X - Bridging/Bracing
X - Bridging/Bracing
(OP)
I have read through a couple of posts regarding this topic and I still seem to be slightly confused.
Does this procedure seem correct or am I off base?:
I have a canopy designed with steel beams that span between 2 girders and and are cantilever over one girder. To brace the beams I want to put in a row x bracing between the beams which will be attached to a masonry shear wall (or something rigid).
It seems that I should use appendix 6.3.1b. and 6.3.2a. Both of which are for nodal bracing.
And design for which ever gives me larger required axial force (for 6.3.2a P=Mbr/d) and stiffness criteria. However these equations do not seem to consider the number of spaces in which I will have x-bracing (or number of columns, not number of nodal braced points) or do they?
I also I have looked over the attached paper which seems to be for a different situation but it does consider number of girders and spacing .
Thanks
Does this procedure seem correct or am I off base?:
I have a canopy designed with steel beams that span between 2 girders and and are cantilever over one girder. To brace the beams I want to put in a row x bracing between the beams which will be attached to a masonry shear wall (or something rigid).
It seems that I should use appendix 6.3.1b. and 6.3.2a. Both of which are for nodal bracing.
And design for which ever gives me larger required axial force (for 6.3.2a P=Mbr/d) and stiffness criteria. However these equations do not seem to consider the number of spaces in which I will have x-bracing (or number of columns, not number of nodal braced points) or do they?
I also I have looked over the attached paper which seems to be for a different situation but it does consider number of girders and spacing .
Thanks
EIT






RE: X - Bridging/Bracing
RE: X - Bridging/Bracing
RidgidXIXIXIXRidgid
And this is more of an "academic" question as I would like to know if there was no deck would this be correct however there is and it will give lateral support to the beams for the gravity load but I do want to brace the beams as there is uplift causing negative moments.
EIT
RE: X - Bridging/Bracing
BA
RE: X - Bridging/Bracing
Thanks
EIT
RE: X - Bridging/Bracing
RE: X - Bridging/Bracing
BA
RE: X - Bridging/Bracing
RE: X - Bridging/Bracing
RE: X - Bridging/Bracing
RE: X - Bridging/Bracing
RE: X - Bridging/Bracing
EIT
RE: X - Bridging/Bracing
RE: X - Bridging/Bracing
Lateral unsupported distance being the distance the between supports which restrain lateral movement, torsional movement.
I understand that nodal bracing bracing uses transverse bracing supported to rigid supports. Where as relative bracing controls movement in relation to adjacent brace points.
and....
feel free to elaborate.
EIT
RE: X - Bridging/Bracing
BA
RE: X - Bridging/Bracing
I wanted to make sure I was using AISC correctly.
EIT
RE: X - Bridging/Bracing
I assume you will be bracing the beams at middle of backspan, at the girders and at the end of the cantilevers.
BA
RE: X - Bridging/Bracing
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Consider top flange loading of the cantilever beam: using top and bottom flange bracing of the beam at the girder support, this reference says that with or without an end brace for the top flange of the cantilever beam, you will need to use a unbraced length of 250% more than the cantilevered distance. With end bracing of both top and bottom flanges of the cantilevered beam you will need to use and unbraced length of 150% more.
RE: X - Bridging/Bracing
Your reference differs from one I have seen (which is also page 32 of the CISC publication "Roof Framing with Cantilever (Gerber) Girders & Open Web Steel Joists:
http:/
When the top and bottom flanges are restrained at the root, there appears to be no difference in the K value for top flange restrained or unrestrained. That seems contrary to all I have read about the value of bracing the top flange. In any case, the K value is 2.5 for top loading, so the unbraced length of the cantilever is 2.5*Lc where Lc is cantilever length.
When load is placed at the neutral axis of the beam, K is 1.0 and 0.9 respectively for top flange unbraced and braced. Huge difference based on the position of the load.
When flanges are restrained at the tip, K is 1.2 and 0.7 respectively for load at top flange or n.a. Again, a substantial difference based on position of load.
I believe the jury is still out when it comes to determining the effective length of a cantilevered beam. When that is the case, it pays to be conservative.
BA
RE: X - Bridging/Bracing
FWIW, the table in the Modern Steel Construction article is also printed in the 4th edition of Guide to Stability Design Criteria for Metal Structures edited by Galambos.