Lateral restraint to prevent LTB 3

[MikeDartmoor]

I am carrying out a loft conversion with two UBs spanning between
and built into masonry party walls 6.7m apart. The two beams will be parallel approximately 6.2 metres apart.
A third beam will be located at approximately mid way between and parallel two the first two beams supported at one end 2.0metres in from one party wall by a fourth beam spanning between the first two beams. Is it reasonable to assume that the fourth beam provides lateral support to the compression flanges of the first two thus reducing their effective length to 4.7 metres

 

[kieran1]

yes but make sure you account for destabilizing loads on the beams if the existing/proposed joists are inducing torsional loading on outer beams

Kieran

 

[rscassar]

Depends, what is the nature of loading. If you are relying on each beam which are equally loaded and both on the brink of LTB than I would not rely on the beams to restrain each other.

 

[JAE]

If the beams are connected to floor framing throughout then they are laterally braced (assuming the floor framing has connectivity to the beams and has some rigidity.

kikflip is right that you have a sort of relative bracing which is provided by the perpendicular beam but both longitudinal beams can buckle in LTB together despite the presence of the intermediate perp. beam. The perpendicular beam will help somewhat as the connection between the beams might resist rotation to a degree.

In the US, AISC has a section in their specs (Appendix 6) which has provisions for relative bracing. Not sure what code/spec you are under and whether it has similar provisions.

 

[RFreund]

I have wondered about this situation where both beams buckle in a global fashion are there any good examples or publications on this calculation. The AISC manual does not really address this situation...
Or do they?
I understand that they give stiffness and strength requirements for the bracing member, so in this case are the 2 parallel beams the braces for each other?
They give the stiffness and strength requirements so would that mean that the beams would need to satisfy the combined load for the strength and stiffness requirement in the horizontal (braced direction / weak axis) direction and also the forces from any gravity load?

EIT

 

[psmaxtor]

A few good references,
Global Lateral Buckling of I-shaped Girder Systems. - Joesph Yura
(Journal of Structural Engineering ASCE- Sept. 2008)

Fundamentals of Beam Bracing- Joesph A Yura
(Aisc-engineering Journal-first quarter-2001)

Both of these documents may be downloaded...

Guide to Stability Design Criteria for Metal Structures- Galambos
There is a newer version of this book on SSRC Website.

Hope this helps

 

[csd72]

The steel construction institute has an old publication called 'lateral stability of steel beams and columns' which covers these things to the old BS. I would also think they are equally applicable to eurocodes.

The code specifically stipulates that these things must be braced back to a rigid point of restrain (such as a braced crosswall) so as long as you have the load path then you should have the bracing.

 

[JAE]

RFreund - look at AISC specification 341-05, Appendix 6 - for relative bracing.

Found here:

http://www.aisc.org/WorkArea/showcontent.aspx?id=17640

Go to page 16.1-191 in the document (or page 249 of the pdf)

 

[RFreund]

JAE -
I read through what the AISC recommends for relative bracing but from what they give, I'm not sure how it applies in this situation. It seems they are giving the strength and stiffness requirements for the brace. Which, in this case, is the beam (6.2m long) between the to parallel (6.7m long) beams. It doesn't seam to address how stiff or strong each parallel beam needs to be. Thats what I was trying to get at.
Or I'm I missing something.

Also thanks for the recommended literature.
@psmastor: Where can they be downloaded at?

Thanks again.

EIT

 

[psmaxtor]

The documents must be purchased:
Global Lateral Buckling of I-shaped Girder Systems. - Joesph Yura
(Journal of Structural Engineering ASCE- Sept. 2008) From ASCE WEBSITE under Structural journals.

Fundamentals of Beam Bracing- Joesph A Yura
(Aisc-engineering Journal-first quarter-2001)
AISC Website-

 

[JAE]

RFreund - The AISC stability requirements that I linked to were developed based upon Yura's work - which is the book that psmaxtor references - so they sould be on common ground.

 

[FinnB]

An old question is: Do timber floor joists provide restraint against lateral torsional buckling?

Looking at this in the past the codes/guidance say that the top flange of the beams must be restrained against lateral movement. Personally I think this is a bit strict in the case of floor joists as the joists will provide resistance to the beams moving laterally should the joists be fixed to a braced wall. Lateral loads due to LTB are described as being very small based on lab testing, around 1-2% of the vertical load. This force would not exert a big lateral load on a supporting wall.

Throw in timber nogging between the joists, screw down the floor boards, preferably into the steel beams as well, and the floor is now very stiff.

If I was putting a loft in my own house I would not be too bothered about LTB of beams when using a timber floor. Working for a client is different and I would be calculating for the effects of LTB. It's a litigious world we live in.

 

[RFreund]

Thanks guys.

EIT