Estimation of Forces In "Tension Only Bracing" 3

[mjohan]

I have a technical question with regards to calculating the compression forces in columns at "tension only bracing" intersections.

The bracing is for townhouses with steel stud walls. They are straps, welded at the intersections, and tack welded to each stud. For simplicity, I would like to give a very generic loading schematic.

3rd floor -------- 3 kip

2nd floor -------- 2 kip

1st floor -------- 1 kip

Floor to floor spacing is 10'
Length of wall is 20'

I would like to know what an expected compression load (in the column at the x braced location, @ first floor) would be due to the lateral load?

I have had suggestions to estimate by two methods. Summing the moments, and sum of forces only. Summing the moments (analyzing the shear wall with overturning at the base), from my estimation yields a compression force of 7 kips, while summing the forces at each floor would give 3 kips.

I appreciate your comments.

 

[JAE]

Take another look at your 3kip method. I think that if you simply take each level's lateral force (3k, 2k, 1k) and "sum F's and M's" at each level, independent of the other levels....you get a total of 3 kips added up down the columns.

But....keep in mind that there is a tension force in the top brace, that "pulls" on the second floor beam, adding to its total pull...etc.

In other words, your floor shears at each level are:

3rd level: 3 kips
2nd level: 3+2 = 5 kips
1st level: 5+1 = 6 kips

Axial force in column 2-3: 1.5 kips (tan 26.56 x 3)
Axial force in column 1-2: 2.5 kips (tan 26.56 x 5)
Axial force in column 0-1: 3.0 kips (tan 26.56 x 6)

Total force = 7 kips....not 3.

 

[mjohan]

Thank you JAE.

My confusion is related to the top chord. My understanding, or lack of, is that the strap bracing was achieved without designing a top chord for the forces.

I have asked a panel manufacturer how he would classify the system of strap bracing and his response was rather vague. My intent was to supply him with seismic loads at each level. In order for me to do that, I needed an "R" value. So I read him TABLE 1617.6 and 1A and 1L seemed to be the most appropriate. His answer to me is that he would not consider strap bracing as a braced frame. So that left me with "light frame walls with shear panels - all other materials" (R = 2).

Utilizing the top chord, to me, seems like a braced frame. Is it possible to use just the straps, to transfer a compression or tension force to the columns by the tension in the straps alone?

 

[VoyageofDiscovery]

Hi mjohan,

In order to have "tension-only" forces in a diagonal, typically the top chord (horizontal) must be designed for compression from the floor shear. If your floor shear transfers to each column some other way then this would not be the case.

Regards

VOD

 

[JAE]

VOD is right from my view. You have to have a complete system (including your horizontal collector) participating in the system for it to be a braced frame.

Also, in higher seismic zones, it is not appropriate to use a tension-only system.

 

[mjohan]

I had asked the panel manufacturer if the top chord of the wall was designed for compression forces. He said it was not. I then asked if that is why he did not feel the system could be classified as a braced system, he said yes.

Maybe I am not understanding your response. I am interpreting your answers as the following: to have a tension only bracing system, their must be a top chord, therefore making it a braced system. But if it is a braced system, an R value of 4, in lieu of 2, "IBC 2000" may be used.

I will try to explain the strapping and column system I have seen.
1) A flat strap, say 6" wide x 14 gauge is welded to grouped studs at each end.
2) They are diagonal straps.
3) The grouped studs extend through the floor system to the top of the concrete slab.
4) The concrete slab is poured around the column.
5) The only transfer of lateral force (that I can see) is from the surface area of the grouped studs to the concrete slab.
6) This detailing repeats floor to floor.

"If your floor shear transfers to each column some other way then this would not be the case." Do you think that this would qualify as an alternative way? And if so, would summing the vertical components at each strap be correct in predicting the compression force in the column?

If an R value of 4 was used in lieu of 2, the compression force would be half, yielding 3.5 kips by sum of the moments. Where as using R = 2 yields a compression force of 3 by summation of forces only.

 

[VoyageofDiscovery]

Hi mjohan,

It appears that the original designer accounted for diaphragm action in each floor level to transfer the loads to the columns. This is also done. However the strapping detail is new to me. I would check the strapping for shear across its own section at the bottom of the concrete slab as this may be its weakest area.

This detail seems hokey to me and I wonder if it has been tested to see if it really performs for the design loads. Studs welded to 14 gauge steel will present its own problems as the weld integrity is questionable and cannot be expected to yield for earthquake. One would not know if it is still intact after a light earthquake since it is embedded in the slab.

Regards

VOD

 

[JAE]

mjohan:
The use of straps in light gage metal framing (that is the system, right?) was actually prohibited by the Corps of Engineers some years ago due to the "hokey" nature of the detailing, the fact that no competent body had produced any standards or design criteria, and the fact that the Corps had a project (in Kansas City, I believe) where strapping was used in a very poor manner.

The straps extended down to the base of the wall and were simply screwed to the bottom track section flanges. So no complete load path was accounted for.

There have been some efforts at researching alternative details - which I've seen and seem to match what you are talking about. These involve a large, flat strap that is screwed to a bundled set of columns - but this was a specifically tested system.

See if you can get a hold of the following document:

Design of Cold-Formed Load Bearing
Steel Systems and
Masonry Veneer / Steel Stud Walls
TI 809-07
I think you can find it here:

http://www.hnd.usace.army.mil/techinfo/ti.htm

This document had a chapter on diagonal strapped shearwalls and should be very helpful to you.

But your individual cannot state that no compression is taken by a collector along the floor level. He cannot simply dismiss basic statics. A diagonal strap in tension pulls on the bundled column and this diagonal force must be resisted by a horizontal component...the force in the edge track, beam, or whatever along the floor.

So this horizontal component is added to that stories base shear and the force builds up as you work down the floors. You keep referring to a difference in the sum of moments vs. sum of forces. My post above showed that with a proper consideration of the load path, there is no difference.

 

[mjohan]

Thank you both for your input!

JAE, I have printed a copy of that document. Many thanks for that one.

 

[jheidt2543]

Interesting thread, I'm not very familiar with steel stud wall design, but know that the American Iron and Steel Institute (AISI) publishes a "Cold-Formed Steel Design Manual" that certainly must cover this. I know I have seen this detail used quite often in projects that provided shop drawings and engineering calculations, I will try to find some documentation.

You might try contacting AISI at their website for more information, sss.steel.org

 

[jheidt2543]

Interesting thread, I'm not very familiar with steel stud wall design, but know that the American Iron and Steel Institute (AISI) publishes a "Cold-Formed Steel Design Manual" that certainly must cover this. I know I have seen this detail used quite often in projects that provided shop drawings and engineering calculations, I will try to find some documentation.

You might try contacting AISI at their website for more information,

http://www.steel.org

(sorry about that slip of the finger)

 

[bjb]

The Light Gauge Steel Engineers Association has a technical note for Design Values for Vertical and Horizontal Lateral Load Systems, where they briefly discuss the design of flat strap bracing. Both the LGSEA and the IBC have guidelines for the design of the strap connections, which basically require either designing for the amplified seismic load or the tensile strength of the member, whichever is lower. I have seen this detail used a lot for load bearing light gauge steel in upstate NY, and have not heard of any problems with it. Of course, we are not an area of high seismic activity. Regarding quality control, the construction of diagonal strap bracing will be covered by the special inspection requirements of the IBC (if that code applies for you), so shoddy construction shouldn't be allowed to remain. I think that if the welding procedures outlined in the AWS for sheet steel are followed and you determine the forces using basic statics, you should be fine.