Beam Stability Factor CL 2

[BadgerPE]

Hey all,When checking a 2x6 purlin in RISA 3-D against my hand calcs, I noticed that I get significantly more capacity out of the member in 3-D than I do by hand. I have determined this difference to be associated with the effective span length le. Based upon Table 3.3.3 in the 2005 NDS, the effective length for a simply supported bending member follows one of the two equations:

lu/d<7, le=2.06lu
lu/d>or=7 le=1.63*lu+3*d
I get a slenderness ratio, RB, of 21.7 when calculating by hand and 16.25 when using RISA. This effects my result significantly as the purlin is about 7% understressed using RISA as opposed to about 15% overstressed when using my hand calc. If I insert the RB value of 16.25 into my spreadsheet, I get the same result as RISA.

Has anyone else run into this issue? Am I doing something wrong by using Table 3.3.3? Any help would be greatly appreciated.

Sorry for the cross post. I thought I was in the structural eng forum.

 

[Lion06]

Could it be that you are using nominal dimensions and RISA is using dressed dimensions in calc'ing RB (or that you are using dressed dimensions and RISA is using nominal dimensions)?

 

[Cadair]

I would love to help, but I don't have RISA, so you will have to give me more information...

 

[BadgerPE]

SEIT,
Nice thought, but I checked. RISA is using dressed dimensions for determining area, moment of inertia, etc. so I would assume they used dressed dimensions for RB. I used dressed dimensions as well.

When I enter Lu into my spreadsheet as the effective lenght Le, I get the same result as RISA. So it would appear that RISA is using the unbraced length as the effective lenght oft the bending member.

 

[JAE]

Here's the quoted section from the RISA help file:

The values Le1 and Le2 represent the unbraced length for the member with respect to column-type (Euler) buckling about the member's local z and y axes, respectively. These values are used to calculate Le1/d and Le2/b, which in turn impact the calculation of Cp, the column stability factor. These length to thickness ratios gauge the vulnerability of the member to buckling. Refer to Section 3.7 of the NDS for more information on this. This section also lists the limiting values of the length to thickness ratios.

The Le-bend values, Le-bend-top and Le-bend-bot, are the unbraced lengths of the member for bending. This unbraced length is the length of the face of the member that is in compression from any bending moments. This value should be obtained from Table 3.3.3 in the NDS code. The Le-bend value is used in the calculation of the slenderness ratio, RB, which is used in the calculation of CL, the beam stability factor. CL is then used to calculate the allowable bending stress. Refer to Section 3.3.3.6 in the NDS for more information on this and note that the value of RB is limited to 50.

Note that RISA calls them Le (not Lu).

 

[BadgerPE]

Well I guess I should have read a little more about it in the User's Manual. Thanks for the help!

 

[ash060]

Also using NDS 4.4.1.2 if you have a 2x6 all that is needed is an end support to avoid worrying about the CL factor