TTUengr51
Structural
- Jul 19, 2005
- 63
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?
Nick Deal, PE, SE
Michael Brady Inc.
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.