Integrated Buildings & Structures

​Design criteria from one of our US Customers specifies  60m/s as hurricane wind speed (as per ASCE 7-10) and it also states that the hurricane can occur 8 times over a span of 50 years. Can anyone help me in understanding this additional requirement? Does this additional clause have any effect on design wind speed (like modification of return period or additional factor etc.) or I can simply take design wind speed as 60m/s and design the structures as usual (i.e. simply ignore the second statement)?

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Neeraj Agrawal
Gurgaon
India
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Odd way to put it if that's all there is - you may want to ask where that comes from.  It may intend to speak to the probability distribution of the event(s). It suggests a 6.25-year mean return interval or 16% chance of occurrence per year. US code(s) generally aim for a 2% mean chance of occurrence depending on the risk level - the acceptable risk level is a variable country by country it seems. If the 50 (or 100-year or the like) are still the goal, you'll have to rework the gamma's or load factors if you are using some form of partial factor design. Melchers or Cornell are good references among several. Be prepared for some reading.

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C Michael Donoghue P.E.,M.ASCE
Principal
Maritech Engr. ,Inc.
Austin TX
(512) 326-3232
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In order to reduce probability from 16% to 2%, I have to increase the wind speed (or use additional gamma factor as you suggested). This sounds logical. I will now work on this idea. Thanks for the idea.

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Neeraj Agrawal
Gurgaon
India
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Original Message:
Sent: 06-21-2019 07:34
From: C Michael Donoghue
Subject: Hurricane wind on structures

Odd way to put it if that's all there is - you may want to ask where that comes from.  It may intend to speak to the probability distribution of the event(s). It suggests a 6.25-year mean return interval or 16% chance of occurrence per year. US code(s) generally aim for a 2% mean chance of occurrence depending on the risk level - the acceptable risk level is a variable country by country it seems. If the 50 (or 100-year or the like) are still the goal, you'll have to rework the gamma's or load factors if you are using some form of partial factor design. Melchers or Cornell are good references among several. Be prepared for some reading.

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C Michael Donoghue P.E.,M.ASCE
Principal
Maritech Engr. ,Inc.
Austin TX
(512) 326-3232
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Figuring out the intended or implied COV of the distribution in order to come up with a revised partial factor on the load side is going to be a headache if the numbers are arbitrary or not base on some actual data set. Actually the partial factor in the ASCE 7-16 parlance will be one as the risk calibration is built into the wind speed itself. That's where understanding the intent and source is. The NIST site cited by another colleague is a good source. A big COV is going to translate to a big partial factor or statistical wind speed.


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C Michael Donoghue P.E.,M.ASCE
Principal
Maritech Engr. ,Inc.
Austin TX
(512) 326-3232
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Original Message:
Sent: 06-22-2019 04:49
From: Neeraj Agrawal
Subject: Hurricane wind on structures

In order to reduce probability from 16% to 2%, I have to increase the wind speed (or use additional gamma factor as you suggested). This sounds logical. I will now work on this idea. Thanks for the idea.

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Neeraj Agrawal
Gurgaon
India
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Original Message:
Sent: 06-21-2019 07:34
From: C Michael Donoghue
Subject: Hurricane wind on structures

Odd way to put it if that's all there is - you may want to ask where that comes from.  It may intend to speak to the probability distribution of the event(s). It suggests a 6.25-year mean return interval or 16% chance of occurrence per year. US code(s) generally aim for a 2% mean chance of occurrence depending on the risk level - the acceptable risk level is a variable country by country it seems. If the 50 (or 100-year or the like) are still the goal, you'll have to rework the gamma's or load factors if you are using some form of partial factor design. Melchers or Cornell are good references among several. Be prepared for some reading.

------------------------------
C Michael Donoghue P.E.,M.ASCE
Principal
Maritech Engr. ,Inc.
Austin TX
(512) 326-3232

IMPACT OF HEAVY WINDS ON STRUCTURES
Wind is the flow of gases on large scale, containing bulk movement of air. It is caused by difference in the atmospheric pressure, such that, encourages movement of air from the higher to lower pressure area, resulting in winds of various speed. 

Wind forces are complex, and the effect on building structures depend on the interaction of many variables. Natural variables include features and properties of the air. These variables create differences in pressure that push and pull on the exterior surfaces of the building, through the following processes, Uplift, Tilting, Overturning etc.

Hurricane or Tropical cyclone, is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms, that produce heavy rainfall. The impact on buildings can be dangerous and hazardous, which includes lateral or sideways movement of the structures and collapse.

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Olusegun Afolabi R.Eng
University of Lagos
Lagos
234803 4248600
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Perhaps, NIST Information Technology Laboratory website will be a good source: https://www.itl.nist.gov/div898/winds/homepage.htm.

It provides a great guidance with data, analyses, publications and other links for wind load designs.

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Dr. Dilip Barua, Ph.D, P.Eng, M. ASCE
Vancouver, BC, Canada
Website: https://widecanvas.weebly.com
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Original Message:
Sent: 06-21-2019 03:48
From: Olusegun Afolabi
Subject: Hurricane wind on structures

IMPACT OF HEAVY WINDS ON STRUCTURES
Wind is the flow of gases on large scale, containing bulk movement of air. It is caused by difference in the atmospheric pressure, such that, encourages movement of air from the higher to lower pressure area, resulting in winds of various speed. 

Wind forces are complex, and the effect on building structures depend on the interaction of many variables. Natural variables include features and properties of the air. These variables create differences in pressure that push and pull on the exterior surfaces of the building, through the following processes, Uplift, Tilting, Overturning etc.

Hurricane or Tropical cyclone, is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms, that produce heavy rainfall. The impact on buildings can be dangerous and hazardous, which includes lateral or sideways movement of the structures and collapse.

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Olusegun Afolabi R.Eng
University of Lagos
Lagos
234803 4248600
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There must be an error with the specified wind speed; 60 mps is not appropriate; check your units; 60 mps would be equal to 3600 mph. (Miles per hour is the commonly used ​units for design wind speed.) A typical design wind speed would be on the order of 90 mph with a 2% chance of recurrence in 100 years.

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Robert Crossn, LMASCE
MASCE-SEI
Albuquerque, NM

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Hurricane design wind speed is a lot more than 90 mph.  Anyway, 60 mps equates to 135 mph, more like a hurricane wind speed, although certainly not the highest.  Not sure how you got to 3600 mph, other than there are 3600 seconds in an hour.  Also, with the way the weather is changing (along with climate change and global warming), I wonder if recurrence intervals are becoming meaningless

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Richard Lenz M.ASCE
West Chester OH
(513) 777-5525
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Original Message:
Sent: 06-22-2019 16:20
From: Robert Crossno
Subject: Hurricane wind on structures

There must be an error with the specified wind speed; 60 mps is not appropriate; check your units; 60 mps would be equal to 3600 mph. (Miles per hour is the commonly used ​units for design wind speed.) A typical design wind speed would be on the order of 90 mph with a 2% chance of recurrence in 100 years.

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Robert Crossn, LMASCE
MASCE-SEI
Albuquerque, NM

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Indeed past may not be prologue on the statistics. Folks are cudgeling brains on this. Last estimate I read had future wind events increasing not infinitely but on the order of about 8% or so ... but who knows and on what time scale and distribution. Structurals need to think in terms of a hundred years. So we are in that game like it or not.

cmd

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C Michael Donoghue P.E.,M.ASCE
Principal
Maritech Engr. ,Inc.
Austin TX
(512) 326-3232
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Original Message:
Sent: 06-23-2019 13:38
From: Richard Lenz
Subject: Hurricane wind on structures

Hurricane design wind speed is a lot more than 90 mph.  Anyway, 60 mps equates to 135 mph, more like a hurricane wind speed, although certainly not the highest.  Not sure how you got to 3600 mph, other than there are 3600 seconds in an hour.  Also, with the way the weather is changing (along with climate change and global warming), I wonder if recurrence intervals are becoming meaningless

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Richard Lenz M.ASCE
West Chester OH
(513) 777-5525
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Original Message:
Sent: 06-22-2019 16:20
From: Robert Crossno
Subject: Hurricane wind on structures

There must be an error with the specified wind speed; 60 mps is not appropriate; check your units; 60 mps would be equal to 3600 mph. (Miles per hour is the commonly used ​units for design wind speed.) A typical design wind speed would be on the order of 90 mph with a 2% chance of recurrence in 100 years.

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Robert Crossn, LMASCE
MASCE-SEI
Albuquerque, NM

​The wind speed mentioned by me is in meters per second hence 60m/s is equal to 135 miles per hour.

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Neeraj Agrawal
Gurgaon
India
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Original Message:
Sent: 06-23-2019 13:38
From: Richard Lenz
Subject: Hurricane wind on structures

Hurricane design wind speed is a lot more than 90 mph.  Anyway, 60 mps equates to 135 mph, more like a hurricane wind speed, although certainly not the highest.  Not sure how you got to 3600 mph, other than there are 3600 seconds in an hour.  Also, with the way the weather is changing (along with climate change and global warming), I wonder if recurrence intervals are becoming meaningless

------------------------------
Richard Lenz M.ASCE
West Chester OH
(513) 777-5525
------------------------------


Original Message:
Sent: 06-22-2019 16:20
From: Robert Crossno
Subject: Hurricane wind on structures

There must be an error with the specified wind speed; 60 mps is not appropriate; check your units; 60 mps would be equal to 3600 mph. (Miles per hour is the commonly used ​units for design wind speed.) A typical design wind speed would be on the order of 90 mph with a 2% chance of recurrence in 100 years.

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Robert Crossn, LMASCE
MASCE-SEI
Albuquerque, NM

If it is a project in the U.S., why not just follow ASCE 7?  If you haven't found ASCE's Online Hazard Tool, you can find it at https://asce7hazardtool.online/ .  Just log in as a Guest and you can see the wind speed for your project location and your structure's Risk Category.  For example, I randomly chose Miami with a Risk Category IV, and the wind speed is 189 mph (which is 91.4458 psf basic wind pressure for Exposure C; 0.00256 x Velocity^2).  You can click on the "Details" button and see more information such as lower MRIs and the details such as what you are looking for.  Also, you can slide the "Overlay" button, zoom out, and you can see the wind contours including the Hurricane Zone delineation.  You can pay a small fee to see all of the other information that you probably need to consider in the design of your structure.  I find this website very helpful in our industry and I hope this helps you (and others) out.

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Otto J. Lynch P.E.,F.SEI,F.ASCE
President & CEO
Power Line Systems
otto@...
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The original question from Neeraj, states a wind speed of 60 m/s, which is 60 meter per second. India uses a metric system.
The 60 meter per second equals @ 135 mile per hour, which is Category 4 storm per Saffir-Simpson scale.

Refer to the following Wikipedia article for additional information.

htpps://en.wikipedia.org/wiki/Tropical_cyclone_scales


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Atul Patel
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For complete understanding you need to ask the background of the wind speeds being asked.  If your customer is following ASCE-7 after 2010   60 m/s represents a 3sec  gust wind speed at the ultimate load level.  When dealing with projects in other countries I attempt to clarify the reliability and loads used.  For wind speeds or pressure I find out sampling periods (3 sec gust to one hour wind speed) and return periods.  As important is how the material (such as steel) is designed in the country.

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David Thompson P.E.,M.ASCE
Principal
KTA Structural Engineers Ltd.
Calgary AB
(403) 246-8827
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