth, which would provide resistance to lateral movement and torsional movement.
I also occasionally see beams that were field welded into place after erection and use the lateral support on either the top flange or directly underneath it. This is where my question came from.
Dave I'm going to follow your lead on this one with the plate. I like that idea.
Thank you all for your insight, Greatly appreciated,
weron4u
The most common practice I see for providing lateral support during erection is using a seated connection from the end of the lateral support to the web of the beam in question, with the depth of the connection being approximately 3/4 the web depth, which would provide resistance to lateral movement and torsional movement.
I also occasionally see beams that were field welded into place after erection and use the lateral support on either the top flange or directly underneath it. This is where my question came from.
Dave I'm going to follow your lead on this one with the plate. I like that idea.
Thank you all for your insight, Greatly appreciated,
weron4u
i have a thought, i think the effective distance for resisting LTB is the 1/6 of beam depth from the beam extreme compression side fibers
i got this thought from the value ot (Rt) stated in the AISC maual 9th edition which use the interia of compression flange + (1/6 of the web) about there vertical axe.
This value is used when i reduce bending strenght due to given limits of unbrced lenght.
i got this thought from the value ot (Rt) stated in the AISC maual 9th edition which use the interia of compression flange + (1/6 of the web) about there vertical axe.
This value is used when i reduce bending strenght due to given limits of unbrced lenght.
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- #12
If the beam were braced against lateral translation, which can be placed anywhere (top flange, bottom flange, web), then all that's left to resist is the torsional rotation. If the beam cross-section is strong enough to resist the torsion, then the lateral translation is the only thing to resist with bracing. Since the force on LTB is typically taken as 2% of the axial load, this shouldn't be too much to resist.
vmirat -
What I hear you saying is the following (please check me on this and confirm):
1. Place a translational brace on either the top, middle, or bottom of the beam.
2. Use Lb = spacing of this translational brace.
3. Ensure that the beam has torsional rigidity (how much?)
4. Design for Lb brace spacing.
I don't agree with this. If you have a beam with some level of torsional stiffness, the torsional stiffness will only be engaged if there is something external to the beam that can take this torsional force and transfer it to other elements of the structure (i.e. the beam can't brace itself).
What torsional couple are you using? You cannot use the translational brace in item 1 above - it is not affixed to the beam in a way to take moment - such as if you attached a lateral brace at the bottom tension flange.
If you use the ends of the beam, say at each column connection, you would now have Lb = span of the beam, not the spacing of the translational brace since it is a "pinned" connection at the tension flange.
What is necessary, is that whatever external brace you have attached to the beam be EITHER:
1. Torsionally/rigidly affixed to the beam anywhere on the beam, with adequate stiffness and strength (granted that the degree of stiffness and strength is small), or
2. Translationally affixed to the beam at the compression flange.
What I hear you saying is the following (please check me on this and confirm):
1. Place a translational brace on either the top, middle, or bottom of the beam.
2. Use Lb = spacing of this translational brace.
3. Ensure that the beam has torsional rigidity (how much?)
4. Design for Lb brace spacing.
I don't agree with this. If you have a beam with some level of torsional stiffness, the torsional stiffness will only be engaged if there is something external to the beam that can take this torsional force and transfer it to other elements of the structure (i.e. the beam can't brace itself).
What torsional couple are you using? You cannot use the translational brace in item 1 above - it is not affixed to the beam in a way to take moment - such as if you attached a lateral brace at the bottom tension flange.
If you use the ends of the beam, say at each column connection, you would now have Lb = span of the beam, not the spacing of the translational brace since it is a "pinned" connection at the tension flange.
What is necessary, is that whatever external brace you have attached to the beam be EITHER:
1. Torsionally/rigidly affixed to the beam anywhere on the beam, with adequate stiffness and strength (granted that the degree of stiffness and strength is small), or
2. Translationally affixed to the beam at the compression flange.
Ok I'll give my 2 pennys worth.
If we're talking about a lateral brace, usually you place it central to a beam to halve it's effective length. There's a good chance that your max moment and zero shear are at the centre?
British standards say that your lateral brace has to resist 2.5% of the compressive force in your flange. With the web effectively redundant you can look at the restraint force and see if it can be resisted in a punching shear senario. In a similar way that you look at tying forces in beam to column web connections (load dispersal and plasic hinge mechanism formation with associated capacity).
That's doing it the hard way. I'd simply have a profiled stiffener between the flanges and connect my bracing to that.
If we're talking about a lateral brace, usually you place it central to a beam to halve it's effective length. There's a good chance that your max moment and zero shear are at the centre?
British standards say that your lateral brace has to resist 2.5% of the compressive force in your flange. With the web effectively redundant you can look at the restraint force and see if it can be resisted in a punching shear senario. In a similar way that you look at tying forces in beam to column web connections (load dispersal and plasic hinge mechanism formation with associated capacity).
That's doing it the hard way. I'd simply have a profiled stiffener between the flanges and connect my bracing to that.
JAE,
I agree that lateral bracing at the top flange is the best. It prevents both rotation and translation. However, I assumed that there were extenuating circumstances that prevent attachment to the top flange. The torsional stress would transfer to whatever supports the ends of the beam. I would use the full 2% axial load that is used to size the lateral brace and apply it to the top flange at each lateral brace. The moment arm would be based on the location of the lateral brace. If the brace were placed at the bottom flange, then the moment arm would be the full depth of the beam.
It is assumed from weron4u's original question, that the beam in question is a wide flange steel section. Due to the complex shape of a wide flange, there are warping stresses induced in a beam in addition to the torsion and lateral bending. The actual stress effects and beam strength could only be determined by finite element analysis. Perhaps this question would be easier to consider from a theoretical standpoint if the beam in question were a wooden 2x12. It has already been stated that Joseph Yura's research has indicated that bracing at the neutral axis is not effective, apparently due to the torsional factor. We know that bracing at the top of the compression zone is the most effective in that it prevents torsion from occurring. Any brace point below the compression edge would allow torsion. Again, I would apply the 2% lateral load to the top edge and use the distance from the top edge to the brace point as the moment arm. The design limit would be the capacity of the end restraint of the beam to resist the torsional stress.
I agree that lateral bracing at the top flange is the best. It prevents both rotation and translation. However, I assumed that there were extenuating circumstances that prevent attachment to the top flange. The torsional stress would transfer to whatever supports the ends of the beam. I would use the full 2% axial load that is used to size the lateral brace and apply it to the top flange at each lateral brace. The moment arm would be based on the location of the lateral brace. If the brace were placed at the bottom flange, then the moment arm would be the full depth of the beam.
It is assumed from weron4u's original question, that the beam in question is a wide flange steel section. Due to the complex shape of a wide flange, there are warping stresses induced in a beam in addition to the torsion and lateral bending. The actual stress effects and beam strength could only be determined by finite element analysis. Perhaps this question would be easier to consider from a theoretical standpoint if the beam in question were a wooden 2x12. It has already been stated that Joseph Yura's research has indicated that bracing at the neutral axis is not effective, apparently due to the torsional factor. We know that bracing at the top of the compression zone is the most effective in that it prevents torsion from occurring. Any brace point below the compression edge would allow torsion. Again, I would apply the 2% lateral load to the top edge and use the distance from the top edge to the brace point as the moment arm. The design limit would be the capacity of the end restraint of the beam to resist the torsional stress.
vmirat - so in your concepts here, you are assuming that the lateral brace that is attached to a bottom flange is fixed so as to take the lateral torsional twisting moment?
I don't have a problem with that...just was concerned that you were giving an impression that the lateral bottom flange brace was just a lateral, translational brace (no fixity to the beam against torsion in the beam) and you were advocating using Lb = brace spacing.
I don't have a problem with that...just was concerned that you were giving an impression that the lateral bottom flange brace was just a lateral, translational brace (no fixity to the beam against torsion in the beam) and you were advocating using Lb = brace spacing.
JAE,
I AM advocating using the brace for only lateral translation and relying on the torsional resistance of the beam to resist twisting. Again, I agree this is not the best way to handle this situation. I was trying to provide weron4u a possible solution to attaching a lateral brace below the top flange that has no resistance to torsion. I suppose this would be no different than a beam which is loaded eccentrically, inducing a torsional stress in the beam.
I AM advocating using the brace for only lateral translation and relying on the torsional resistance of the beam to resist twisting. Again, I agree this is not the best way to handle this situation. I was trying to provide weron4u a possible solution to attaching a lateral brace below the top flange that has no resistance to torsion. I suppose this would be no different than a beam which is loaded eccentrically, inducing a torsional stress in the beam.
With that, I don't see how the tension flange brace is doing anything at all. The beam can quite nicely rotate outward, buckling at a loading based on Lb = span of beam.
What weron4u should do if top flange bracing is not permitted, is to torsionally brace the beam by rigidly attaching the brace to the beam with a vertical web stiffener and ensuring that the brace stiffness is adequate.
You cannot adequately brace a beam by using a "loose" pinned tension flange brace. It is too close to the center of twist which is just beyond the face of the tension flange.
What weron4u should do if top flange bracing is not permitted, is to torsionally brace the beam by rigidly attaching the brace to the beam with a vertical web stiffener and ensuring that the brace stiffness is adequate.
You cannot adequately brace a beam by using a "loose" pinned tension flange brace. It is too close to the center of twist which is just beyond the face of the tension flange.
In addition - Yura has put on numerous seminars dealing exactly with this subject. They have researched numerous types and modes of bracing - relative lateral, discrete lateral, continuous lateral, lean-on lateral, discrete torsional and continuous torsional.
All of these attempt to prohibit the beam from rotating about the center of twist, located just beyond the face of the tension flange. The center of twist, being where it is, initiates translation of the compression flange and rotation of the whole section. Little, if any, translation occurs at the tensino flange so any translational brace does nothing to reduce the Lb. This has been proven in research.
So you can effectively brace the beam (or in other words, reduce the effective Lb) by prohibiting translation of the compression flange or prohibiting rotation of the entire section.
You also cannot add a tension brace, and somehow count on a WF beam's torsional stiffness (which is very small) and somehow realize a reduced Lb less than the full span of the beam. So if the Lb = span of the beam, and the tension flange does nothing, why even bother with the tension flange.
All of these attempt to prohibit the beam from rotating about the center of twist, located just beyond the face of the tension flange. The center of twist, being where it is, initiates translation of the compression flange and rotation of the whole section. Little, if any, translation occurs at the tensino flange so any translational brace does nothing to reduce the Lb. This has been proven in research.
So you can effectively brace the beam (or in other words, reduce the effective Lb) by prohibiting translation of the compression flange or prohibiting rotation of the entire section.
You also cannot add a tension brace, and somehow count on a WF beam's torsional stiffness (which is very small) and somehow realize a reduced Lb less than the full span of the beam. So if the Lb = span of the beam, and the tension flange does nothing, why even bother with the tension flange.
- Thread starter
- #20
JAE
It sounds to me that I could possibly have the top flange of an S10x25.4 bolted to the bottom flange of two W14x30 beams, and add intermediate stiffeners to the W14x30 beams directly above the bolted connections, and have that translationally and torsionally restrain the W14x30 and the S10x25.4.
The W14x30 beams are restrained at the ends, where the S10x25.4 beams are only supported by the W14x30's. This may mean that the W14x30 beams are not translationally restrained. Does this sound right, or is translational movement of the tension flange not probable, to where the only concern is the compression flange?
Thank you all for your posts.
It sounds to me that I could possibly have the top flange of an S10x25.4 bolted to the bottom flange of two W14x30 beams, and add intermediate stiffeners to the W14x30 beams directly above the bolted connections, and have that translationally and torsionally restrain the W14x30 and the S10x25.4.
The W14x30 beams are restrained at the ends, where the S10x25.4 beams are only supported by the W14x30's. This may mean that the W14x30 beams are not translationally restrained. Does this sound right, or is translational movement of the tension flange not probable, to where the only concern is the compression flange?
Thank you all for your posts.
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