flat plate tension braces

[shaheed]

Hi Guys

I am designing a single storey steel framed buiding clad with cavity brickwork(two leaves of brickwork with a 50mm cavity)and I am planning to use flat plate tension cross braces in the brick wall cavity. The braces are aprroximately 75mm by 5mm plate.

Does anyone have any comments or advise on using this type of bracing system.

Thanking you all for your kind assistance in advance

 

[csd72]

Impossible to pretension and very easy to install slack - use with caution.

 

[jike]

Provide a detail on how to pre-tension and you should be all set! Prevent slapping against the metal studs.

 

[BAretired]

They must be pretensioned. The attached detail has been used successfully where clearance for bracing is restricted.

BA

 

[shaheed]

Thanks for the replies but can some one explain the importance of pretensioning. The reason I ask this is because if the plates were replaced by, say, angles there would be no pretensioning required.

Why is this any different?

 

[hokie66]

Using cavity brick construction in this manner is very unusual. The brick would be much stiffer than the bracing, so would tend to take all the shear. At any rate, rods which can be tightened are a better solution than plates.

 

[dhengr]

shaheed:

Listen to Hokie66 and think about what he has to say, re: rods not bars and relative stiffness of brick wall and steel frame, and design and detail accordingly. Show us a wall section, how does the steel column end up in the middle of a double wythe brick wall? I assume ‘two leaves’ = two wythes in my lingo. Steel x-bracing can not be installed tight enough to be effective, it must be retensioned after the steel structure is erected, and possibly latter in the life of the building.

 

[BAretired]

shaheed,

Pretensioning is required only where the tension brace is extremely flexible as it is with a flat shape. If the brace is stiff, pretensioning is not necessary.

Rods are another solution as noted by hokie66. Whether or not they are a better solution is a matter of opinion. I have used both, but when confronted with clearance problems, I have found that pretensioned flat plates are a better solution to the problem.

dhengr,

Sorry, but steel x-bracing CAN be installed tight enough to be effective with the detail I suggested earlier.

BA

 

[dhengr]

BA:
I absolutely agree, your detail is just fine, but you do have provide a means of tensioning the x-brace after it is installed and bolted up, at the columns. Be careful to tighten the two bolts in your detail an equal amount so as not to induce a secondary moment in the tie bar and at the welds. That may not be a life and death issue, but it doesn’t exist with a rod and turnbuckle.

 

[BAretired]

dhengr,

Agreed, but there are times when two rods cannot be accommodated in the space available. In these cases, the flat plate is a practical alternative.

BA

 

[csd72]

shaheed,

The real issue with cross brace pre-tensioning is the lateral flexibility of the brace. It will wobble around during installation and will definitely not be straight at the time the bolts are done up. An angle doesn't have this problem due to its bending stiffness.

 

[Stillerz]

I have no experience with this type of construction, but would it not be possible to detail this situation such that frame and bracing accommodate the shear independent of the brick?
I would think this would not be too different than a wood stick-framed house with plywood sheathing (shear walls) and brick veneer, no?
Again, no need to jump me as I am only offereing some thoughts here.

 

[shaheed]

For all those believe that pretensioning is required I have a hypothetical scenario for you all to consider. Please dont think I am trying to talk my self into ignoring your great advice, that comes from years of experience, but I am only trying to get a clearer understanding.

Here goes - if an angle tension brace or a plate tension brace was used in the same location interchangeably(is there such a word? anyway...)would both elements not be fabricated to the exact same length and bolt hole size and locations. Does this not mean both members would mobilize and start carrying load in a similar fashion - i.e. after the slack in the bolt holes is taken up. Also, in a tension only X brace system, be it angle or flat plate, would the "non load carrying element" simply buckle out of the way to allow the tension brace to mobilize and carry the load. And because the geometry of the plate or angle is exactly the same the buckling characteristic of either the angle or the plate would need to be the same to enable the tension brace to mobilize. Therefore any "flapping or slapping" that would occur with a flat plate would also necessarily need to occur with an angle brace as well. I suppose this could be overcome by bolting the X braces together where they cross each other.

Apologies for the long post but your comments on the above will be much appreciated.

 

[jike]

I developed a detail in the past with a 5"x5" tube with 1/2" gussets on each side that the flat braces would then get field welded to. In other words, a pair of x-braces, one on each side of the column. The column would be centered in a 6" metal stud wall, the 1/2" gussets would bring it out to the face and then the plate braces would go on the outside of the studs. I would then run furring channels at 16" oc across the studs (stopped at the braces) to provide a space for the flat plate braces. The sheathing then would fasten to the furring channels.

I would have one pretensioning device on one gusset plate.

Field weld the intersection of the braces, to prevent slapping.

 

[csd72]

Stillerz,

The wall can be isolated from the columns using compressible material and sliding brick ties.

shaheed,

You may be correct. I have actually used flat steel braces in the past with no pre-tension but I would be hesitant to do so any more due to the above stated reasons. I have seen these easily damaged and buckled out of shape on site giving even more slack.