I am being asked to design a custom truss with a completely unbraced bottom flange. The dead load on the truss is so light that there is a net uplift with wind load. This puts the bottom chord in compression. I don't like this condition, but I would like to have more than "I don't like it" for a answer, and if there is a safe way to design for this condition I would like to know how. I checked the bottom chord as a unbraced column the length of the joist and it worked fine. Also, KL/r would be less than 200 if K is 1.0. Is there some reason that K would be more than 1.0? Is there some reason other than structural stability to brace the bottom chord? The truss is basically a bar joist made out of tube steel. I have two competing ideas in my mind. The one is bar joists where any joist with uplift always has uplift bridging. The other is a crane beam where the compression flange has no bracing. Is there a way to calculate a stiffness that would make compression chord bracing unnecessary?
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Unbraced compression truss chord 2
- Thread starter WCW
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I found this reference elsewhere on the web. While it doesn't provide the answer to this post, they describe the same question everyone here is wrestling with (page 20 1st paragraph for those skimming the report).
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- #22
wcw-
That's the same issue as the pretensioned column. The applied load is via a tension force which is inherently stable.
Try THIS, have two infinitely rigid concrete walls 10' apart (horizontally), now place a horizontal HSS in that 10' space. Let's say that the HSS is placed on small ledges that support it vertically and both ends have rollers that allow it to slide along the length of the wall (the small ledge keeps it in place along the height of the wall) - this is now braced in one plane and not in the other. Now, let's say the walls start to get closer, thereby compressing the HSS. Lets also say that the installer put the HSS in so that it is at an angle of 89.9 degrees to the walls, and not 90. Once the HSS feels any compression, it will rotate as a rigid body (not take a buckling mode) and will just become unstable.
Do you see that? Do you agree? I believe there is a difference between an externally applied load and an internal load. The two examples that seem to say it's ok (the prestressed column and the bow) have no external load or reaction to be resisted. The forces are completely internal from the prestressing of the cable or bowstring. This is why you can have internal forces in a prestressed beam but not have any external reaction due to the prestressing, because the forces are all internal, not external.
That's the same issue as the pretensioned column. The applied load is via a tension force which is inherently stable.
Try THIS, have two infinitely rigid concrete walls 10' apart (horizontally), now place a horizontal HSS in that 10' space. Let's say that the HSS is placed on small ledges that support it vertically and both ends have rollers that allow it to slide along the length of the wall (the small ledge keeps it in place along the height of the wall) - this is now braced in one plane and not in the other. Now, let's say the walls start to get closer, thereby compressing the HSS. Lets also say that the installer put the HSS in so that it is at an angle of 89.9 degrees to the walls, and not 90. Once the HSS feels any compression, it will rotate as a rigid body (not take a buckling mode) and will just become unstable.
Do you see that? Do you agree? I believe there is a difference between an externally applied load and an internal load. The two examples that seem to say it's ok (the prestressed column and the bow) have no external load or reaction to be resisted. The forces are completely internal from the prestressing of the cable or bowstring. This is why you can have internal forces in a prestressed beam but not have any external reaction due to the prestressing, because the forces are all internal, not external.
WCW,
Interesting discussion. I agree the last panel point of your truss bottom chord needs to be braced. Suggest thinking about it as like a "pony truss" in a road bridge. That is, a truss with its top, compression chord unbraced except by the truss webs cantilevering from the bridge deck members.
Interesting discussion. I agree the last panel point of your truss bottom chord needs to be braced. Suggest thinking about it as like a "pony truss" in a road bridge. That is, a truss with its top, compression chord unbraced except by the truss webs cantilevering from the bridge deck members.
hokie66,
Excellent suggestion! This avoids having unsightly bracing on the bottom chord. Each end of the bottom chord would be laterally braced by a vertical member fixed at the top to a rigid member spanning between joists.
Best regards,
BA
Excellent suggestion! This avoids having unsightly bracing on the bottom chord. Each end of the bottom chord would be laterally braced by a vertical member fixed at the top to a rigid member spanning between joists.
Best regards,
BA
Without some form of bridging at or near the ends of the joists, the structure is unstable under uplift loading. It is unstable for reasons other than stated thus far.
The spreader beam shown in the attached photo is in compression. Neither end is laterally braced, yet the effective length is the length of the beam. Like the prestressed HSS mentioned in an earlier post, the compressive forces applied to each end are precisely aligned. But that is also true for the bottom chords of the joists in question.
Under uplift loading, the end diagonals, acting in compression transfer all of the uplift to the supports. Assuming these members to be pin-ended, there is no resistance to overturning. It is the same situation as a bottom bearing joist under gravity load. There is nothing to prevent the joists from racking.
So we all seem to be in agreement that bridging is required at each end of the joist but for different reasons.
Best regards,
BA
The spreader beam shown in the attached photo is in compression. Neither end is laterally braced, yet the effective length is the length of the beam. Like the prestressed HSS mentioned in an earlier post, the compressive forces applied to each end are precisely aligned. But that is also true for the bottom chords of the joists in question.
Under uplift loading, the end diagonals, acting in compression transfer all of the uplift to the supports. Assuming these members to be pin-ended, there is no resistance to overturning. It is the same situation as a bottom bearing joist under gravity load. There is nothing to prevent the joists from racking.
So we all seem to be in agreement that bridging is required at each end of the joist but for different reasons.
Best regards,
BA
BA:
Another example like your spreader bar is the boom on a jib crane, the far end is laterally unbraced and the other end at the column is pinned. The k for the boom in compression is 1.
It just goes to prove that the things we build don't always fit neat little engineering models but they work....
Another example like your spreader bar is the boom on a jib crane, the far end is laterally unbraced and the other end at the column is pinned. The k for the boom in compression is 1.
It just goes to prove that the things we build don't always fit neat little engineering models but they work....
I've had this paper for quite a while now, but just recently had the opportunity to read it. I was quite pleasantly surprised to find an example regarding truss bottom chord bracing. Using the techniques in this paper, I have come to the conclusion that bottom chord bracing is required to prevent buckling (not necessarily of just the compression diagonal, but LTB of the section), but have also convinced myself that the required bracing is minimal at most.
I would be interested in hearing others' opionions if you get a chance to read the paper and play around with some examples.
I would be interested in hearing others' opionions if you get a chance to read the paper and play around with some examples.
On a related note, if you would like other assorted papers from Shankar (including a clean version of the paper StructuralEIT uploaded) you can find them here:
A space frame like Hokie66 suggested should work well, and it's depth could likely be reduced relative to an open web joist. Architects generally like them for their "uniqueness" in open ceiling applications. Cost might be offset by size and spacing changes.
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Refer to attached paper by James Fisher. This paper also talks about the effect of lateral buckling of bottom chord on compression webs.
I am thinking as the BM changes from center to end the compressive force in bottom chord also changes with maximum at center (assuming ss) and zero at ends. So this case is not exactly similar to a column in uniform compression. What are your thoughts about zero force at ends does that mean this "free" end does not displace lateraly?
In my opinion the simplest solution, if the architect is opposed to continuous bridging, is as suggested by StructuralEIT (5th post from top) i.e. to extend the ends to supporting column or wall with a sloted connection to avoid tension transfer (similar to joist bottom chord extention at column osha requirement).
Another solution would be to provide kickers extending from bottom end panel point to top of adjacent truss panel point.
I am thinking as the BM changes from center to end the compressive force in bottom chord also changes with maximum at center (assuming ss) and zero at ends. So this case is not exactly similar to a column in uniform compression. What are your thoughts about zero force at ends does that mean this "free" end does not displace lateraly?
In my opinion the simplest solution, if the architect is opposed to continuous bridging, is as suggested by StructuralEIT (5th post from top) i.e. to extend the ends to supporting column or wall with a sloted connection to avoid tension transfer (similar to joist bottom chord extention at column osha requirement).
Another solution would be to provide kickers extending from bottom end panel point to top of adjacent truss panel point.
StructuralEIT,
The example of the truss in the Nair paper is not exactly the same scenario as we have here. His truss is continuous with the columns, developing compression in members L1L2 and L6L7 under gravity load applied at the top of the truss.
In the case we have, the load is upward. Members L1L2 and L6L7 are unstressed axially. The remainder of the bottom chord is in compression throughout, increasing toward midspan. If you assume, as Nair did, that all truss connections are pinned in both directions, then we require lateral bracing at every panel point on the bottom chord.
The best locations for bracing in the problem at hand is at the bottom of the end diagonals. Then the bottom chord does not have to be extended to the support. This, of course, would not please the architect.
Whether or not more bracing is required between these points is a question we cannot answer without knowing the geometry and the loading. If additional bracing is not required, then the bottom chord must be continuous between braced points (no pins) and we would be relying on the buckling capacity of the bottom chord over the length between braced points.
Best regards,
BA
The example of the truss in the Nair paper is not exactly the same scenario as we have here. His truss is continuous with the columns, developing compression in members L1L2 and L6L7 under gravity load applied at the top of the truss.
In the case we have, the load is upward. Members L1L2 and L6L7 are unstressed axially. The remainder of the bottom chord is in compression throughout, increasing toward midspan. If you assume, as Nair did, that all truss connections are pinned in both directions, then we require lateral bracing at every panel point on the bottom chord.
The best locations for bracing in the problem at hand is at the bottom of the end diagonals. Then the bottom chord does not have to be extended to the support. This, of course, would not please the architect.
Whether or not more bracing is required between these points is a question we cannot answer without knowing the geometry and the loading. If additional bracing is not required, then the bottom chord must be continuous between braced points (no pins) and we would be relying on the buckling capacity of the bottom chord over the length between braced points.
Best regards,
BA
I agree with Fisher's opening statement on tension (bottom)chord bracing requirement: " 1. To control the slenderness ratio (L/r)...3.., to brace the bottom chord for uplift (as it is under compression with inherent lateral instability)..". (my own words)
However, he admits ".., in most ordinary situations, adequate bracing has been provided BY DEFAULT, because designers have followed bridging and bracing requirement based on the slenderness criteria of AISC and SJI, which may inherently provide the necessary bracing strength and stiffness."
What is "BY DEFAULT"? My take is follow the traditional design approaches to design the compression members with K = 1.0 for all truss members, the result is a set of well/properly-sized members with adequate strength in the plan of loading. Then apply resulting compressive forces on the full length chord (again, with K=1.0) to ensure it will yield prior to buckle under side sway mode. The longer the span, the lower the allowable compressive stress, thus, it often results in adding braces to reduce the unbraced length. The "DEFAULT", therefore is to simply keep stress (compression) low in the compression members, the INHERENT stiffness will likely to prevent catastrophic failure mode -buckling.
After all said, as Fisher pointed out, K maybe greater than 1.0. Given consideration to side sway mechanism, it can be 1.2, 2.0..., the result (with larger K) is bigger member size, which is not necessary, nor economical. So, for practical reasons, I stick to K=1.0 for designing truss members. Fisher's method provides excellent means to check the final design to ensure laterl stability.
Thanks kbbandw for providing the excellent reference.
However, he admits ".., in most ordinary situations, adequate bracing has been provided BY DEFAULT, because designers have followed bridging and bracing requirement based on the slenderness criteria of AISC and SJI, which may inherently provide the necessary bracing strength and stiffness."
What is "BY DEFAULT"? My take is follow the traditional design approaches to design the compression members with K = 1.0 for all truss members, the result is a set of well/properly-sized members with adequate strength in the plan of loading. Then apply resulting compressive forces on the full length chord (again, with K=1.0) to ensure it will yield prior to buckle under side sway mode. The longer the span, the lower the allowable compressive stress, thus, it often results in adding braces to reduce the unbraced length. The "DEFAULT", therefore is to simply keep stress (compression) low in the compression members, the INHERENT stiffness will likely to prevent catastrophic failure mode -buckling.
After all said, as Fisher pointed out, K maybe greater than 1.0. Given consideration to side sway mechanism, it can be 1.2, 2.0..., the result (with larger K) is bigger member size, which is not necessary, nor economical. So, for practical reasons, I stick to K=1.0 for designing truss members. Fisher's method provides excellent means to check the final design to ensure laterl stability.
Thanks kbbandw for providing the excellent reference.
I am not familiar with AISC or SJI requirements for joist design. They are probably similar to the CISC requirements which states that the radius of gyration of the tension chord shall not be less than L/240.
Fisher is saying that, in most situations, adequate bracing has been provided "by default".
Default is defined as:
failure to perform a duty;
failure to pay on time;
failure to appear in court.
I would interpret this to mean that, without performing the necessary calculations (failure to perform a duty), most engineer's designs will work satisfactorily because they followed the simple rule of L/240 for bracing spacing on the bottom chord. Perhaps we should all take a closer look at this situation in the future.
In my opinion, open web steel joists should always be braced at the junction of the end diagonal and the bottom chord and at maximum L/240 beyond. Otherwise, the joists may be unstable. This is a tough message to get across to the joist suppliers who are reluctant to comply.
Best regards,
BA
Fisher is saying that, in most situations, adequate bracing has been provided "by default".
Default is defined as:
failure to perform a duty;
failure to pay on time;
failure to appear in court.
I would interpret this to mean that, without performing the necessary calculations (failure to perform a duty), most engineer's designs will work satisfactorily because they followed the simple rule of L/240 for bracing spacing on the bottom chord. Perhaps we should all take a closer look at this situation in the future.
In my opinion, open web steel joists should always be braced at the junction of the end diagonal and the bottom chord and at maximum L/240 beyond. Otherwise, the joists may be unstable. This is a tough message to get across to the joist suppliers who are reluctant to comply.
Best regards,
BA
The OP asked in the begining: "...if K is 1.0. Is there some reason that K would be more than 1.0? Is there some reason other than structural stability to brace the bottom chord?)
The questions have answered by Shankar & Fisher, except K for truss design - Fisher implies 1 < K < ?, Shanker explicitly indicates K (conservatively) = 1.
Practically, what K value would you suggest in the begining of a truss design? Or would you design the bracing prior to the truss so that the K is exact?
The questions have answered by Shankar & Fisher, except K for truss design - Fisher implies 1 < K < ?, Shanker explicitly indicates K (conservatively) = 1.
Practically, what K value would you suggest in the begining of a truss design? Or would you design the bracing prior to the truss so that the K is exact?
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