Masonry Wind Loads 1

[SteelPE]

I know I have seen this question on this forum before but I can’t seem to find it at this time.

I have a masonry wall that is 20’ high x 30’ long (no openings), when sizing the wall reinforcing for wind loads (I am using the Components and Cladding requirements) do you use the full area of the wall (600 ft^2) or do you use the tributary area to each bar (say @40 in oc or 66ft^2). I have had some pretty lively discussions with other engineers in the office and was wondering what other people think.

 

[dik]

I would normally treat it as a wall with local additional reinforcing at corner areas to reflect higher wind loadings at those locations.

Dik

 

[spats]

I would use the effective wind area definition from ASCE7, that is the effective width need not be less than one-third of the span. In your case A = 20 x 20/3 = 133 sq. ft.

 

[DaveAtkins]

spats is correct.

DaveAtkins

 

[BAretired]

I would do the same as dik. That is most likely because we are both using a different code than ASCE7. I don't even know what you mean by effective width, nor why this topic would create much in the way of lively discussion amongst engineers.

BA

 

[kslee1000]

I am in line with dik & BA, though I am in the US. Treat the wall as a whole, there is nothing to tribute to.

 

[spats]

BAretired,

The question was about determining area for calculating components & cladding wind loads. dik did not address that at all, so I don't know what you're agreeing with.

The codes I work with in the US all reference back to ASCE7. That was the basis for my suggestion. I would think other codes have similar provisions for determining area for c&c wind loads.

As far as lively discussions... I can definitely see that. The same thing goes for designing tilt-up walls. Is the area the area of the entire panel, or is it the span times the spacing of the reinforcement?

 

[DaveAtkins]

dik did not answer the question. In order to determine wind loads at typical areas or at corners, you must know the effective wind area. Per ASCE 7, this is height squared divided by three.

DaveAtkins

 

[OldPaperMaker]

If you are designing a freestanding wall or sign using ASCE 7-05 I suggest that you get the wind loads from Section 6.5.14, formula 6-27.

It send you to Fig. 6-20 to find Cf

 

[dik]

Sorry, I misunderstood the question. I thought the question was based on the tributary area of the wall vs. the tributary area provided by a single bar @ 40" o/c and my reply was correct based on that understanding.

There was no reference to ASCE7.

Dik

 

[BAretired]

spats,

In the National Building Code of Canada, pressure coefficients are given for walls and roofs of varying slopes. A wall would be designed for a pressure derived from the algebraic sum of the internal and external pressure. These in turn would be determined in accordance with the code, taking into account q, the reference velocity pressure which is listed for many locations in Canada, Ce the exposure factor, Cg the gust factor and Cp the external pressure coefficient.

These various factors are all tabulated in the Structural Commentaries to the NBC. The area affected would be the entire wall area, although factors in the corner regions are greater than the central area. I believe that is why dik indicated he would design the wall for a uniform pressure with increased pressure in the corner regions.

There is no mention in our code of "effective area" and in fact, I still do not understand what you mean by the term. DaveAtkins says it is height squared divided by three which does not help in my understanding. Please elucidate.

BA

 

[kslee1000]

spat:

I have missed the original question as strength (reinf) rather than component concern, though the OP has indicated he was using such pressure.

 

[spats]

BAretired,

Under ASCE7, the external pressure coefficient GCp varies, based on the wind zone and effective wind area that the component supports. Wind area for a girt, for instance, would be the girt span x tributary load width (spacing between girts). ASCE7, however, adds the "effective" description by stating that the tributary load width (effective width) does not need to be taken as less than span/3. That is why DaveAtkins says height (span) squared, divided by three.

I thought all codes required that components supporting smaller wind areas had to be designed for higher winds. This relates to the principles of probability... the smaller an area the member supports, the more likely the member is to be fully loaded over it's entire area.

 

[SteelPE]

I am referencing ASCE 7. I missed the part about “effective wind area being the span length multiplied by an effective width that need not be less than one-third the span length” when calculating wind pressures using the charts of ASCE7.

Just so that I am clear. This question refers to the sizing of the reinforcing in a masonry wall due to wind normal to the wall. If you have bars spaced at 32 in o.c. what area do you use when entering the Components and Cladding charts of ASCE 7? Is the area based off the entire wall? Or is the area based off the bar spacing times the height of the wall?

Spats has cleared it up.

Sorry for the confusing question.

 

[inju]

Spats is on target. But, I wouldn't dispute dik's point either. According to ASCE 7-05, depending on the enclosure classification, you may have edge and interor zones. the edge zones require addional reinforcing due to higher wind pressure. But I have found this tricky as to how to address it in your design documents. You may call out, say #5@16" at the edge then #5@32" (elsewhere). No surprise it can easily be missed during construction.

 

[strucguy]

I have always used effective wind area of span length multiplied by an effective width (which is onethird span length or actual width of wall whichever is smaller). It would be an overkill to use bar spacing as effective width. I can't imagine one bar being overstressed without adjacent bars not contributing to resisting the localised wind pressures.

-strucguy

 

[OldPaperMaker]

SteelPE,
In your original post you mentioned two things; use of Components & Cladding (C&C) loads and the width to use in designing a masonry freestanding wall.

I pointed you toward Section 6.5.14 & Fig. 6-20 because I think that you are designing the Main Wind Force Resisting System,MWFRS and not C&C.

The Freestanding Walls section will ask you to investigate three load cases, A,B,& C and the resultant F value is the total wind load on the wall. How you proportion that total load to the wall and the reinforcing is up to you.

I hope that this clears up my initial response.

 

[hokie66]

There are a lot of things we can have international discussions about, but apparently wind loading is not one of them. The nomenclature used in the ASCE code is completely different from the Australian Standard, and I gather that is true also of the Canadian code.