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(2) C & C questions

(2) C & C questions

(2) C & C questions

(OP)
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!!

RE: (2) C & C questions

I would use 4).

Why wouldn't you use the L*L/3 for the roof as well?

RE: (2) C & C questions

(OP)
Thanks StructuralEIT,

If everybody agreed that #4 was the way to go, then applying that same logic on the roof would basically achieve what I'm after.  The effective tributary width of of using L*L/3 is 6'-8" o.c. with 24" o.c. purlins spanning 20'.

Would everybody concur with StructuralEIT that #4 is the correct trib area for determining the external pressure coefficients, regardless of how many interior supports a particular girt or purlin may have?

Thanks.

RE: (2) C & C questions

I would agree the #4 is correct for design of the girt itself.  However, when sizing you connection you will have to increase your loads based off the smaller tributary area to the connection.

As far as you roof question goes, as long as you are considering wind you would calculate your tributary area the same way.
 

RE: (2) C & C questions

where can i find this L * L/3 provision?  

RE: (2) C & C questions

#3--span squared/3=effective area. If you had a five span continuous beam you wouldn't use the length of five spans.

RE: (2) C & C questions

(OP)
Stillerz, it is found in the definition of "Effective Wind Area" in Chapter 6 of ASCE 7.

Now we have ron9876 offering a differing viewpoint to StructuralEIT.  Any other thoughts?  Anybody have any documentation to support why it should be looked at like either of these responders?  

ron9876 notes that a five-span beam shouldn't use the length of five spans to determine the trib area.  That makes sense.  I can also understand StructuralEIT's thoughts about using the entire member, even if it has more than one span, but five spans feels like too much.  Any other thoughts on any of this???

RE: (2) C & C questions

My thought is that the wind doesn't know how many times the beam is supported below.  Use the full length of the beam times either its trib width, or length/3.   

RE: (2) C & C questions

i see...you are only trying to determine Gcp...not acutally designing the member for that trib area...correct?  

RE: (2) C & C questions

the definition here seems to be flawed...
You'd think it would be worded "...SHALL not be less than one-third the span length..." rather than "...need not be..."
Goofy.  

RE: (2) C & C questions

still, i say #3 is correct....the span is what it is = 10'-0"
If you have a multi-span beam continuous over supports you dont consider the beams full length as the "span". I'd say "span" is the distance between supports.  

RE: (2) C & C questions

I think "need not" is fine.  That gives the engineer the option to be conservative and use the actual trib width instead of L/3.

If you have a single girt spanning 25', does it matter if it's simply supported or if it have (5) 5' continuous spans?  I don't think so.

RE: (2) C & C questions

so, if you were to design a girt as simply supported at the ends and continuous over 4 interior supports that would the same as designing a single span simply supported beam?
i'm lost.
maybe i should take statics again

RE: (2) C & C questions

(OP)
Good arguement Stillerz.  Good point too WillisV regarding the wind not knowing how many times a girt is being supported.

As much as I'd like to take the full beam length, I think I have to conclude that the span is the distance between supports, not the entire length of the beam...........unless someone else can provide a compelling reason to use the full length.

Thanks all.   

RE: (2) C & C questions

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.  

RE: (2) C & C questions

#3 is correct in my view.

Stillerz - the NEED NOT is correct in that the code is saying you don't have to use a smaller area (thus a larger C&C load) if it gets less than L/3 in trib width.

 

RE: (2) C & C questions

Also very important is that in the begining stages of desing you may not know your actual trib and sometimes girt spacings vary ...therefore the L/3 becomes convenient when working up wind profiles in the calc stage of the game./  

RE: (2) C & C questions

Stillerz-

I meant that only for determination of wind loads, not for designing the member.

RE: (2) C & C questions

For girt design, the tributary area is 1) i.e. 10' x 2' = 20 SF.   

BA

RE: (2) C & C questions

If you use the whole length of the member to get the Cp, what do you use for the connection?

If it is one member do you design the connections for the trib area of the connection or do you design it with the reactions obtained from the analysis of the member?

Because one would result in higher connection loads than the other, but if you design the connection for the higher reaction and the member itself can not put that load into the connecting elements what is the point of designing for the higher connection force.

Or would you design the member so that it can withstand the reaction, which in turn means using a larger Cp coefficent.

RE: (2) C & C questions

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?

RE: (2) C & C questions

ash060....
good  

RE: (2) C & C questions

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

RE: (2) C & C questions

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.

RE: (2) C & C questions

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".

RE: (2) C & C questions

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.

RE: (2) C & C questions

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

RE: (2) C & C questions

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?

RE: (2) C & C questions

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

RE: (2) C & C questions

That makes sense to me, johnycash.

BA

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