Wind Load

[MOOK]

In ASCE page 74 (Figure 6-21)

In order to determine the shape (force) coefficient Cf, you need to get h/D from the table.

My question is regarding the definition of h.

Case (I)

If we have tank with height "L = 10ft" on the ground, so the term "h" which will be used in the table

h = 10 ft.

No problem in this case.

Case (II)

If we have tank with height "L=10ft" on a building with height "H= 20ft". There are two opinions:

1- The the term "h" which will be used in the table = H + (L/2) = 25ft. Actuall I found this answer in few references. I really do not understand how did they came up with assumption.

2- ASCE define "h" as height of the structure in feet (look down in the notation) and according to this we should use h = 10 ft.

I have been working for long time with non building structures and I always use h as the height of the structure and I even asked all of my co-workers and all of them use it the same way as me.

Could you please guys give me your feedback.

Thanks

 

[teda]

For sure, 10ft is correct answer. Code is trying to specify dimension ratio for exposed surface. For figure 6-21, Cf does not relate to elevation. Elevation is included in velocity pressure qz already. No matter tank is at ground or 200ft above ground, Cf will be same for same tank. That is my understanding.

In ASCE page 74 (Figure 6-21)In order to determine the shape (force) coefficient Cf, you need to get h/D from the table.

My question is regarding the definition of h.

Case (I)

If we have tank with height "L = 10ft" on the ground, so the term "h" which will be used in the table

h = 10 ft.

No problem in this case.

Case (II)

If we have tank with height "L=10ft" on a building with height "H= 20ft". There are two opinions:

1- The the term "h" which will be used in the table = H + (L/2) = 25ft. Actuall I found this answer in few references. I really do not understand how did they came up with assumption.

2- ASCE define "h" as height of the structure in feet (look down in the notation) and according to this we should use h = 10 ft.

I have been working for long time with non building structures and I always use h as the height of the structure and I even asked all of my co-workers and all of them use it the same way as me.

Could you please guys give me your feedback.

Thanks

 

[ARLORD]

Case (II)If we have tank with height "L=10ft" on a building with height "H= 20ft". There are two opinions:

1- The the term "h" which will be used in the table = H + (L/2) = 25ft. Actuall I found this answer in few references. I really do not understand how did they came up with assumption.

According to a seminar on Wind Design, Case II as you described it, is the correct method, H + L/2. I can't explain why. I have always done it the other way, but I have since changed. The code is not clear on this.

 

[whype]

According to a seminar on Wind Design, Case II as you described it, is the correct method, H + L/2. I can't explain why. I have always done it the other way, but I have since changed. The code is not clear on this.

ARLORD,

Looks like the information your got from the seminar is completely wrong.

 

[ARLORD]

ARLORD,
Looks like the information your got from the seminar is completely wrong.

Why do you say so? no pun intended.

 

[whype]

Why do you say so? no pun intended.

I mean for Case II, we should still use h/D = 10'/D, instead of (H + L/2)/D = 25'/D, to determine Cf, as you always used before you atteded the seminar.

 

[ARLORD]

I mean for Case II, we should still use h/D = 10'/D, instead of (H + L/2)/D = 25'/D, to determine Cf, as you always used before you atteded the seminar.

How do you know that is the correct way?

 

[ARLORD]

I mean for Case II, we should still use h/D = 10'/D, instead of (H + L/2)/D = 25'/D, to determine Cf, as you always used before you atteded the seminar.

See Example 2 on the attached handout from the seminar. Example 2 starts on page 32. On page 43, Part 2, #1 starts the solution for the wind load on the roof top unit: h = 63.5'(bldg ht) + 0.5"(curb) + 6/2(unit mid-ht) = 67'.

On page 43, Part 2, #3 calculates Cf using h=67'

 

[MOOK]

Actually, I have the book

Structural Load determination under 2006 IBC and ASCE/SEI 7-05 by David Fanella

He get Cf as H+(L/2) and I really do not understand why.

I remember I saw the same thing in another reference but I do not remember it at the moment.

 

[whype]

See Example 2 on the attached handout from the seminar. Example 2 starts on page 32. On page 43, Part 2, #1 starts the solution for the wind load on the roof top unit: h = 63.5'(bldg ht) + 0.5"(curb) + 6/2(unit mid-ht) = 67'.
On page 43, Part 2, #3 calculates Cf using h=67'

ARLORD,

Thanks very for the information. I can only say that it is a conservative approach, but not necessarily the right approach by using H + (L/2) as shown in the example. If a structure (non-building structure) is located on the roof of a building, qz will take care of the overall height (building height + 50% of the roof top structure height here), and Section 6.5.15.1 will take care of the roof top structure, by using a multiplier up to 1.9. According to the notes in Figure 6-21, h should be the height of the non-building (or roof top) structure itself. I will use h/D, instead of (H+L/2)/D to get cf from Figure 6-21. After all, engineering judgement is needed sometimes.

I don't think I can convince you, but some debate is quite useful. Thanks again for the information.

I have a book "Guide to the Use of the Wind Load Provisions of ASCE 7-02" by Metha and Delahay, but I do not have the guide for ASCE 7-05. I am also dealing with a lot of wind issues for non-building structures for my work.

 

[McEngr]

I'm on board with whype. Even though a guy interprets a problem a certain way (i.e. Alan Williams) in a seminar doesn't make it gospel. I'm convinced that this is a conservative approach. Why even calculate qz? Why not assume that qz is at 15' and then apply your h at 63' (or whatever it is) and use exposure B so that you get a 0.7 kz coefficient? It just seems counterintuitive, but it is good to see another perspective.

 

[teda]

What showing on seminar is not always correct. Just like any books or sample exams, you can see a lot of errors. We, engineers, should have our own solutions per code. Code clearly stated H=10ft for cases you mentioned.

 

[MOOK]

I totally agree with you that it should be h/D rather than (H+L/2)/D. This what I used to do in my work for long time.

I am still curious to know why the author insists to present it in this way. As I said I have the book for the seminar presenter of the seminar and he solved lengthy example using (H+L/2)/D

ARLORD, did not anybody ask him in the seminar about that?

 

[ARLORD]

I totally agree with you that it should be h/D rather than (H+L/2)/D. This what I used to do in my work for long time. I am still curious to know why the author insists to present it in this way. As I said I have the book for the seminar presenter of the seminar and he solved lengthy example using (H+L/2)/D

ARLORD, did not anybody ask him in the seminar about that?

It was a webinar, and no one to my knowledge questioned it. I am curious what ICC would say. I don't have a membership. But if you do, they give interpretations.

 

[McEngr]

It was a webinar, and no one to my knowledge questioned it. I am curious what ICC would say. I don't have a membership. But if you do, they give interpretations.

Hey ARLORD,

I hope you're not taking all of these posts personally. I'm sure that all of us here, including myself, respect your abilities and experience.

For what it's worth.

 

[ARLORD]

Hey ARLORD,
I hope you're not taking all of these posts personally. I'm sure that all of us here, including myself, respect your abilities and experience.

For what it's worth.

McEngr,

I take every post personally, but offence to none, and I'm glad you respect my abilities and experience. If only I could convince my co-workers, family, friends, etc.

 

[ARLORD]

I totally agree with you that it should be h/D rather than (H+L/2)/D. This what I used to do in my work for long time. I am still curious to know why the author insists to present it in this way. As I said I have the book for the seminar presenter of the seminar and he solved lengthy example using (H+L/2)/D

ARLORD, did not anybody ask him in the seminar about that?

I contacted D. Fanella(seminar instructor) via email, and his response to his use of h=(H+L/2) follows:

"In Figure 6-21, h is defined as the height of the structure, and I’m basing it on that definition. If you look in Commentary Section C6.5.11 on page 300 (about half-way down the left column), it becomes evident that h is the height of the building.

Hope that helps.

Dave Fanella"

 

[McEngr]

I contacted D. Fanella(seminar instructor) via email, and his response to his use of h=(H+L/2) follows:
"In Figure 6-21, h is defined as the height of the structure, and I’m basing it on that definition. If you look in Commentary Section C6.5.11 on page 300 (about half-way down the left column), it becomes evident that h is the height of the building.

Hope that helps.

Dave Fanella"

Wow! You actually received a response! Well, all I know is that the tables from 6-21 to 6-23 are almost identical to the TIA/EIA-222-G tables for antenna support structures. I've done many reanalysis of existing cellular carriers which hold antennas. With these, all of the existing calculations provided by very reputable firms use the "height of the structure" as the height of the area in question, not the height relative to the ground level.

The way the TIA/EIA-222-E deals with these is by applying a higher basic wind and using h/w like an aspect ratio of the discrete appurtenance. This usually ranges between 1.4 and 2.0 for tall skinny structures. The TIA/EIA also includes a solidity ratio much like it does in figure 6-22 for all shapes whether round square or cone shaped. The TIA/EIA has much more research on these issues, and in my opinion, should be the referenced standard. However, they have decided to take an approach that is not consistent, yet borrowed from, the TIA/EIA standard. According to the letter of the law, Mister Fanella is correct, but that will not stop me from doing it the "wrong" way.

Good job ARLORD. I just hope that I don't have a problem like this one on the SE II. If I do, I'll get it wrong.

 

[MOOK]

I contacted D. Fanella(seminar instructor) via email, and his response to his use of h=(H+L/2) follows:
"In Figure 6-21, h is defined as the height of the structure, and I’m basing it on that definition. If you look in Commentary Section C6.5.11 on page 300 (about half-way down the left column), it becomes evident that h is the height of the building.

Hope that helps.

Dave Fanella"

Thanks ARLORD for your care and interest.

Still, from page 300 I can not interpret that h is he height of the building.