Let's take our example as a column in the middle of a one-story building, say the size of a grocery store. It supports girders that support roof joists. It has assumed pinned ends and is not part of a moment frame, braced frame or shear wall. In other words it resists only gravity and uplift from the roof, no lateral forces. It's tributary area is, say, 400 square feet. Do you use MWFRS pressures or C&C? I say it is C&C but some in my office including my boss say to use MWFRS. It's tributary area is less than the 700 square feet that the ASCE 7 says is ok to use MWFRS pressures. What are your opinions on this, what pressures would you or do you use? This is purely a made up example though I have been in conflict with those in my office about this point before with similar set-ups. My logic is it does not provide stability to the structure as a whole by taking the main wind forces and therefore is not main wind. The opposing logic is it does not take wind forces directly but receives it from other members and therefore is main wind.
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Is it MWFRS or C&C? 1
- Thread starter UcfSE
- Start date
structuresguy
Structural
I would design this column as main wind force, no question in my mind. By the C&C logic of your description, then even the footings would have to be designed for C&C. I find the code is quite clear actually, though I have also had these discussions in my office. If it takes wind directly (cladding), or supports the cladding (component - girts, purlins, bar joists, etc) then it is C&C. If it supports the components, then it is MWFRS. Otherwise, everything in the structure eventually could be considered C&C.
By stability, you say it does not contribute. Well, maybe not to lateral stability, but certainly to uplift and gravity stability it does. If you remove the column, a very large section of roof will be comprimised, as there would be no support for the girders and joists and decking, etc... MWFRS is more than just lateral resisting system.
Of course, some can be both, depending on direction or type of loading. Load bearing masonry walls are a perfect example of C&C for out-of-plane bending and MWFRS for in-plane shear.
By stability, you say it does not contribute. Well, maybe not to lateral stability, but certainly to uplift and gravity stability it does. If you remove the column, a very large section of roof will be comprimised, as there would be no support for the girders and joists and decking, etc... MWFRS is more than just lateral resisting system.
Of course, some can be both, depending on direction or type of loading. Load bearing masonry walls are a perfect example of C&C for out-of-plane bending and MWFRS for in-plane shear.
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- #4
ASCE 7 provides some good definitions in the commentary (Section C6.0 of ASCE7-02) where it does make a distinction between MWFRS and CC based on function of the element verses a purely statistical basis relative to tributary area. So any part of a structure that serves to translate lateral wind to the foundation is part of the MWFRS and the MWFRS loads should then be used regardless of tributary area.
The wind loads are generated numerically based on tributary area (small areas have high spikes in wind pressure which requires high pressures), but ASCE7 then separates the elements based on function, ASSUMING that all MWFRS elements are taking loads from large tributary areas.
For your column, I would not use MWFRS unless the At got large. The column is an element, a component, and not part of the lateral bracing system. However, as At values get large, the CC pressures approach the MWFRS pressures anyway.
The wind loads are generated numerically based on tributary area (small areas have high spikes in wind pressure which requires high pressures), but ASCE7 then separates the elements based on function, ASSUMING that all MWFRS elements are taking loads from large tributary areas.
For your column, I would not use MWFRS unless the At got large. The column is an element, a component, and not part of the lateral bracing system. However, as At values get large, the CC pressures approach the MWFRS pressures anyway.
- Thread starter
- #5
I appreciate your responses and am glad to see some agree with me. I definitely understand structuresguy's point also.
When reading the definition of MWFRS in the ASCE 7 it states, "The system generally receives wind loading from more than one surface." Does that mean more than one surface of the building, like north and south walls or two walls and the roof? It would seem that by that interpretation of the definition that there is no doubt as to whether my example is C&C, not considering the tributary area. Still, the definition is not worded so that it makes a clearly defined statement that is not open to interpretation. The term "generally" for instance does't clearly define what is and is not main wind.
When reading the definition of MWFRS in the ASCE 7 it states, "The system generally receives wind loading from more than one surface." Does that mean more than one surface of the building, like north and south walls or two walls and the roof? It would seem that by that interpretation of the definition that there is no doubt as to whether my example is C&C, not considering the tributary area. Still, the definition is not worded so that it makes a clearly defined statement that is not open to interpretation. The term "generally" for instance does't clearly define what is and is not main wind.
Try this link to a paper written on wood design which discusses this subject.
Yes, I know that, but FSS was claiming that since the column took loads (gravity) from multiple other elements, the column should therefore be designed with MWFRS wind loads...
The decision between MWFRS and CC wind is based on whether the element is part of a lateral system bracing the structure against wind and taking wind from other pieces, not whether its part of a structure taking gravity loads from other pieces.
An interior column as described by UcfSE is not part of the MWFRS.
The decision between MWFRS and CC wind is based on whether the element is part of a lateral system bracing the structure against wind and taking wind from other pieces, not whether its part of a structure taking gravity loads from other pieces.
An interior column as described by UcfSE is not part of the MWFRS.
Our company's approach is to apply MWFRS winds to the column if the roof members that it directly supports are designed for MWFRS winds.
Is the column design really going to be controlled by the tension case due to uplift? Seems we might be aggravating over a case that is unaffected by the decision.
Is the column design really going to be controlled by the tension case due to uplift? Seems we might be aggravating over a case that is unaffected by the decision.
I just wanted to point out that the uplift loads on the column will be transferred from the roof joits and trusses, through the girder and finally to the column. This, in my opinion, means that the column should be designed for MWFRS not C & C.
The code says that MWFRS is " 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".
Although this isn't the clearest definition, I think that this puts a column supporting roof trusses and a roof girder as MWFRS.
One rule of thumb I use is to detrmine if failure of this member woild significantly affect the overall structure. If the girts (C&C) fail, the overall structure is still stable even if the wall sheeting might collapse. If this column should fail, there will be more than a local effect. Hence MWFRS.
Thanks.
The code says that MWFRS is " 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".
Although this isn't the clearest definition, I think that this puts a column supporting roof trusses and a roof girder as MWFRS.
One rule of thumb I use is to detrmine if failure of this member woild significantly affect the overall structure. If the girts (C&C) fail, the overall structure is still stable even if the wall sheeting might collapse. If this column should fail, there will be more than a local effect. Hence MWFRS.
Thanks.
To clarify my earlier post, I was basing my opinion on uplift loads coming from multiple members.
Also, for those who have not taken the ASCE 7 Wind Load seminar, I highly recommend it. This is one of the questions that comes up and the guys with their names in the standard are the ones who teach it.
Also, for those who have not taken the ASCE 7 Wind Load seminar, I highly recommend it. This is one of the questions that comes up and the guys with their names in the standard are the ones who teach it.
Note that ASCE 7-02, in the commentary, states that some components may act as both MWFRS elements and C&C elements. Therefore, the engineer may have to check multiple loadings for a single element to determine the worst case.
Also note, that most building codes require that all elements that are part of a lateral system be designed and detailed for seismic effects even if seismic does not govern. Therefore, it may not be prudent to define all members as part of the lateral system due to the detailing required. Please note that special inspections may also be required if the member is considered part of the Lateral system.
In the case of an exterior column that is not directly defined as part of the lateral system, check the column for MWFRS loads when you are analyzing the structure for overall lateral stability. These loads may cause p-delta effects in the column that would cause global and local instabilities. In addition, check the column for C&C load based on the trib area. This will show local instabilities and deficiencies.
With a computer modeling program, it is faily easy to do, however the wind load portion of your load combinations just became huge.
Good Luck.
Also note, that most building codes require that all elements that are part of a lateral system be designed and detailed for seismic effects even if seismic does not govern. Therefore, it may not be prudent to define all members as part of the lateral system due to the detailing required. Please note that special inspections may also be required if the member is considered part of the Lateral system.
In the case of an exterior column that is not directly defined as part of the lateral system, check the column for MWFRS loads when you are analyzing the structure for overall lateral stability. These loads may cause p-delta effects in the column that would cause global and local instabilities. In addition, check the column for C&C load based on the trib area. This will show local instabilities and deficiencies.
With a computer modeling program, it is faily easy to do, however the wind load portion of your load combinations just became huge.
Good Luck.
- Thread starter
- #15
Looking at the definition about receiving wind load from more than one surface, it would seem that receiving load from only one surface, such as the roof, would classify the member as C&C. One than one surface does not imply more than one member or a system of members.
It does seem like an academic point when you consider that the column isn't designed by tension anyway. Here in Florida, particularly south Florida the roof pressures can get very high. Making a distinction between MWFRS pressures and C&C pressures has a big impact not so much on the column but rather on the base plate, anchor rods and especially the footing size as well as other connections. For one project in south Florida I worked on a while back the C&C roof uplift was about 45 psf versus about 25 psf for MWFRS (both zone 1). That's a huge difference in footing size after you add the 1.5 factor of safety.
It does seem like an academic point when you consider that the column isn't designed by tension anyway. Here in Florida, particularly south Florida the roof pressures can get very high. Making a distinction between MWFRS pressures and C&C pressures has a big impact not so much on the column but rather on the base plate, anchor rods and especially the footing size as well as other connections. For one project in south Florida I worked on a while back the C&C roof uplift was about 45 psf versus about 25 psf for MWFRS (both zone 1). That's a huge difference in footing size after you add the 1.5 factor of safety.
Being in Florida UcfSE - were there lots of building failures during the hurricanes due to uplift on a column/footing?
Generalizing here, but all the failures were probably due to the loss of cladding and/or poor details. I'm a big fan of making realistic decisions. Realistically (going to invoke my engineering judgment as I am wont to do) if the design of the column/footing/connections has been addressed and with proper details it's not going to fail if it has designed for MWFRS loads rather than C&C loads.
Following with some of your example numbers (combining from a couple different posts) and making one up:
If your column has At=400 sf @ 45 psf (C&C)=18 kips uplift. An adjacent column in the same building has At=720 sf @ 25 psf (MWFRS)=18 kips uplift. So the foundations would be designed for the same uplift value? While that may be completely defensible according to the reference - guess intuitively I can't help but have a problem with that.
Generalizing here, but all the failures were probably due to the loss of cladding and/or poor details. I'm a big fan of making realistic decisions. Realistically (going to invoke my engineering judgment as I am wont to do) if the design of the column/footing/connections has been addressed and with proper details it's not going to fail if it has designed for MWFRS loads rather than C&C loads.
Following with some of your example numbers (combining from a couple different posts) and making one up:
If your column has At=400 sf @ 45 psf (C&C)=18 kips uplift. An adjacent column in the same building has At=720 sf @ 25 psf (MWFRS)=18 kips uplift. So the foundations would be designed for the same uplift value? While that may be completely defensible according to the reference - guess intuitively I can't help but have a problem with that.
- Thread starter
- #17
You would have to understand that wind pressures are higher over a smaller area because of the nature of the wind itself. That seems odd intuitively but wind is higly localized in reality and certainly does not behave statically nor does act with a single average value over an area like we assume. The hurricanes that passed through here recently were not strong enough to be design storms except maybe in some local areas near the coast. In Orlando for instance we saw winds in the range of 90-100 pmh for the most part with some gusts a little higher but nothing around the 120 mph we design with. In that case I would not expect failure of footings nor significant uplift on them but rather failure of poorly designed/constructed cladding. There are lots of shingles missing ;-)
I definitely understand what you are saying and have heard it from a few others as well, I just don't buy into it. I'm not comfortable with letting something slide in a manner of speaking because something else should fail first. If I did my job right the cladding and everything leading to the footing won't fail and instead will allow the footing to do its job. If it isn't constructed right I can't help that. You are right though, a footing designed with MW is not likely to fail. Then again, a steel beam with 20% overstress isn't likely to fail either, but that's not ok right?
I definitely understand what you are saying and have heard it from a few others as well, I just don't buy into it. I'm not comfortable with letting something slide in a manner of speaking because something else should fail first. If I did my job right the cladding and everything leading to the footing won't fail and instead will allow the footing to do its job. If it isn't constructed right I can't help that. You are right though, a footing designed with MW is not likely to fail. Then again, a steel beam with 20% overstress isn't likely to fail either, but that's not ok right?
I'll keep plugging my opinion into this...I think we are making this way more complex than it needs to be.
MWFRS
This is the MAIN system of members that work together to brace the building against wind loads in an overall sense....the diaphragm of the roof or floor, the collector elements, and the columns and beams that serve to make up either moment frames or braced frames that laterally brace the whole structure.
C&C
This is a series of wind loads (not "a" wind load) but a series depending on tributary area. Any part or portion of the structure that resists wind from a limited area must be designed for this wind load. As the trib. area increases, the value of C&C wind approaches MWFRS wind.
For the interior column that started this thread, this is NOT and NEVER WILL BE a part of the main wind force resisting system. Its a dang interior column that supports gravity loads and takes wind pressure from a discreet area. Just take the C&C loads and apply the area and get a pressure - voila - its done.
For an exterior column that is part of a brace, say an X-brace, it would be checked for load combinations using MWFRS in the direction of the brace. And in the perpendicular direction would be checked for all load combinations for C&C wind since this wind is from a discreet, portion of the wind load.
MWFRS
This is the MAIN system of members that work together to brace the building against wind loads in an overall sense....the diaphragm of the roof or floor, the collector elements, and the columns and beams that serve to make up either moment frames or braced frames that laterally brace the whole structure.
C&C
This is a series of wind loads (not "a" wind load) but a series depending on tributary area. Any part or portion of the structure that resists wind from a limited area must be designed for this wind load. As the trib. area increases, the value of C&C wind approaches MWFRS wind.
For the interior column that started this thread, this is NOT and NEVER WILL BE a part of the main wind force resisting system. Its a dang interior column that supports gravity loads and takes wind pressure from a discreet area. Just take the C&C loads and apply the area and get a pressure - voila - its done.
For an exterior column that is part of a brace, say an X-brace, it would be checked for load combinations using MWFRS in the direction of the brace. And in the perpendicular direction would be checked for all load combinations for C&C wind since this wind is from a discreet, portion of the wind load.
JAE,
As a metal building engineer my typical frame of reference may be a little unique. Let me ask your opinion of this example - which could occur readily in our day-to-day design of metal buildings.
Example: We have 150' wide multi-span rigid frames spaced at 20' centers with 30' column modules. The sidewall columns and rafter form a continuous member using moment connections. The frame tributary area is 150'x20'=3000 sf (not including wall areas). So of course we design the frame using MWFRS wind coefficients on all wall and roof surfaces. However the interior columns (pin-pin) have At=600 sf doing a simplistic calc. Would you use C&C coefficients for the uplift on the column?
Thanks
As a metal building engineer my typical frame of reference may be a little unique. Let me ask your opinion of this example - which could occur readily in our day-to-day design of metal buildings.
Example: We have 150' wide multi-span rigid frames spaced at 20' centers with 30' column modules. The sidewall columns and rafter form a continuous member using moment connections. The frame tributary area is 150'x20'=3000 sf (not including wall areas). So of course we design the frame using MWFRS wind coefficients on all wall and roof surfaces. However the interior columns (pin-pin) have At=600 sf doing a simplistic calc. Would you use C&C coefficients for the uplift on the column?
Thanks
This is a great reason why I enjoy Eng-Tips so much. The diversity of opinion here is very rich and everyone has articulated their points very well.
I will beg to disagree with the notion that MWFRS somehow only means members that brace the building against wind load in an overall sense. My understanding is that MWFRS are members that contribute to the overall stability of the structure.
If you base your definition of MWFRS solely on trib area, then theoretically if I have a frame that has small enough trib area, those main columns should be designed for C&C, not MWFRS.
From the ASCE 7 code commentary:
MWFRS: Transfer wind loads to the foundation
C&C: Transfer wind loads to other MWFRS members.
Thus a girt would transfer loads to a column (C&C) a column would transfer loads to the ground (MWFRS).
I will beg to disagree with the notion that MWFRS somehow only means members that brace the building against wind load in an overall sense. My understanding is that MWFRS are members that contribute to the overall stability of the structure.
If you base your definition of MWFRS solely on trib area, then theoretically if I have a frame that has small enough trib area, those main columns should be designed for C&C, not MWFRS.
From the ASCE 7 code commentary:
MWFRS: Transfer wind loads to the foundation
C&C: Transfer wind loads to other MWFRS members.
Thus a girt would transfer loads to a column (C&C) a column would transfer loads to the ground (MWFRS).
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