AISC Appendix 6 - Beam Bracing 5

[JAE]

Through past work by Yura the AISC specification has added appendix 6 which provides requirements for bracing.

Section 6.3 is for beams and includes both relative and nodal bracing requirements. The general concept I understand but we've been trying to understand the explicit application of this section to brace designs.

The main issue is that the brace strength equations ((A-6-5 and A-6-7) provide values for required brace strength, Pbr. This is given as a force in pounds as a function of bending moment and beam depth (h_o).

But if we provide braces at 4 feet on center, we get a Pbr value. If we put braces at 8 feet on center we still get a similar Pbr value since the moment is the same. The beam might be a bit deeper since Lb would be larger, but that means that the brace force actually gets smaller since h_o is in the denominator....and that seems counter-intuitive...fewer braces means less brace strength required.

Anyone have any views on this?

 

[sundale]

So you get X kips per beam bracing force and have N number of beams. If all these N # of beams want to LTB in the same direction, then N*X summation of beam bracing force goes into the diaphragm and then into your lateral system?

The column stability formulae considers the effects of gravity "leaner columns" on the overall stability of columns in lateral elements, e.g. braced frames or moment frames. Depending on what stability methodology you use, you can either ignore the notional load Ni (0.2% of the total gravity load) if it is smaller than wind or seismic or sometimes you consider the notional load Ni to be additive to the the transient wind or seismic lateral loads.

Is the notional load concept really a proper way to view the summation of beam bracing forces on the lateral system?

 

[ARLORD]

sundale,

I do not fully understand you post, but if I understand correctly, the brace force, Pbr, calculated is not additive for multiple bracing members for the same beam. For example, for a simply suported beam with the max moment at the center, with 3 braces at quarter points. The max brace force would be at the max moment, center brace. That force goes into the diaphragm. If you have adjacent beams, all with the center brace, then you would add that center brace force for all the adjacent beams into the diaphragm, but you would not add the brace force of 3 per beam.

 

[BAretired]

Article 9.2.8 of CSA S16-01 has provision for accumulation of bracing forces when the number of beams being braced, n is greater than 1. It is a statistical approach which recognizes that not all beams will have the same initial misalignment. The factor (0.2 + 0.8n^-1/2) is used with initial misalignment and brace force Pb.

Thus, if there were 4 beams, Pb per beam would be 0.2 + 0.8/2 = 0.6 Pb for a single beam.

When there are two or more brace points, the forces, Pb alternate in direction so adjacent brace forces are not cumulative.

BA

 

[ARLORD]

Also see the following:

http://www.steeljoist.org/graphics/...006 JMFisher Bracing using OWSJ V43_01_25.pdf

 

[cap4000]

JAE

Chapter 7 on page 311 in the 2008 Galambos Structural Steel Stability Book he does exactly a beam braced with two perpedicular beams framed into shear walls. Where he differs from you is that the stiffness of the 2 braces is B=AE/L. He then calculates the A value or area of steel required with a 15 foot long brace combined with the Pbr value. The bottom line is the area of steel is very small so many shapes will typically brace a 30 foot long beam with a moment of 465 kip-feet braced at the 1/3 points. Hope this helps.