ASCE 7-10 Wind Uplift (Roof Suction) 1

[SteelPE]

I am in the process of designing a single-story building that is 225’ x 200’ with 56’ x 40’ bays. Mean roof height = 30’-0”. The building is to be designed in accordance with IBC 2015/ASCE7-10. Wind speed is 130 mph. Monoslope roof with pitch = 1/4"/foot The building is to utilize steel decking, open-web steel joists and joist girders.

I am now in the process of trying to design the foundations. I need check the foundations on the interior footings for uplift due to roof suction. Section 30.2.3 of ASCE 7 says if the area of the component is greater than 700 square feet then you shall be permitted to design the member using the provisions of the MWFRS (interior footings are supporting 2,000+ square feet of roof).

So am I am to follow this correction I can use equation 27.4-1 using the roof pressure coefficients of figure 27.4-1?

This method seems to reduce the wind uplift loads significantly as you get towards the center of the building as Cp reduces from a maximum of -0.9 at the perimeter(0'-0" to 30'-0" in), to -0.5 (30'-0" to 60'-0"), to -0.3 in the center (60’-0” away from the edge of the building).

 

[SAIL3]

not sure a col/footing could be considered a component.....could be mistaken, though....the OP should check the definition of "component" to be certain...

 

[SteelPE]

The definition of Components and Cladding : Elements of the building envelope that do not qualify as part of the MWFRS
The definition of MWFRS: An assemblage of structural elements assigned to provide support and stability for the overall structure. The system generally receives wind loading from more than one surface.

So a footing in the middle of the warehouse only receives loading from one surface (the roof) and does not provide stability for the overall structure. So I wold think that a footing would comply for components and cladding.

That being said, the footings in question have a tributary area greater than 700 square feet and in accordance with section 30.2.3 would be designed using the MWFRS parameters and the question still applies.

 

[Ron]

SteelPE....interior footings can carry more than just a roof load. If they intersect any diaphragm action, they pick up a portion of the lateral load as well. In general, I consider footings to be primary structure, not components.

 

[SteelPE]

OK, I'm not sure I agree with that assessment, but never the less, what uplift loads would you use to size the footing (ie what section of the code). From what you are saying you would treat them as part of the MWFRS. So is my interpretation correct when looking at uplift on footings?

 

[JAE]

If the interior, non-lateral column/footing, is affected by less than 700 sf you use the C&C pressures based on its effective area.
If >700 sf then, as you stated above, you use the MWFRS wind.

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[SAIL3]

overall, I have always considered the primary difference between the catagory "component & cladding" and "primary" members ,at least in wind loading, as the latter being essential to the integrity of the structure while the former being of a sacrificial nature....

 

[gte447f]

SteelPE, I agree with your method to use MWFRS. Equation 27.4-1 and Figure 27.4-1 seem to me to be applicable. I guess there is some debate over whether or not a gravity column footing is a "component", but I agree with you that in your case it doesn't matter because of the 2,000 sf tributary area.

 

[Graham Knapp]

Yes this provision is allowing for the fact that the peak wind load will not affect the whole roof at any one time or for the same wind direction so any structural element which supports large areas of roofing can be designed assuming a lower load averaged over the whole area.

Wind Engineer
CSTB Nantes

http://recherche.cstb.fr/en/services/specialist-area/major-structures/

 

[a2mfk]

SteelPE - depending on where you are, a way to reduce mass concrete just for wind uplift resistance is to use a helical pile at each footing to supplement the dead load of the footing. They can be surprisingly inexpensive, especially with a big job like yours.

 

[SteelPE]

a2mfk,

That's not a bad idea, however if I started adding helical piles to project the client would not be very happy. Also, on this particular project, we have ledge issues. But it's still an idea I can bring up on future projects.

In the end I designed the footings using the MWFRS loading with Cp as shown in fiure 27.4-1. I ran the uplift in my modeling software as a fake snow load so I could track in down to the foundations.

 

[T_Bat]

I know I'm late to the party but I have usually always designed foundation uplift for MWF pressures. There may be some cases with some very small structures where I wouldn't. My thinking is that even if a column isn't part of LFRS (L for Lateral) - it will be part of VFRS (V for Vertical) for wind. So the main wind force in the vertical direction will have to be resisted by each individual column. Basically, every vertical element (column) is part of main wind force resistance in the vertical direction.

 

[Graham Knapp]

I know I'm late to the party but I have usually always designed foundation uplift for MWF pressures. There may be some cases with some very small structures where I wouldn't. My thinking is that even if a column isn't part of LFRS (L for Lateral) - it will be part of VFRS (V for Vertical) for wind. So the main wind force in the vertical direction will have to be resisted by each individual column. Basically, every vertical element (column) is part of main wind force resistance in the vertical direction.

Hi T_Bat. The 700sq ft cutoff is significant because the gusts in the atmosphere which generate loads on the building have a specific physical size. Small elements of cladding and roofing will be fully engulfed in the strongest local gusts, so you cannot apply the MWFRS provisions but should rather apply the higher cladding pressure value.
Merry Christmas to all !

Wind Engineer
CSTB Nantes

http://recherche.cstb.fr/en/services/specialist-area/major-structures/

 

[T_Bat]

I agree for elements such as roof beams, decking, ect. But for columns it seems reasonable to use MWF pressures. I know my explanation wasn't exactly clear on that. I'm only suggesting the logic from my previous statement applies to the columns...

 

[StrucDesignPE]

The definition of Components and Cladding : Elements of the building envelope that do not qualify as part of the MWFRS
The definition of MWFRS: An assemblage of structural elements assigned to provide support and stability for the overall structure. The system generally receives wind loading from more than one surface.

So a footing in the middle of the warehouse only receives loading from one surface (the roof) and does not provide stability for the overall structure. So I wold think that a footing would comply for components and cladding.

That being said, the footings in question have a tributary area greater than 700 square feet and in accordance with section 30.2.3 would be designed using the MWFRS parameters and the question still applies.

The C&C definition refers to "elements of the building envelope." While ASCE doesn't seem to directly define what the building envelope is, the commentary to ASCE 7-10 regarding the definition of C&C would seem to indicate that it consists of roof joists and decking, exterior wall framing, wall coverings, doors and windows, etc. As such, I don't think you can consider a footing to be an element that would meet the definition of C&C. If the footing had less than 700 sf tributary area, I think I would still design for MWFRS loads.

 

[JAE]

I think a lot of engineers get needlessly hung up on the "definition" of Components and Cladding.

ASCE 7 simply divides the two (MWFRS and C&C) into separate approaches due to their different uses in design - but it is the same wind that causes the forces for both.

The only difference between the two, technically, is that the MWFRS is a global average of forces, simplified a bit to allow the designer to design the large lateral system of the structure.
The C&C, meanwhile are for [red]ALL[/red] the other components of the structure. ALL of them.

Since some components receive wind from small areas - you get much larger peak loads on them vs. components that receive wind from larger areas. Thus the C&C effective area parameter.

Despite the misguided commentary in ASCE 7, where they talk about components receiving wind from "only one surface", etc. you simply use the C&C wind for ANY component with less than 700 s.f. of effective area. Any component with greater than 700 sf you CAN use the MWFRS load - or not - as they are almost the same value anyway at that point.

In this thread - the issue is an interior column/footing. If it is NOT part of the MWFRS then what is it? You would simply use the C&C pressure.

It's not that complicated.



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[abusementpark]

I really don’t think the code is clear on this. I asked my college professor (who was a wind expert and involved with ASCE 7 wind provisions) about this once and he said that if it receives load from more than one surface then it definitely can be considered MWFRS. He used a gable truss as an example for how some members of the truss would be designed for C&C and others would be designed for MWFRS. Seems like if the code intended for anything and everything with a wind area less than 700 square feet to be to designed for C&C, then it would plainly state that.

 

[JAE]

Well if anything and everything is NOT a part of the MWFRS then what else do you use BUT C&C.

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