(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!!

 

[Lion06]

I would use 4).

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

 

[PostFrameSE]

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.

 

[SteelPE]

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.

 

[Stillerz]

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

 

[ron9876]

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

 

[PostFrameSE]

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???

 

[WillisV]

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.

 

[Stillerz]

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

 

[Stillerz]

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.

 

[Stillerz]

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.

 

[Lion06]

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.

 

[Stillerz]

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

 

[PostFrameSE]

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.

 

[Stillerz]

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.

 

[JAE]

#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.

 

[Stillerz]

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./

 

[Lion06]

Stillerz-

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

 

[BAretired]

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

BA

 

[ash060]

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.