ASCE 7-05 Wind Load Methods # 1 & # 2 1

[cap4000]

Has anyone ever compared the results of these methods. The Analytical Method which is more detailed is very different from Method # 1's Simplified results. The MWFRS loads don't agree. Any input would be appreciated.

 

[OldPaperMaker]

cap4000,
The few projects that I have run both methods seem to indicate that Method 2 results in lower base loads than Method 1. My trials were done on smaller, two story office buildings. It appears to me that the lower loads in Method 2 are a result of not having to use the higher loads in zones A & B on the end zones.
I practice in the Puget Sound area of Washington where the design winds are only 85 to 90 MPH. I don't know what the comparison would be for the higher wind speed areas of the country.

 

[Taro]

That would make sense. Method 1 is the simplified method. So the loads are more conservative in order to envelope the results of a more rigorous analysis.

 

[UcfSE]

Method 1 is based on scaled results from wind tunnel tests. Method 2 is analytical. In some cases method 1 should give you better results the closer ou get to meeting the ideal conditions as used in the tests.

 

[msquared48]

The same phylosophy is true for seismic when comparing the Simplified and Equivalent force methods. Simplified runs generally about 30 to 35% more.

Mike McCann
McCann Engineering

 

[cap4000]

Check out the link below and the seminar book which does a real nice job explaining the different methods. With typical square buildings under 60 feet high use Method 1 and the charts provided in ASCE7-05 on pages 38 and 39. The Simplified Analytical Method results and wind tunnel tests is where it all comes from.

https://secure.bluehost.com/~skghosha/onlineorder/show_book.php?isbn=20070314152

 

[NJEngineer1]

It appears to me, something is amiss in the ASCE 7-05. Figure 6-10 should state definitively whether the pressure shown is perpendicular to the surface (therefore horizontal & vertical components of the pressure should be calc'd using sine & cosines of the roof/surface angle) or it should state the contrary. As it reads right now, the figure is vague.

This is an email I recently sent to Mehta & L. Griffis, who are both on the ASCE 7 Wind Committee. I have yet to receive a reply.

It appears that Fig 6-10 (External Pressure Coeff) can be used to calculate the columns of Fig 6-2 using the givens on pg 283 of Commentary as shown below:

Where (1) GCpi= positive. (2) The positive direction is in the direction of coefficient 1 and up.

horizontal loads
p.A = qh[(GCpf.1E-GCpi)+(-GCpf.4E+GCpi)]
p.B = qh[(GCpf.2E-GCpi)+(-GCpf.3E+GCpi)]
p.C = qh[(GCpf.1-GCpi)+(-GCpf.4+GCpi)]
p.D = qh[(GCpf.2-GCpi)+(-GCpf.3+GCpi)]
vertical loads
p.E = qh[(GCpf.2E-GCpi)]
p.F = qh[(GCpf.3E-GCpi)]
p.G = qh[(GCpf.2-GCpi)]
p.H = qh[(GCpf.3-GCpi)]

However, where as the pressures found in fig 6-2 are based on horizontal and vertical projected surfaces, fig 6-10 appears to be based on pressure coefficients normal to each surface (as indicated in your Guide to the use...of ASCE 7-98 Fig 3.8.2 etc..). The later would suggest that the vertical and horizontal components of pressure coefficents should be used as follows:

horizontal loads
p.A = qh[(GCpf.1E-GCpi)+(-GCpf.4E+GCpi)]
p.B = qh[(GCpf.2E-GCpi)+(-GCpf.3E+GCpi)]sin(roof angle)
p.C = qh[(GCpf.1-GCpi)+(-GCpf.4+GCpi)]
p.D = qh[(GCpf.2-GCpi)+(-GCpf.3+GCpi)]sin(roof angle)
vertical loads
p.E = qh[(GCpf.2E-GCpi)]cos(roof angle)
p.F = qh[(GCpf.3E-GCpi)]cos(roof angle)
p.G = qh[(GCpf.2-GCpi)]cos(roof angle)
p.H = qh[(GCpf.3-GCpi)]cos(roof angle)

[If anyone knows someone on the ASCE 7-05 committee, I would appreciate their current email address in an effort to resolve this. Or you can ask the question at an ASCE 7 seminar.]

 

[OldPaperMaker]

NJEngineer1,
I think that in Figure 6-2 Method 1, note #1 sez the pressures shown are applied to the horizontal & vertical projections. In Figure 6-10 Method 2 (Low-rise Walls & Roofs), note #1 sez "plus and minus signs signify pressures acting toward and away from the surfaces, respectively".It is a poorly written sentence but if one compares the two notes I think that the pressures in Figure 6-10 are to be applied perpendicular to the roof surfaces.
Please keep us posted if you ever hear back from the Dark Lords that created that mess called ASCE-7.

 

[cap4000]

OldPaperMarker

In an effort to better understand all the Wind and Seismic Provisions of ASCE7-05 I have put together 2-24x36 sheets and all the different methods and formulas right on them. It consolidates everything onto one sheet so that I don't have to keep flipping pages. I also have flow charts set up. In addition I also have Meca software to calculate the loads. A lot of time is really required unlike the old BOCA Code. I agree what a mess.

 

[NJEngineer1]

I know my original post was a bit wordy, but the short of it is that Fig 6-2 is calculated using Fig 6-10. Try it yourself.

Given: Gable roofed building mean roof height 30 feet, I=1, Kz=0.7, Kzt=1, Kd=0.85, V=100mph, Roof Angle =20 deg. Enclosed Building GCpi= +/-0.18

qh=0.00256(0.7)(1)(0.85)(100)^2(1)=15.23 psf

p=qh[(GCpf)-(GCpi)]

Use GCpf from figure 6-10 (Transverse Direction) and Fig 6-2 nomenclature and adding contributions from both front and back wall(roof) surfaces with the proper signs to get Fig 6-2 pressure values.
Calc press/Actual Fig 6-2
p.A=15.23[(0.8-0.18)+(0.64+0.18)] = 21.93/22.0
p.B=15.23[(-1.07-0.18)+(0.69+0.18)] = -5.79/ -5.8
p.C=15.23[(0.53-0.18)+(0.43+0.18)] = 14.62/14.6
p.D=15.23[(-0.69-0.18)+(0.48+0.18)] = -3.20/ -3.2
p.E=15.23[(-1.07-0.18)] =-19.04/-19.1
p.F=15.23[(-0.69-0.18)] =-13.25/-13.3
p.G=15.23[(-0.69-0.18)] =-13.25/-13.3
p.H=15.23[(-0.48-0.18)] =-10.05/-10.1

The problem here is that no sine/cosine was used to combine the vertical or horizontal components of the roof surface. This implies the pressures are not taken perpendicular to the roof surfaces as was done in Mehta's Asce 7-98 wind guide but are oriented with the ground as is done in Fig 6-2. That is why I think something is amiss.

 

[cap4000]

NJEngineer1

The building you are using I would recommend the pressure load charts provided with Simplified Method. You are using the Simplified Analytical Method # 2 which in this case is quite intensive. The basis of the ASCE7-05 roof loads comes from wind tunnel tests. They even have 2 load cases for the steeper roof angles. Don't use the sine or cosine.

 

[NJEngineer1]

1. Can't use Fig 6-2 Method 1 Simplified Procedure for Partially Enclosed buildings GCpi = +/-0.55. The GCpi used to create Fig 6-2 was +/-0.18 Enclosed Building.

2. Furthermore, ASCE 7-98 Fig 6-4 is ASCE 7-05 Fig 6-10(with a few updates). In ASCE 7-98 the use of sine/cosine was clearly intended-the arrows shown on the fig are perp to the building surfaces and in the example from Mehta's 98 Wind Guide using fig 6-4, he shows the pressure perp to the roof and wall surfaces. For some reason the arrows are dropped in the ASCE 7-05 fig 6-10. The calculation of Fig 6-2 (first seen by me in IBC 2000 as Table 1609.6.2.(1)) doesn't use sin/cos. In the past when using the IBC 2000 table I would use sin/cos of the hor & vert pressure load to determine the pressure perp to the roof surface so I could calc the uniform load on the top chord of trusses, joists or beams. The horiz/vert pressure values from the table could be used directly to calc uplift from those members on the tops of walls or horizontal diaphragm loads. I am reticent about forging ahead with dropping the sin/cos without clarification from the ASCE 7 committee itself based on the previous usage of the same figure in a past ASCE 7.

 

[OldPaperMaker]

NJEngineer1,
1. Just for the record, I don't really understand why the folks that developed the stuff in ASCE-7 made things so complicated.
2. But, the Structural Engineers Association of Washington (SEAW) and the Applied Technology Council (ATC) have publised three volumes: SEAW/ATC 60, volumes 1 & 2 and SEAW RSM-03.
3. SEAW/ATC volumes 1&2 are a commentary on wind code provisions. Volume 2 has numerous examples of the use of the various methods with comparisons of results.
4. SEAW RSM-03 is a Handbook of a Rapid-Solutions Methodolgy for Wind Design. This is basically a graphical solution to the ASCE Method 2, All Heights process in figure 6-6 of ASCE 7-05. The book covers MWFR and C&C.
5. You can find the cost and details about the books at the ATC wqebsite which is

http://www.ATCouncil.org

Regards