Torsional Bracing of Beams at Points of Support 1

[CollinW]

I am attempting to analyze a monorail beam that is supported from above with vertical HSS tubes. It has 5 points of support, 4 internal spans, and one cantilever span at the end. Per the steel manual F1 and App. 6.3 the beam must be restrained against rotation in the longitudinal axis. This beam does not have any bracing along the bottom flange, so the only option that I see is to check it for Torsional Bracing per Eq (A-6-9) and (A-6-10). However, Eq. (A-6-11) has the variable "n" that doesn't seem to apply in my case because there are no braces in between supports. When I use n=1 and L="each span" then the calculation works, however when I try to do something like n=3 and L="sum of spans," it fails.

I was wondering what other people thought of this and if anyone has run into this same issue. None of the other equations in App. 6 have this "n" variable. The structure and beam have already been built, but no lifts have been done yet.

 

[human909]

The fortunate thing about monorails is that, while restraining the bottom flange might be hard or impossible, the load position is generally in a position that decreases of prevents LTB. Many codes I've seen don't have explicit allowances for stabilising loads, so I can't point to your particular code and say that is how you "make it work".

The other aspect I'd like to point out is monorail trolleys generally load the bottom flange in quite an 'abusive' way. As a result I find myself designing some pretty heavy flange beams to achieve suitable flange performance. The maximum bending stress in the beam only ends up to be around 50mPa and LTB never dominates in my experience.

 

[WARose]

I've generally treated hung monorails like spreader beams in lifting applications (as far as buckling goes). ASME's BTH-1 is useful in that regard.

 

[CollinW]

Thanks for the responses.

Human909, I have heard of reducing the LTB for the load position and it makes sense. I was wondering if you have ever found a source that quantifies the reduction? I have not been able to. I also completely agree about flange bending, which is why we picked an S shape with high flange bending.

WARose, I looked at the BTH-1 and I saw lots of info about designing the beam and the C_LBT factor. It seems very helpful, however I find myself running into the same issue: I am unsure about how well the supports restrain the beam against twist.

 

[human909]

Hey Collin.

Have a read of this the attached document:
Lateral Buckling of Monorail Beams

It is for AS code rather than US. But as it describes the approach can apply to other codes.

 

[WARose]

WARose, I looked at the BTH-1 and I saw lots of info about designing the beam and the C_LBT factor. It seems very helpful, however I find myself running into the same issue: I am unsure about how well the supports restrain the beam against twist.

The C[sub]LTB[/sub] factor can compensate when no rotational restraint is present. It equals 1.0 where beams are restrained against twist....but they give you a equation for situations where it isn't. We kick this around in the thread below (see WinelandV's post).

https://www.eng-tips.com/viewthread.cfm?qid=446703