ASCE 10-97 triangulated hip bracing 3

[pennacchietti]

Good morning,
I have been searching old posts on the subject, but I couldn't find any answer.

I am trying to fully understand the requirements of ASCE 19-97 with reference to the need or advantage of triangulation of hip members.
My concern derives from the possibility to reduce leg slenderness in case of X and K bracing patterns when redundant members are added without having hip members or not triangulated hip bracings.

In the event of a square section tower, and wind at 45deg, both diagonals and legs are in compression.
To me, using non triangulated redundant to reduce leg slenderness is therefore allowing compressed members (diagonals) to brace another compressed element (leg) without the needed rigidity in the perpendicular plane.
it seems that TNX Tower software is also highlighting the issue, see below the comments from its User Manual

"Secondary Horizontal Braces Leg. Secondary horizontals ordinarily are not considered to be able to brace leg members. When the secondary horizontals are sufficiently triangulated to have this capability, then you may check this box"

Additionally , other software like MStower is requesting additional care in case of not fully triangulated structures (plan + hip bracing)
I found some documentation on the subject, referring to power transmission tower collapse and the effect of triangulation, but nothing that really gives clear guidelines on the subject.

Paolo

 

[JoshPlumSE]

Well, if not one else response I'm willing to give it a partial response.

AISC requirements for bracing and tower requirements will be a certainly bit different from what you get with the tower codes. Different industries, different assumptions. The AISC requirements will (I believe) be a more stringent. That's because they're primarily aimed at building type structures.

For communication towers or such, the industries tend to be a bit more liberal. They're looking at towers with a history of successful performance and trying to justify that they should be allowed under the newer codes.

Plus, we know that if you triangular the bracing then it should provide SOME stability to the member. Is it fully braced at that point.... I'm not sure that it is. But, is that okay? If they're requiring you to do a geometrically non-linear analysis does that allow them to get away with less conservatism with their unbraced lengths?

If I wanted to really research the issue, I'd go through the code and commentaries (both ASCE and AISC) and see what technical papers they are referencing on this subject. Then I'd start collecting these papers and reading through them in detail.

 

[KootK]

To me, using non triangulated redundant to reduce leg slenderness is therefore allowing compressed members (diagonals) to brace another compressed element (leg) without the needed rigidity in the perpendicular plane.

This is going to be difficult to explain but I'll give it a go anyhow:

1) A compressed member can definitely brace another compressed member. In buildings, this happens all the time. A vertical braced frame, in compression, will brace the tops of all the gravity columns which are also in compression. The trick to it is to ensure that the thing doing the bracing is stiff and strong enough that it can share it's stiffness with the members that it is providing bracing to.

2) With your tower, it is not the diagonals that are bracing the tower legs. Rather, it is the vertical trusses formed by the diagonals, the legs, and the web members that are bracing the tower legs. And while the diagonals on their own may be too close to buckling themselves to provide bracing to the legs, the trusses just mentioned will be nowhere close to buckling in plane.

3) From what I can see, all of the horizontals connected to the legs of your tower are in fact triangulated in the planes shared by the legs and the diagonals. Thus, I believe that all of those horizontals can be counted as bracing.

This is the same principle that allows us to assume that building truss compression chords are braced in the plan of the truss at each panel point. Even in tension, the opposite truss chord would be nowhere near stiff enough to provide bracing to the compression chord on its own.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.