KL/r Ratio for staggered bracing

[nitin36537]

AS per ASCE 10-97 while calculating l/r for leg member which is continuously staggered as shown in fig 3.2 b and 3.2 c of ASCE 10-97 , we have to consider kl/r = 1.2 l/r.

Does this apply only to leg members or it will be applicable to lattice member also. (Lattice p7T as shown in attachement) ?

If only to leg members then why ? Your valuable comments on this will be highly appreciated.


Thanks

Nitin Patel

 

[transmissiontowers]

This must be one big tower to require bracing like that. FWIR, the staggered Leg bracing was used after a lot full scale testing by SAE from somewhere in Europe, Spain I think. I'm not sure if you can use the 1.2 factor in this case. The SAE tests were done on leg (corner post) members where there were 2 planes of support from the 2 tower faces where the stagger was used. In your case, the stagger is in one plane and out of plane buckling is the big question. If member P7 is the corner post leg and this drawing is the longitudinal face lacing pattern, then member P7T is in the same plane as P7 and P7L. If I am reading the drawing correctly, then P7T has staggered bracing in this plane but where is it braced for out of plane buckling?

If you use the latest version of PLS-CADD software, you can model all the redundants in the TOWER model and the nonlinear analysis will show where this bracing pattern will buckle. I would model this panel in PLS-TOWER with all members and use the 1.2 factor and apply a big compression load on leg P7 and a little wind perpendicular to the face using ASCE-74-2009M so you get wind on all the redundants and lacing members, then I would run a NL analysis and see if the panel is stable. Members P7T and P7L will be beam members.

I guess it depends if you want to gamble that the staggered bracing factors can be used and pick smaller members for P7T, or you can be conservative and assume P7T buckles out of plane where it is braced.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[nitin36537]

P7 is corner leg , P7T is lattice member of transverse face and P7L is lattice member of longitudinal face.
P7 is supported from both face. ( Not staggered ). Member P7T and P7L are not supported from two side at every point. We face problem during tower testing with this pattern during transverse load conditions ( No longitudinal load and very small vertical load on tower). Design of P7T and P7L members are considering KL/r = L/r.

Finally designer added few more members on transverse ,longitudinal and inclined face to support P7T and P7L at all points. With revised arrangement tower tested successfully recently.

Does it mean we have to consider KL/r = 1.2 L/r for bracing members also if we wanted to test tower without adding extra members ?

Nitin

 

[msquared48]

I would think that it would apply also to the main diagonals (K- Bracing) and X-bracing, as well as the legs, but not any redundant diagonals or horizontals - anything that sees compression loads not associated with just the lateral restraint of a member.

Are you designing in accordance with TIA-222-G? What tower analysis program are you using?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[transmissiontowers]

Mike; With the designation transverse and longitudinal, this lattice tower is most likely an electric transmission tower not subject to TIA. The OP listed ASCE 10-97 so he is probably using NESC if he is in the USA or the tower code from his country.

Nitin;
The dashed members between P7T and P7L, do they connect the transverse and longitudinal faces of the tower? Is the tower square? If you use PLS-CADD software, I suggest you model all the joints and connect them as they are in the real world as beam members and run a nonlinear analysis to see how they behave. Your Lx or Ly are probably greater than 1.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[nitin36537]

This is 500KV S/C tower. Loadings are as per IEC. Tower is square base. we are using PLS Tower for analysis. During testing some redundant members are added ( Refer attached line diagram ). Tower is successfully tested with added redundant members.

My point is , Even if ASCE 10-97 clause is for leg member ( KL/r = 1.2L/r) , it should be applicable for lattice members if we are not connecting from two direction.

 

[transmissiontowers]

The staggered bracing formula adjustments were based on SAE full scale testing. I think you can use them since the lacing P7T is braced out of plane by the members in your View L. If you had a successful test with the added redundants in View L, you should add them into the tower details and build the tower this way in the future.

_____________________________________
I have been called "A storehouse of worthless information" many times.