Web Sidesway Buckling from Column Reactions on Continous Beam 1

[kar108]

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)

 

[BAretired]

kar108 said "I want to design the lightest possible beam for web sidesway buckling without using a web stiffener".

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

 

[Lion06]

I'm not even sure I am buying that this is a web sidesway phenomenom. Web sidesway is when the tension flange moves laterally as a result of a large concentrated force on the compression flange. Your situation is not the same, because your tension flange (top flange) is braced. This is a LTB issue. What is the problem with providing the stiffener?

 

[KootK]

Structural EIT:

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.

 

[KootK]

The sidesway phenomenon is about the two flanges moving relative to one another. Whether it is the tension or compression flange that does the physical moving, it's still sidesway buckling.

 

[BAretired]

The stiffener is an extension of the column through the beam. In effect, the column spans from footing to top of beam by virtue of stiffeners or the beam web if it is stiff enough. This prevents rotation of the bottom flange about a horizontal axis. A stocky beam may provide a web stiff enough to achieve this without a stiffener.

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

 

[csd72]

I think Kootenaykid is spot on with regards to the reasons why these dont fail in residential construction.

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.

 

[CentrePACE]

StructuralEIT:
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?

 

[ishvaaag]

All elements in one structure brace -stabilize- ones to others. The overall general buckling of a framed structure, a reality for weak ones, is generally omitted from consideration in our piece by piece designs, by methods usually safe. Notional loads by diverse derivations, not all consistent in the proposed value, or different in-member imperfections from fabrication or construction are applied. Relative bracing is accepted, yet for some cases (not precisely for those it is recommended) the path of the forces appearing as a requirement of stabilization are not properly followed to foundations. In all, that if not for enough safety, for proper understanding, we need a thick to be seen two tomes manual to fix the many shortcomings generally appearing in the field. Whilst, keep tight to the code, but even this is no easy task, for as we see here, interpretations vary even between seasoned and well taught engineers.

 

[KootK]

CentrePACE,

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.

 

[Lion06]

CenterPace-

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.

 

[OzEng80]

Structural EIT - Would you mind listing the stiffness requirements from AISC App.6? My code (Australian) has a 2.5% bracing requirement with no provision for stiffness. I would appreciate it if I could be armed with some figures for this obvious requirement.

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.

 

[Lion06]

Oz-
I'll post it tomorrow morning. I was out of the office today.

 

[apsix]

OE80
AISC 360-05 can be downloaded from here;

http://www.aisc.org/content.aspx?id=2884

 

[OzEng80]

Thanks! Got some reading to do. I must confess i was hoping for something along the lines of: brace deflection limit of beam depth/1000 or column height/500...

Cheers

 

[csd72]

OzEng80,

Just as a curiosity, look at the references on page 439 and you will see reference to the old Australian Standard AS1250.