Professional and Career Topics

  • 1.  WIND ANALYSIS WITH SPEED

    Posted 8 days ago
    I came across another Engineer report that claimed that using the Longer Pass theory that is also know as the Equal Transit theory to claim wind over a house speeds up higher than the local reported wind speeds. This was done in an effort to claim wind damage to a roof at low wind speeds. The theory was disproven years ago and never used for a house. The issue is that he has been using it for years and never challenged on it till now.
    How does everyone feel about using a method not intended for a house, not approved for design use of a house, not proven true by any scientific community, and never proven factual in any testing methods.

    ------------------------------
    George Miles P.E., M.ASCE
    President
    Alligator Engineering Inc
    Edgewater FL
    ------------------------------


  • 2.  RE: WIND ANALYSIS WITH SPEED

    Posted 7 days ago
    Considering this is a residential application and claim, does an engineer need to be involved?  An experienced architect may be able to weigh in.  Roof manufacturers and installers are often challenged to back their warranty only to deny claims for one reason or another.  An insurance adjuster would have experience in advising what is covered or not.  The decision makers (owners, insurers, contractors, lawyers) are not bound to accept stamped calculations (correct or incorrect) from any source nor are they prevented from seeking a second opinion.  Ultimately, the local code official and board of design professionals is responsible for oversight.  The code official may rely more on drawings and site visits than calculations.  The solution is likely prescribed by residential building code rather than formulated in design calculations.

    ------------------------------
    Chad Morrison P.E., F.ASCE
    Professional Engineer
    Greenville RI
    ------------------------------



  • 3.  RE: WIND ANALYSIS WITH SPEED

    Posted 4 days ago
    Personally, I think it's a reasonable case for a complaint against the engineer to the Board. This in my opinion is a breach of ethics. Using a disproven method that has never even been used on houses to facilitate in an insurance claim borders on fraud.

    George Runkle, P.E. S.E. M.ASCE
    President
    Runkle Consulting, Inc
    Grayson, GA

    ------------------------------
    George Runkle, MS, PE, SE, M.ASCE
    President
    Runkle Consulting, Inc
    Lawrenceville, GA
    ------------------------------



  • 4.  RE: WIND ANALYSIS WITH SPEED

    Posted 3 days ago
    I did file a complaint while the board decided that he did not do anything wrong. I personally feel the board did not do a proper investigation. Working on disproving the method to stop the issue of fraud that is going on is  my current goal.

    ------------------------------
    George Miles P.E., M.ASCE
    President
    Alligator Engineering Inc
    Edgewater FL
    [Phone]
    ------------------------------



  • 5.  RE: WIND ANALYSIS WITH SPEED

    Posted 3 days ago
    [Pardon me in advance. Up late working]
    George, after reading this my initial thoughts went to the topographic factors in calculating wind loads where there is a speed-up at the top of a hill. I had used topography factors to calculate an increase in wind speed for a splash wall at the top of a "hill".  I may be the only one challenged to comment on an element of a report without a few more details (i.e., elevated house on piles, low pitch roof, etc.) and characterization of terrain, topography, etc.

    When it comes to various analysis methods, I believe we are often challenged by the idea of "exactness" when attempting to find "the answer". I imagine that the gap between the theoretical answer is aided by the use of a factor of safety and reality falls within or under the theoretical envelope. My tendency is to provide the comparison data as to how the  non-conventional method stacks up against conventional or accepted methods when analyzing or assessing another's work. Testing of scaled models close the gap with reality, but are not exact.

    I believe that when there are deviations, it is the deviator's responsibility to provide the justification as to why the method is acceptable. [Note: My first time task with checking a more "senior" engineer's work resulted in me marking it up and asking for technical references for those items he marked "GBI" (GBI - Good by Inspection). I even had to ask what "GBI" stood for. He basically said that I had not been there long enough to question his analysis.]  We were taught in school that anyone should be able to pick up your work and understand how and why you came to your conclusion. With the exception of universally accepted formulas, I documented and referenced sources. It was easier for me to spend an extra minute, as I was working, to note the source than it was for the checker to spend hours trying to comprehend and then returning to me to ask for the source.

    If you performed calculations using an accepted method, how do your numbers compare to his?

    It has been a few moons since my last wind load calc involving topographic factors and I looked up the "Equal Transit Theory" for information on the nasa website (https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/foilw1/#air-molecules-travel-faster-over-the-top-to-meet-molecules-moving-underneath-at-the-trailing-edge). This is what I captured.  It speaks of under estimation but makes note of

    • "The lift predicted by the "Equal Transit" theory is much less than the observed lift, because the velocity is too low. The actual velocity over the top of an airfoil is much faster than that predicted by the "Longer Path" theory and particles moving over the top arrive at the trailing edge before particles moving under the airfoil."
    • "The difference in pressure across the airfoil produces the lift. As we have seen in Experiment #1, this part of the theory is correct. In fact, this theory is very appealing because many parts of the theory are correct. In our discussions on pressure-area integration to determine the force on a body immersed in a fluid, we mentioned that if we know the velocity, we can obtain the pressure and determine the force. The problem with the "Equal Transit" theory is that it attempts to provide us with the velocity based on a non-physical assumption as discussed above."
    By the way, thank you.  I had been searching my computer folders for a 2013 presentation by Donald R. Scott, S.E. on "Wind Loads on Non-Standard Building Configurations" for over a year. I found it due to this discussion.

    ------------------------------
    James Williams P.E., M.ASCE
    Principal/Owner
    POA&M Structural Engineering, PLC
    Yorktown, VA
    ------------------------------



  • 6.  RE: WIND ANALYSIS WITH SPEED

    Posted 3 days ago
      |   view attached
    Thanks James for that, I had what you were looking for. I have been working for over 8 months to fully understand wind loads and how they affect roof tiles. It is interesting that concrete and clay tiles are moved by moment forces and not the direct lift of wind. Some of the studies that have been done really layout the areas that are first damaged.
    The sad part is that I have come across multiple Engineers that are writing reports with the longest path method to claim wind damage. The fact is that in all states we are only to use methods that are proven to be factual by testing, studies, and documented reports in combination with our experience. At the end of it all this method is not true and there were multiple studies that prove that. The sad part is all of this was done on my part to prove I was correct in my analysis. The attached study helps disprove it.

    The real issue I have is using a disproven method in an attempt to claim wind damage. The wind can speed up over a home at the ridges and hips as noted in ASCE 7. This is well documented while the overall affects must be considered. The affects of bluffs, scarps, etc. only come into place under certain conditions. The notes for the section explain that and the speed up that can occur actually came from fire studies. Wind was speeding and causing the fire to move faster. Actually found an old report about it dating back to the mid 70's.

    The short of all this is that no one has supporting using a method that is disproven and not been approved for use on a house.


    ------------------------------
    George Miles P.E., M.ASCE
    President
    Alligator Engineering Inc
    Edgewater FL
    [Phone]
    ------------------------------

    Attachment(s)

    pdf
    how wings work.pdf   356 KB 1 version


  • 7.  RE: WIND ANALYSIS WITH SPEED

    Posted 2 days ago

    It is interesting to see that NASA has gone length to disprove the 'Equal Transit Wind Theory'. But they didn't have to – for some simple reasons like:

    • It seems to me Equal Transit Wind Theory is sort of a heuristic argument (if I may say so) rather than the established flow physics.

    • Vertical lift force resulting from uneven horizontal fluid flow speeds is simply laid down by the unsteady Bernoulli (Daniel Bernoulli, 1700 – 1782) Equation (DBE). If one recalls, the DBE is derived from the elaborate and complete Navier-Stokes Equation (NSE) by balancing the steady convective flow acceleration against the pressure gradient force and gravitational force. The beauty of DBE lies in depicting the reciprocity of pressure and speed (e.g low speed underneath giving rise to high pressure there).

    • For those, who feels aversion to deal with differential equations – I have presented the NSE in simple terms in my Seabed Roughness paper. One can request the paper from Springer (they will charge a small fee to recover the publishing cost), or can write to me for a pdf reprint of the author's copy. Also my website piece Common Sense Hydraulics has simple summaries of some basics (hydraulics and airflow are all governed by fluid flow physics laws).

    • The implication is that while DBE is easy to use for simple surfaces – one must be cognizant about its limitations. NASA and other similar entities dealing with vertical lift of aircrafts – handle this limitation by conducting elaborate experiments to determine suitable coefficients. Or, perhaps in modern times – solving the complete NSE by numerical models or by fast digital computing.

    • Now, let us imagine the real case of the lift forces on house roofs which are not simple surfaces. The lift force on such uneven surfaces (there may be many configurations in response to owner's request or the contractor constraints and works, etc) is not a simple function of DBE without suitable coefficients. Additionally, the configurations including the neighborhood effects (which can be complicated, by its own right) might play a significant role.

    James, I like when you say things like four moons ago or many moons ago – it is very uncommon for engineers. We are serious people (perhaps all professionals are). Aren't we used to say many years ago – but we do not have to follow the lead always, do we?

    -----

    Dr. Dilip K Barua, PhD

    Website

    Google Scholar