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Civil/Structural Global Forum

Main Topic

Author: Michael Barbetta/PH/Fluor Last Modified: 08/03/1999 02:29 PM

Topic: cross bracingClick for Instructions

Category: STRUCTURAL ENGINEERING TOPICS

Sub Category: steel

Keywords: Click for Instructions Status: Active

bracing

Discussion:

If diagonal cross bracing is pinned at the intersection where both braces meet, whatlength is to be used in determiningthe KL/r value for each member? Would it be the full length of the member, 1/2 thelength of the member, or something in between?

Thread:

>cross bracing by Bryan Galloway on 8/4/1999 11:11:29 AM

Concerning X-bracing effective lengths...

We did our own investigation on my current job, and we calculated a theoretical

value for k = 0.58 (Based on technical paper "Design of Diagonal Cross

Bracings, Part 1: Theoretical Study" by Picard & Beaulieu). However we used a

design value of k=0.65 & applied it to the full length of the brace in

compression.

If you would like me to fax you a copy of our investigation (it's only about 10

pages), drop me a note & a fax number. I would recommend taking a look at the

study & working through the numbers before using the k-value.

There are other approaches & justifications, but this one made the most sense

to us.

Hope this helps,

Bryan

>another response by Ed Haninger on 8/4/1999 8:30:26 AM

Under normal situations, where the two braces are of equal length and the

forces are opposing (one in compression and one in tension), the stiffness of

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the tension member plus the restraining effect of the tension force is enough

to force the compression member into the second mode buckling shape -

effectively providing a brace at midpoint.

There are articles (which I tossed before I moved) which discuss this.

Timoshenko and Gere, in "Theory of Elastic Stability", Second Edition, cover

the subject of a beam-column with a flexible support, on pages 70 to 74.Although, he does not cover the restraint of the tension force. Also, the

latest draft of the new LRFD spec has some additional information on

requirements for bracing of compression members which may help. Also, see the

previous response by Wislocky (1997, in this database) to this similar

question.

>Effective Length for Cross Bracings by Janos Toth on 8/5/1999 2:44:40 PM

There is a paper written by Sayed H. Stoman in the ASCE Journal of Structural

Engineering, Vol. 115, No. 12, December 1989 discussing this topic. A good

summary of references and some practical formulas also could be found in this

article finding the value of k.

>K factor depends on tension in tension brace by Marian Podlovsky on 8/20/1999 12:07:27 PM

(the following is extracted from Journal of Structural Engineering, Vol 115,

December 1989)

For bracing pinned ( both, tension and compression) both ends.

--------------------------------------------------------------------------------

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Pec = 3.14^2 * Ec * Ic / (Lc)^2

Ec - modulus of elasticity

Ic - moment of inertia in direction perpendicular to the cross brace plane

Lc - Full length of the brace

c subscript indicates compression

Pcr = 2 * Pec + Q

Q - tension in the other (tension) brace

Gamma = Q/Pcr = Q / ( 2 * Pec + Q )

K = ( ( 1 - Gamma) / 2 )^0.5 but >or = 0.5

K will vary between 0.5 and 0.7

For bracing with fixed ends

---------------------------------------

Pcr = 8 * Pec + Q

K = (( 1 - Gamma)/8)^0.5 but > or = 0.35

With only the tension brace ends fixed ( not a very practical case, if lateral

forces are reversible...)

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Replace the 8 with 5 and K should be at least 0.5.

So, based on tension in the tension member the K factor will be at least 0.5

for pinned braces and maximum 0.7.The 0.6 or 0.65 value should be a good value to use.