(2) C & C questions

[PostFrameSE]

When calculating the tributary area of a girt for the determination of the appropriate external pressure coefficient, I'm wondering do I use the entire length of a multi-span girt or do I look at the tributary length as though it was from support to support? In other words, if I had 10' o.c. columns with a 20' girt spaced at 2' o.c., would my trib area be?:

1) 20sf - 10' x 2'
2) 40sf - 20' x 2'
3) 33.3sf - 10' x 10'/3
4) 133.3sf - 20' x 20'/3

Number 3 and 4 of course are using the provision of 1/3 the span length.

Also, for purlins spaced 24" o.c. on a roof structure with corrugated steel sheathing attached to each one, is it possible to take the tributary width as something greater than the spacing of the girts due to the load-distributing steel sheeting?

Thank you!!

 

[Stillerz]

EIT:
That would imply that "span" has multiple definitions. If you had a bridge 1000' long and had 2 interior equally spaced suports, do you call that a 1000 foot span bridge? Or, do you call it a 1000 foot long bridge with 333' spans?

 

[Stillerz]

ash060....
good

 

[Stillerz]

it is a good thing they can't roll or ship channels 1000' feet long, because the trib width would be 333' resulting in an effective wind area of 333,000 sq. ft. or roughly 8 acres.
I kid

 

[Lion06]

I just read through the commentary for ASCE 7-05. Before I read it, I thought it was going to change my mind. After reading it, however, I believe that option 4 is correct. page 281 (The second full paragraph on the right) says, "..The area served by the component may become long and narrow. To better approximate the actual load distribution in such cases, the width of the effective wind area used to evaluate GCp need not be taken as less than one third the LENGTH of the AREA."

To me, that says that if you have a girt that has 4 supports, the length of that area = the length of that member = the length of all (3) spans, and that the effective width (only for calc'ing GCp) is one third of that total length.

 

[Lion06]

ash-

ASCE 7 is pretty clear that the connections/fasteners should be looked at with a different GCp. That is regardless of where you come down on this issue. The reason has only to do with the total effective area and the "evening out" of higher localized wind pressures. If you have an effective area of 500 sq ft, the chances of all of that area seeing the highest wind pressure is low. As that area decreases (from C&C member design to C&C fastener design) that probability increases and you have to account for it.



Stiller-
Point taken about the use of the word "span". That is actually why I thought the commentary would change my mind. It is curious that the commentary uses "length" of area, and chapter 6 uses "span".

 

[Stillerz]

EIT:
That's why i had this commentfrom before:

my guess is, for components and cladding, since they have lesser stiffness compared to the MWFRS, the l/3 provision is there to be sure the trib is not to small ONLY in determining GCp.
For design, is still say trib. width is trib width.

Basically, if the girt undergoes "relatively" large deflections it would follow that the trib width would become larger due to relative stiffness.....HOWEVER, that does not change the span as noted in my smart-assed 1000 foot long girt comment.

 

[Stillerz]

If the area goes over 700 sq ft and you can use MWFRS provisions anyway.

 

[PUEngineer]

I know it's hard to do because sometimes it seems there is no logic, but what if you think about the intent of the code. The reason for the trib areas and it's effect on the pressures. The smaller the trib area the greater the pressure.

I'm not sure I have an opinion on which option is better, so I would just use the span over three times the span of the member to get the worst pressure, excluding using the actual trib width. I guess if it were a continuous multispan memeber then you could use the whole length of the member and apply that load to each span of the member, then run the loads for smaller trib and just apply that to one span of the continuous member. Sort of patch load like. I don't do much girt design so I don't have a gut feel for what load cases control the design of girts.

Was that confusing?

 

[bootlegend]

The MBMA Metal BuildingSystems Manual uses each individual span length and not the total member length in the example problems. FYI.

 

[BAretired]

That makes sense to me, johnycash.

BA