Truss / Joist Girder Stability Bracing
Truss / Joist Girder Stability Bracing
(OP)
I realize there are other past threads that have attempted to address this topic, but I wanted to take the time to get some feedback on an item I am currently contemplating. I have attached a sketch to follow along with.
I currently have designed a couple of 120' clear span, 10' deep steel trusses for a heavily loaded roof. Typically I would assume the supported bar joists/metal roof deck would provide ample stiffness to laterally brace the truss. However, since this truss is 40' longer than any I have designed in the past, and the diaphragm depth is 1/3 of the truss span, I thought it would be worth while to look into this issue further to make myself more comfortable.
I originally assumed the bar joists at 5' o.c. braced the top chord of the truss, thus making my Ly = 5'. While summing the brace forces (0.004*chord force at each joist - treated as column per App. 6) required along the truss at the bar joist locations (see 2nd page of attachment), I came up with a total lateral brace force on the diaphragm of 76.6k, which puts 38.3k in my end walls. With only a 40' diaphragm depth, this requires the decking to resist 958 plf shear for D+L stability bracing. Obviously, this seems ridiculous.
So my first red flag was is the 0.004*Pr required to be calculated at each brace point? Or is this force taken at the maximum chord force and then distributed amongst the brace points? The second item involves the LFRS. Is the diaphragm required to be designed to resist these cumulative loads and therefore distribute to the LFRS? Or is this the purpose of the required brace stiffness criteria listed in Appendix 6 and no further assessment of the load transfer is required?
In moving forward, I decided to be conservative and utilize brace trusses at the quarter and mid-span locations of the truss (Ly now is 30' - see 1st page of attachment) in order to significantly reduce the lateral stability forces transmitted to the diaphragm. While this makes the D+L forces in the diaphragm fairly negligible (150 plf), it obviously makes the compression chord of the truss much larger in size. While this is probably viewed as very conservative, I felt comfortable and moved on.
To complicate matters, during the bidding phase, I had steel fabricators request that a joist girder be explored as an alternate to the custom truss I had designed to try to cut cost. I had considered this early on, but thought the span/depth and loads would not make this a viable option. After discussions with Vulcraft, I had asked about their assumptions regarding compression chord bracing. As I suspected, they assume each bar joist braces the truss. When asked how they account for the brace forces transmitted to the bar joists and beyond, they were not able to really give me an answer other than it's just assumed to work, which did not make me feel very comfortable.
In conclusion, I'm hoping to get feedback for:
1) How do you interpret the requirements listed in AISC Spec Appendix 6 in regards to placement/distribution of the brace loads, and cumulative effects on the diaphragm?
2) Dealing with joist girders/open web joist bracing?
I currently have designed a couple of 120' clear span, 10' deep steel trusses for a heavily loaded roof. Typically I would assume the supported bar joists/metal roof deck would provide ample stiffness to laterally brace the truss. However, since this truss is 40' longer than any I have designed in the past, and the diaphragm depth is 1/3 of the truss span, I thought it would be worth while to look into this issue further to make myself more comfortable.
I originally assumed the bar joists at 5' o.c. braced the top chord of the truss, thus making my Ly = 5'. While summing the brace forces (0.004*chord force at each joist - treated as column per App. 6) required along the truss at the bar joist locations (see 2nd page of attachment), I came up with a total lateral brace force on the diaphragm of 76.6k, which puts 38.3k in my end walls. With only a 40' diaphragm depth, this requires the decking to resist 958 plf shear for D+L stability bracing. Obviously, this seems ridiculous.
So my first red flag was is the 0.004*Pr required to be calculated at each brace point? Or is this force taken at the maximum chord force and then distributed amongst the brace points? The second item involves the LFRS. Is the diaphragm required to be designed to resist these cumulative loads and therefore distribute to the LFRS? Or is this the purpose of the required brace stiffness criteria listed in Appendix 6 and no further assessment of the load transfer is required?
In moving forward, I decided to be conservative and utilize brace trusses at the quarter and mid-span locations of the truss (Ly now is 30' - see 1st page of attachment) in order to significantly reduce the lateral stability forces transmitted to the diaphragm. While this makes the D+L forces in the diaphragm fairly negligible (150 plf), it obviously makes the compression chord of the truss much larger in size. While this is probably viewed as very conservative, I felt comfortable and moved on.
To complicate matters, during the bidding phase, I had steel fabricators request that a joist girder be explored as an alternate to the custom truss I had designed to try to cut cost. I had considered this early on, but thought the span/depth and loads would not make this a viable option. After discussions with Vulcraft, I had asked about their assumptions regarding compression chord bracing. As I suspected, they assume each bar joist braces the truss. When asked how they account for the brace forces transmitted to the bar joists and beyond, they were not able to really give me an answer other than it's just assumed to work, which did not make me feel very comfortable.
In conclusion, I'm hoping to get feedback for:
1) How do you interpret the requirements listed in AISC Spec Appendix 6 in regards to placement/distribution of the brace loads, and cumulative effects on the diaphragm?
2) Dealing with joist girders/open web joist bracing?
Nick Deal, PE, SE
Michael Brady Inc.
http://www.michaelbradyinc.com






RE: Truss / Joist Girder Stability Bracing
Basically, brace forces are not cumulative (they do not need to be resisted by the LFRS) because the truss buckles in a sine curve between brace points so that the brace forces are opposite of each other at each point. The induced diaphragm shear is also not cumulative, it is a panel by panel phenomenon as addressed in the paper. You also need to check the stiffness provided by the diaphragm - again as addressed in the paper.
RE: Truss / Joist Girder Stability Bracing
I am looking at a couple of roof beams that I will have to design as drag struts and I was just thinking about how to design it for compression and what kind of bracing I could assume from the joists/deck. This is perfect. Of course, I also have to consider bending stresses, but for a built up truss you have to consider that too in the top chord- bending between the webs plus axial, correct? (I have never designed a large truss like this before..)
RE: Truss / Joist Girder Stability Bracing
RE: Truss / Joist Girder Stability Bracing
It also assists in erecting the trusses without disastrous results. Trusses are very unstable until all the bracing has been installed. Collapse during erection is not uncommon with trusses of this span.
While erection may be the responsibility of the erector, I would encourage him to connect the trusses to the horizontal braced frames on the ground, then lift the assembly into place and connect to the columns before attaching open web steel joists.
BA
RE: Truss / Joist Girder Stability Bracing
RE: Truss / Joist Girder Stability Bracing
I understand your point concerning the sine wave mode of failure, which would indeed cancel out any cumulative forces at the diaphragm edge. However, for this to occur, wouldn't the brace points have to be assumed to be rigid out of plane? It seems to me you would still have to design the diaphragm/horizontal truss to account for these internal shear forces.
I keep going back to pre-engineered wood/metal truss bracing scenarios. For bottom chord uplift bracing, it is common to run rows of lateral bracing when no ceiling diaphragm is present. The brace forces that accumulate in the lateral braces are distributed to the supporting walls with a bay of horizontal diagonal bracing every 4th to 5th truss. The forces developed in the horizontal diagonal bracing are typically cumulative.
I think you would also want to consider a failure mode in which all the trusses would buckle in the same direction at mid-span. It seems this would put a lot of force into the diaphragm that would not cancel out.
BARetired,
I plan to keep the brace trusses at the quarter and mid-span to provide torsional restraint and to brace the bottom chord to meet tension slenderness requirements and against compression buckling under uplift loads.
In regards to the metal deck diaphragm, I don't have a problem using it in conjunction with the brace trusses/bar joists. I just want to be clear on how much load to induce into it. Using an 18 gage deck with decreased spacings on the puddle welds and sidelap fasteners, you can get shear values that exceed 1000 plf.
Nick Deal, PE, SE
Michael Brady Inc.
http://www.michaelbradyinc.com
RE: Truss / Joist Girder Stability Bracing
RE: Truss / Joist Girder Stability Bracing
Otherwise, where do you stop? Joist buckling forces? Joist girder buckling forces? Top of column buckling forces?
Don't overthink this one.
DaveAtkins
RE: Truss / Joist Girder Stability Bracing
Here is the thread:
thread507-286765: unbraced length question
I will attach the notes that were posted. Hopefully they will help.
EIT
RE: Truss / Joist Girder Stability Bracing
One way I thought about it to make myself more comfortable about not applying the brace forces to the diaphragm was to think about the chord bracing in-plane with the truss. In this axis, you utilize the vertical/diagonal web members as braces, but you do not add additional loads into these elements, thus increasing the load on the truss.
On the flip side, I had posed this question to AISC a few days ago and finally got a response today. I have included a snippet of the response below.
"...something ultimately needs to resist the brace force and this would most likely be the LFRS."
The rest of the response was pretty vague and really didn't lead me either way on whether or not to accumulate these loads into the diaphragm.
Thanks everyone for your responses so far on an issue that leaves a lot to judgment and interpretation.
Nick Deal, PE, SE
Michael Brady Inc.
http://www.michaelbradyinc.com
RE: Truss / Joist Girder Stability Bracing
Good link...Thanks!
Nick Deal, PE, SE
Michael Brady Inc.
http://www.michaelbradyinc.com
RE: Truss / Joist Girder Stability Bracing
How would you calculate the global lateral buckling resistance?
I've seen this article referenced before:
http
How did you decide to design the system?
EIT