ASCE 7-10 Simplified Envelope Method

[jpp06]

Can someone explain to me why the simplified method does not use the Kz, velocity pressure exposure coefficient? I am getting much higher pressures using the simplified envelope method.

Thanks

 

[briancpotter]

Kz is an adjustment based on height above ground level and exposure category, due to how ground roughness affects the wind speed. The simplified method takes these factors into account with the lambda adjustment factor.

Brian C Potter, PE
Simple Supports - The history and practice of structural engineering.
ConstructionPic - Send annotated jobsite photos.

 

[jpp06]

I used lambda and my topographical coeff was 1... my pressures are still a great deal higher than when I use the calculated envelope method.

 

[woodman88]

Simplified = Conservative?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[JAE]

woodman is correct.

 

[jpp06]

I agree, simplified = conservative. A 10-20 psf higher pressure seems too conservative to me. Here is what puzzles me - The simplified method utilizes tables based on a height of 30 feet and a wind design category B and an Importance factor of 1. The wind speed maps it tells us to reference are based on a 33 ft height for wind design category C and importance factor is non existent (we are using risk categories now). It makes sense to me to use the Kz factor when determining the pressures using the simplified method in addition to the lambda factor, but the book does not say to do this. How can you use wind speeds assigned to areas assuming wind design category c at height of 33 ft with a table that assumes wind design category b and a height of 30 ft.

Should Kz be used with the simplified method?

It looks to me like the same tables were used for the simplified method going from ASCE 7-05 to ASCE 7-10 without making any adjustments to the pressures or the adjustment factor (lambda). The pressures should be lower in theory in the newer version than those in the older version, but I am not getting that unless I use the Kz exposure coefficient.

Detailed input would be appreciated.

Thanks

 

[EngineeringEric]

I didn't think ASCE 7-10 had importance factor... there are Risk Categories but those are not factors, they relate to different wind speeds.

In my experience, when calculating the velocity pressure from 05 to 10 there is an increase... so the qz value is sometimes 50% higher... when i go though the full process and apply a load factor my values become much more related... asce 7-10 LF=0.6 were equivalent in 7-05 is 1.0.

I do not have a ton of experience with ASCE 7-10, but i typically calculate it all out completely in a nifty spreadsheet

 

[NOLAENG]

I agree with EngineeringEric as far as getting similar results for the simplified method between 7-05 and 7-10.
I just checked my spreadsheet for 7-10 against Alex T's 7-05 spreadsheet and have similar results.

 

[jpp06]

That's what I ultimately did. I applied a load factor of 0.6 and achieved the pressures I was expecting. If you look at figure 28.6-1 (cont.) it clearly says that the tables assume an I = 1.0. But you are correct, there is no importance factor anymore... only risk categories. This was one of the reasons I was thinking they just copied the tables over from 7-05 and did not modify.

Parks Payne, PE

 

[jpp06]

actually sq.rt. of 0.6 if going from an ultimate wind speed to a wind speed used in allowable stress design.

I am just puzzled as how you can use information from a map that assumes 1 set of criteria to navigate a table that assumes a completely different set of criteria.

 

[woodman88]

"But you are correct, there is no importance factor anymore... only risk categories"

Not quite true. What was done was to apply the important factor to the wind map. So instead of one map and three important factors, we now have three maps. One for each Important Factor that existed before.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jpp06]

but it is not an importance factor. it is now a risk category. there is no importance factor anymore

 

[woodman88]

There never really was an Importance Factor in the first place.
ASCE 7-05 Section C6.5.5 clearly shows that what was called the "Importance Factor" is really the MRI factor that is to be multiplied to the wind speed V, for the difference Occupancy Categories. Rather than creating three wind tables, in the ASCE 7-05, they decided to multiply it to the V in the equations. But since the V is squared in the equations, they squared the MRI's and call it an "Importance Factor".
Now in the ASCE 7-10 they took the wind maps and basically multiply the wind speed by the square root of 1.6 ( the 1.6 is the amount they decreased the effect of the wind speed in the basic load combinations). Then multiplied the new map values by the MRI's, for the "Risk Categories" (which are the old Occupancy Categories) to create two "new" maps.

As for the ASCE 7-10 figure 28.6-1 (cont.) with the I=1.0. They copied the table from the ASCE 7-05 and missed removing the I=1.0 factor note.

As for all the cookie cutter designers (even those who show a PE in their post) who are comparing spreadsheets "by others" for the ASCE 7-05 to the ASCE 7-10 loads. If you can not (or don't want to) do your own check of where the loads are coming from and how they are calculated. Then you should only ask for information here and not making statements about something they know very little about.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jpp06]

The intent of my original question was to find out why the Kz value was not being used with the simplified envelope method. If I had a spreadsheet to go by it would probably help answer my question. I understand what, why and how the "importance factor" was used in ASCE 7-05. I understand that ASCE 7-05 referenced the "Importance Factor" and not "MRI factor" in their wind equations. No matter how they are calculated. This is how ASCE 7-05 referred to this factor. I understand we now have risk categories instead of occupancy categories. Thank you for answering my question regarding ASCE 7-10 figure 28.6-1. The purpose of me starting this thread was to get feedback from other engineers on this particular method because I was having some issues with my own personal calculations.

My question regarding the transition from a map assuming certain criteria to a table that assumes another set of criteria still stands. Pertaining to the statement I made regarding multiplying the wind speed by sq. rt. of 0.6 was taken directly from IBC 2012 and is the multiplier they use to go from an "Ultimate Wind Speed" to a nominal or ASD Wind Speed. I do appreciate the good feedback.

As much as I appreciate your indirect jab, I am just looking for honest feedback.

 

[Archie264]

Hmmm, somehow this seems relevant:

“It will be of little avail to the people, that the laws are made by men of their own choice, if the laws be so voluminous that they cannot be read, or so incoherent that they cannot be understood; if they be repealed or revised before they are promulgated, or undergo such incessant changes that no man, who knows what the law is today, can guess what it will be to-morrow.” (James Madison)