Lateral restraint to prevent LTB
Lateral restraint to prevent LTB
(OP)
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
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






RE: Lateral restraint to prevent LTB
Kieran
RE: Lateral restraint to prevent LTB
RE: Lateral restraint to prevent LTB
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.
RE: Lateral restraint to prevent LTB
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
RE: Lateral restraint to prevent LTB
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
RE: Lateral restraint to prevent LTB
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.
RE: Lateral restraint to prevent LTB
Found here:
http
Go to page 16.1-191 in the document (or page 249 of the pdf)
RE: Lateral restraint to prevent LTB
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
RE: Lateral restraint to prevent LTB
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-
RE: Lateral restraint to prevent LTB
RE: Lateral restraint to prevent LTB
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.
RE: Lateral restraint to prevent LTB
EIT