Intent of existing X-bracing

[apsix]

I need to determine the capacity of existing bracing so that I can match it.
The existing bracing members are 2 - 152x152x10 Angles, starred. These are arranged in the normal cross, in addition to horizontally at mid-height (refer attached).
Bay size is approximately 6.5 x 6.5m.

I suspect that the intent was for the bracing members to act simultaneously in tension and compression.
Does anyone agree?
Does the horizontal member appear sufficient to act as a lateral restraint to the braces in compression? (I will calculate it, I'm just looking for informed opinions at this stage).

 

[hokie66]

A lot depends on the connections. The tensile capacity of the double angles is easy. Will the connections develop that capacity? Then the capacity in compression has to be determined for the other brace. I would start with finding the amount of lateral force the horizontal member can take, assume that is 2% of the compression brace force, and check the compression brace for the resulting force. Are the starred angles connected together?

 

[apsix]

Thanks Hokie

The starred angles are battened at third points.

I take it that you are saying it's not unreasonable to check the capacity due to tension + compression.

Is this a bracing form that you've seen/used elsewhere? (specifically, the reasonably light horizontals?)

 

[rscassar]

Are you able to provide the detail which shows the intersection of all those angles? I am assuming that the 2 angles which form 1 diagonal are back to back with the 2 angles which make the other diagonals, but unsure on how the horizontal members are fixed.

I would say that the design intent was for the 2/152x10EA to be used as a compression member. I am saying this without know the magnitude of compression which the section needs to resist.

By using it as a compression member (instead of tension only) it will double the stiffness of that structures against sidesway.

 

[hokie66]

I think it is reasonable to use both braces, but you may not get much capacity out of the compression braces, as the horizontal is not a very good bending member. But no, I've never seen starred angles used for the intermediate horizontals. Personally, I've never used double angles in that configuration for any type member.

 

[apsix]

The starred angles form a cruciform. Intersection connection is via a plate between the angles, see attached.

Seeing that I need to at least match the existing capacity, I'll conservatively assume that adequate horizontal restraint is provided and base the total capacity on 2x the compression capacity (and also check connections).

 

[hokie66]

As the tension capacity is probably greater than the compression, the total capacity may be greater than 2x compression capacity.

 

[apsix]

I'm assuming the strut will fail when trying to use the extra tie capacity.

 

[ToadJones]

I guess in short you can say that if the compression brace does not buckle out of plane, then the system is tension-compression.
I don't think you'd find cruciform shapes in bracing that was intended to be tension-only bracing.

 

[paddingtongreen]

The cruciform shape looks to have straps at the third points so they should perform well as struts, possibly better than the ties if the ties have mandated section reduction at the bolted connections. The horizontal beam only needs the strength to resist the 2%, deflection is not important.

That's interesting, I haven't seen star angles used in new work ever. I tried to use them here in the States and I got some very strange looks.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dhengr]

Apsix:
You don’t have to suspect that the diags. will act simultaneous in tension and compression, you can count on it, the structure says so. We just have trouble putting our finger on the magnitude of the compression brace contribution. The tension diag. suggests an elongation and drift, and the compression diag. will tend to reduce the drift to the extent that it does not buckle. When the compression member buckles you have reached the capacity of the bracing system. You get to pick the relative stiffness of the two diags. and that’s the tough part.

The lateral capacity of that bracing arrangement will be the sum of the tension diagonal, plus the compression diagonal. The tension diag. capacity is fairly straight forward, given the area of the angles and enough weld or bolts at the end connection plates. The compression diag. is somewhat less straight forward because of the buckling problem. I agree that 2 or 3% of the compression force is a good rule of thumb for a bracing force required, but that is for a continuous compression member, such as a WF or HSS column or strut. In this case, your compression diag. is made up of two shorter struts, about 4.6m long, with semi-rigid connections at each end, a function of plate bending and buckling stiffness.

I don’t think I’m to confused about the above, but I am confused about the diags. Are they each two angles back to back, with spacers btwn. them, or are they each a cruciform shape made up of four angles, all back to back? The diags. as a unit make up what I’d call X bracing, is this starred? And, half the time, for simplicity, we just neglected the compression diag’s. contribution, because of the buckling issue, assuming it might buckle elastically. We knew we had a tension diag. whichever direction the lateral load took. We put the two back to back angles of one diag. on one side of the end connection plates and the two back to back angles of the other diag. on the opposite side of the end connection plate; then used a spacer plate of the same thickness where the diags. crossed. The horiz. angles do offer some bracing, but that’s a tough one too, hokie gave you a good suggestion in his first post.

You could get in trouble if you guess (hardly an exact guess either) to high or to low for the existing x-bracing stiffness and the new bracing system stiffness. At the location in the building where they interact, the building has to be designed to tolerate some differential lateral drift, a movement joint; or the two parts must be tied together, through floor diaphragms or some such, so as to distribute the remaining differential lateral loads and prevent the differential movement.

 

[apsix]

Thanks all.

paddingtongreen
This is modification of a 30 year old building, I wouldn't expect starred angles in new build.

dhengr
The members are made up of 2 angles, in a cruciform shape, I guess you could call it corner to corner, with a 15mm gap.
They were reasonably popular in the UK and Australia in the past, but I didn't expect to see them even in a 30 year old building.

I'm still not happy to use 'The lateral capacity of that bracing arrangement will be the sum of the tension diagonal, plus the compression diagonal'.
I expect that the strut will buckle before the full strength of the tension tie can be utilised (once I've done the analysis).

 

[paddingtongreen]

dhengr, the picture says they are star angles (cruciforms) with battens at the third points, the battens being parallel to the gusset. That's the only thing I see different from the old days, they alternated direction back when. It is an efficient way to use angles. As I said in my post above, when I tried to use them here in the states, I got some crooked looks.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ToadJones]

hmmm... I too thought that the cruciforms were made up of four angles, not two.
I like that configuration.
I guess one could actually use some intermediate clip angles to connect the two long angles together.
I like so much that I am going to use that configuration on my next job

 

[apsix]

Attached is the starred angle detail form this job.
Each battened point uses a pair of batten plates, at 90deg to each other.

 

[hokie66]

I think that whatever assumption you make, it will probably be conservative. My reason is that the tension diagonal provides additional bracing to the compression diagonal. The intersection point can't move laterally unless all the members move.

 

[dhengr]

Apsix & Paddington:
Wow, that’s different, I don’t think I’ve ever seen angles used in that configuration for this purpose. I had never heard the ‘starred angles’ and ‘battened’ terms used this way, and wondered if the ‘cruciform shape’ wasn’t the total X-brace system, which is sort of a cross shape. I can see what Paddington is describing as battens parallel to the gusset pls., although they are not very clear in the photo I see. I suppose that is a reasonably efficient shape, at least symmetrical, for this application, and it can be bolted. If welding, I would put the two angle toe to toe (a box section) welded to the gussets and with battens at the third points, again parallel to the gusset plates, an even more efficient shape.

 

[BAretired]

Starred angles tend to buckle in a torsional mode.

BA

 

[dhengr]

BA:
I was going to comment on unstiffened outstanding legs, a width/ thickness ratio issue, and their potential for buckling; I suspect that would lead to a torsional form of buckling. Somehow that came out as ‘at least they are symmetrical,’ thinking no inferior radius of gyration.

Also.... My best to Mrs. A. I hope she’s keeping you unbraced. Oh, damn, I did it again, I mean embraced.

 

[BAretired]

Thanks again, dhengr...all is well on the home front.

BA