Web Sidesway Buckling from Column Reactions on Continous Beam
Web Sidesway Buckling from Column Reactions on Continous Beam
(OP)
I am designing a 30 ft long (3) span continuous steel beam for a residential basement. The floor joist system provides continuous support for the top flange. The beam is bolted to the top bearing plates of (2) adjustable steel columns spaced approximately 10' apart. The column reaction creates a concentrated load acting on the bottom flange of the beam. This load must be analyzed for concentrated loads acting on the Flanges and Web as described in section J10 of the AISC Steel Construction Manual (13th Edition). Using equation (J10-7), I want to design the lightest possible beam for web sidesway buckling without using a web stiffener.
My question is what should I use for "l"(the largest unbraced length along either flange at the point of load)---
What is the proper choice for the unbraced length???
(1) The entire length of the beam – ~30ft? (I am assuming that the columns do not provide lateral bracing), or
(2) The maximum span between columns and/or foundation walls --- ~10ft?,
or
(3) The distance along the bottom flange where the moment is negative ( measured from each column location to the point of zero moment)--- ~5' for the fully loaded condition; ~12.5 when the live load is removed from the center span? (Since the floor system fully braces the top flange which has a positive moment, we only need to consider the unbraced portion of the bottom flange subjected to a negative moment)
My question is what should I use for "l"(the largest unbraced length along either flange at the point of load)---
What is the proper choice for the unbraced length???
(1) The entire length of the beam – ~30ft? (I am assuming that the columns do not provide lateral bracing), or
(2) The maximum span between columns and/or foundation walls --- ~10ft?,
or
(3) The distance along the bottom flange where the moment is negative ( measured from each column location to the point of zero moment)--- ~5' for the fully loaded condition; ~12.5 when the live load is removed from the center span? (Since the floor system fully braces the top flange which has a positive moment, we only need to consider the unbraced portion of the bottom flange subjected to a negative moment)






RE: Web Sidesway Buckling from Column Reactions on Continous Beam
As for the value to use for "l", my interpretation -- based on diagram C-J10.2 given in the commentary - is that you should use the full length of the beam (~30).
Based on the commentary info, it sounds as though adding stiffeners wouldn't improve matters anyhow.
I have a few related questions of my own:
1) In kar108's case, there are two point loads (reactions) causing sway buckling withing one length of unbraced bottom flange. Does anything additional need to be done to account for the fact that two loads may now contribute to the sway buckling?
2) It has always seemed to me that, if one provided full height stiffeners and a stiff cap plate connection, you could consider the stiffener / web combination to simply be an extension of the column below. In this way you could claim to have braced the beam using the column. Is this rational? Does anyone know of a procedure for checking the cap plate connection in this scenario?
Usually, when I ask this question around the office I get a big fat "Just brace the damn flange".
Adam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
I know AISC provides a nice check for sidesway web buckling for these conditions but over the years, the main structural collapse mechanism that I've seen is simply beams without stiffners over the columns.
Due to tolerances in sweep and web plumbness, installation variability in column plumbness, and unbalanced loads from one side of the beam, you can get a pretty unstable condition without them.
Now to answer the first question - l is defined in section J10.4 as the "largest laterally unbraced length along either flange at the point of load."
So you would go essentially the full length of the beam in your case. One unstiffened column point doesn't brace another.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
In that case, the unbraced length is equal to the span plus the distance from the column to the point of inflection on each side of the span. If the point of inflection is at 2' each side of the column, the unbraced length is 10 + 2 + 2 = 14'.
Such a condition is not likely to occur in residential loading, but it is important to understand the principle involved.
Most codes recognize that, when the moment is variable across the unbraced length, another factor comes into play which acts in your favor. I believe AISC calls it Cb.
BA
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
If the beam was a 10' span with a 2' cantilever each end, each loaded with a concentrated load at the tip, then the unbraced length of the bottom flange would be 14'
BA
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
I disagree with BA, the deflection points are not points of restraint and your effective length is between points of restraint no points of zero moment.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
So, technically, since it is the rotation prevention by the floor what is accepted in many cases as the stabilizing mechanism against the lateral buckling of the referred bottom flange, and seeing the books contemplate the case, one may think it is acceptable practice to go for the beam without stiffeners, since, once the stabilizing rotational loads are passed to the floor, there must not be much difference between a rigdized and not rigidized support point, as long its capacity is proven.
And so one might start to think in what width of the web could be taken for such check. 18 times -9 times each side- the thickness of the beam web plus the width of some quite stiff plate supporting the beam?
Of course, all forces, vertical compression and bending stressses in plane and out of plane (bracing required) force need be taken in to account for the accepted collaborating segment of web. If the loads are small, it might work.
There is also the question on how the working notional panel is working. Given the small dimensions, maybe just a stresses approach might work; you may see the implications and assumptions that need to be taken to analyze one of such panels in whatever the way.
And as well, beyond the science of construction, the crystallized mode of it, code and pathology of construction experience. So one might elect not enter somewhat uncharted territory and put stiffeners.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
I would tend to say NO. You can certainly check to see if the cantilevered column is adequate (for both strength and stiffness) for bracing the bottom flange of the beam, but that detail, in and of itself, doesn't brace the bottom flange. I would just provide the stiffener and call it a day.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
The basic problem is that without the stiffener, you are counting on the beam to brace the column, so you can't count on the column to brace the beam. Unless, like I said, it is a cantilevered column (not a pinned base) and you actually check the strength and stiffness of the column to brace the beam.
Providing stiffeners takes care of both issues.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
I ask this question, because my own observations (and also responses from several steel suppliers) indicate that residential steel basement beams are typically not reinforced at column supports in our area (PA – low seismic risk).
Is this possibly because basement beams are subject to gravity loads only (no wind loads to create drift in members above and eccentric loading situations, and no seismic in the area). The beams are supported by relatively short steel columns (4"x11Ga), only 7' to 8' long that are embedded in 4" of concrete and either bolted or welded to the beam.?
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
kar-
Where in PA are you?
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
You could determine the maximum flange force as a result of the load. Apply two percent of the flange force laterally on the bottom flange at each column. If the beam web tributary to each column has the strength and the stiffness to resist the lateral force in accordance with the bracing requirements of the code, it should be adequate.
BA
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
I'm not sure that I agree with your assesment regarding stiffners / braces. AISC J10.3 commentary reads "If flange rotation is permitted at the loaded flange, neither stiffeners nor doubler plates are effective."
To me, this implies that a discrete brace is required, whether you choose to install stiffeners or not.
I agree, however, that if the top flange is not just braced laterally but torsionally as well, then the stiffeners would provide lateral restraint to the bottom chord. This would be reasonable if the top flange were embedded in concrete and connected to the slab with studs etc. The wood framing situation described here would not provide the necessary rotational restraint in my opionion. It would really only be the dead load on the beam preventing it from rotation. The same dead load, mind you, that may be encouraging the system to buckle in the first place.
If you're going to count on stiffeners for beam bracing, I think that it needs to be via the mechanism that I described in my post near the beginning of this thread.
I've never seen a failure of this sort of system. I have seen a couple that seemed to be barely holding on for dear life however. Always at the beam over column connection too.
I suspect there are two main reasons for the lack of failures in residential construction:
a) Steel members used in residential construction tend to be grossly oversized.
b) Usually residential steel members have stringent depth limitations on them (often why they're steel in the first place). This leads to stocky, shallow members that tend to be relatively stable.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
The cap plate of the column must be capable of resisting the moment resulting from the brace force acting horizontally at the bottom flange.
BA
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
In commercial construction I have read of a few failures contributed to a lack of stiffener over columns.
The one thing that has not been mentioned so far are all the second order effects that could result from the sidesway of the beam web. Short of an FEM analysis these would be hard to truly account for.
We need to remember that codes are only an approximation of reality and there are rare situations where merely following the code will not necessarily make a safe structure. This is the reason why we are taught to understand and not just to know.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
Unless, like I said, it is a cantilevered column (not a pinned base) and you actually check the strength and stiffness of the column to brace the beam.
Thanks. I have analyzed this column as a cantilever (fixed base and free top) and applied a lateral load at the end of 1% of the axial load (which is 310# based on a 31k axial load in one specific case). The column does not fail, but it deflects 1.044". Can I consider this as a lateral brace or is the deflection to high? If not, what degree of deflection would be a limit?
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
I don't think that you want to use your column as a cantilevered member to brace the bottom flange of your beam.
Effective bracing is about strength AND stiffness. And, in the majority of cases, stiffness is far more important.
While your column might be able to muster the 2% strength required to act as bracing, it is likely woefully inadequate with respect to stiffness. Your deflection estimate would seem to support this conclusion.
I believe that the 2% rule is intended to be an indirect way of providing brace stiffness. I suspect that the rule was also derived assuming discrete, strut type bracing. I'm not sure that ANYTHING (column, beam flange, slinkys)that can support the two percent rule can be considered bracing.
An interesting example of this is when engineers try to brace steel framing with wood members. In my opinion, the wood bracing should be designed for the 2% load multiplied by E(steel)/E(wood). Without scaling up the load in this fashion, I don't see how adequate brace stiffness can be ensured.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
I don't think 1% is a good number. Use App. 6 in AISC 360-05 - I believe it works out closer to 2% of the compression force in the flange of the beam. When you get that load, apply it laterally to the top of your column and check it for strength (may not be a problem), and stiffness (this is where the problem will likely be). For the stiffness, just apply a 1k lateral load to the top of the column, check the deflection, and invert it (1/delta) - this will give you the stiffness in K/in. Compare this to the required stiffness per AISC 360-05 App.6.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
Regarding the main post, I always provide stiffeners in these situations. I believe the calculation of the web bending (to restrain the bottom chord) should also account for the large compressive load at this point (support) ie, the web is already tending to buckle under compression prior to adding additional bending stresses.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
I'll post it tomorrow morning. I was out of the office today.
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
AISC 360-05 can be downloaded from here;
http://www.aisc.org/content.aspx?id=2884
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
Cheers
RE: Web Sidesway Buckling from Column Reactions on Continous Beam
Just as a curiosity, look at the references on page 439 and you will see reference to the old Australian Standard AS1250.