X - Bridging/Bracing 1

[RFreund]

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

EIT

 

[Lion06]

Is the X-bracing in the plane of the canopy roof? What exactly is the point of it? What is being used to deck the canopy roof?

 

[RFreund]

The x bracing is is perpendicular to the roof beams =>

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

 

[BAretired]

Can you provide a sketch?

BA

 

[RFreund]

Here is a rough sketch. Once again there is a roof deck however I do also have uplift forces and I would like to know how to analyze this if there was no deck.

Thanks

EIT

 

[darkwing88]

In addition to the x-bracing you have shown in your sketch, also put another line of x-bracing between the beams over the girder. No need for anchoring the x-bracing to side walls. Lateral support distances for the beams will be between end of beam and x-bracing, between x-bracing, and x-bracing and other end girder.

 

[BAretired]

Do not use X-bracing in the two end bays where you attach to the masonry wall. Use only one diagonal brace from bottom of beam to underside of deck at wall. Otherwise, cross bracing is okay.

BA

 

[slickdeals]

Only word of advice would be to not leave your firm's information on scanned sheets online. Don't know what liability issues there might be. Just a thought.

 

[Lion06]

Maybe it's just me, but I wouldn't use any X-bracing with or without the deck. If it's stacked construction, then provide stiffeners and bolts through the girder and beam flanges to brace at the girder lines. If top of steel for girders and beams are equal then the bracing is already taken care of. Also, unless you have a very long backspan that is getting killed for unbraced length for the negative moment at the cantilever I don't see a need for the X-bracing in the backspan either. Bumping the beams by a size or two and eliminating the X-bracing would be cheaper than having all that X-bracing, no?

 

[darkwing88]

Bracing is inexpensive and gets you huge load capacity increases for members. Don't be shy about using bracing.

 

[Lion06]

Is that plate work, welding, and field work for the X-bracing really less expensive than bumping a W12x19 to a W12x26 or something similar? I've always been taught to use more steel instead of welding and field work, that the money to be saved is in labor not in material.

 

[RFreund]

Thanks for the input but what are your thoughts on the design procedure mentioned in the original post? Does the number of x braces not matter?

EIT

 

[darkwing88]

RFreund, do you know the definition of unsupported lateral distance for a beam? Do you know the difference between relative and nodal bracing?

 

[RFreund]

Well..

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

 

[BAretired]

Your sketch does not show dimensions or beam sizes, but if the scale is anywhere near correct, you might want to consider bracing both flanges at the ends of the cantilevers, particularly in the absence of steel deck which, incidentally, is shown in the wrong direction in section.

BA

 

[RFreund]

I believe I have done a poor job of conveying the information. There is a steel deck and cantilever span is approx 10', back span is 28 feet. I should have just said to assume that the deck will not support the beams laterally just for arguments sake. Good call on the deck - sorry, haste makes waste.

I wanted to make sure I was using AISC correctly.

EIT

 

[BAretired]

I can't help you with AISC as I use a different code in Canada.

I assume you will be bracing the beams at middle of backspan, at the girders and at the end of the cantilevers.

BA

 

[darkwing88]

Here's a reference for information on designing the cantilever beam and end bracing options:

(

http://www.modernsteel.com/steelinterchange_details.php?id=197).

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.

 

[BAretired]

darkwing88,

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://www.google.co.uk/url?sa=t&so...DWJogVO8Q&sig2=1QdAOqvBw37f6iaQinU27A&cad=rja

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

 

[darkwing88]

Notice the difference in the 3rd root condition for both tables. Interestingly, one shows lateral restraint for only the top flange while the other shows lateral and torsional restraint for only the bottom flange. The k values are identical except with the tip restaint being both top and bottom flanges (i.e. 4.5 vs. 3.6, and 2.4 vs. 2.1).

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.