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wind continued...

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What Scott and Gary are saying, in essence, is in Hurricane regions the wind speeds are based on a much longer period of time of an occurence versus non-hurricane zones?  IE, it is much more likely that a barn in Wisconsin could get strong winds from a summer thunderstorm then a silo in Florida getting hit by a hurricane. I am not at the office so I cannot read that section but I have always thought about that when looking at the wind speeds and the Importance factor....
 
"Beat me to it. Section C6.5.5 of the commentary talks about the =
importance factor and notes the difference in return periods between the =
non-hurricane zones (50-year MRI) and hurricane zones (500-year MRI =
factored down to 50, and incorporating the old coastal multipliers).
Gary
And Chris and Conrad have been softening my stance on MWFRS vs Component and Cladding. I have not completely been convinced or come around, and need to do a little more studying on my own. However, let me focus the discussion on my own practical encounters. Nobody is checking what pressures I use on my own designs so that is up to our engineering judgment. However, we have to put C and C pressures on our drawings for component design, which include windows, doors, and perhaps trusses and awnings.
 
Now since trusses are designed by another engineer, that could be said it is up to their judgment, though I am the EOR I could insist on one type of wind design versus the other. But I also don't need a truss mfr telling the GC the structural engineer is ridiculous and an idiot, and then they tell my client the same....
 
If we do not have a GC and truss mfr on board during design, we do our own quick analysis using C and C pressures to determine uplift, C and C pressures for out of plane wall loading, and MWFRS for shear. We then size the connections for the truss to wall connection for these different combinations of reactions. When we get the truss shop dwgs and calcs down the road, we compare our reactions with theirs and upsize if necessary. Since the real static analysis of a truss system can quickly get complicated (and they may do a different layout then what we have suggested), we use trib area methods and usually come out on the high side so we then do not have to change the connections or drawings. Nobody likes drawing revisions, the GC may have already ordered the straps, and these are usually $1 or less at the Home Depot so who cares.
 
Chris and Conrad suggest, to paraphrase, trusses should be perhaps analyzed for both C and C and MWFRS. The individual tail at the overhang, the top chord in one section near the ridge, a web, etc.,, may need to be analyzed for C and C pressures. Now the overall bending forces and T & C forces in the truss and the reactions should be analyzed using MWFRS. HOWEVER, what truss designer sitting in front of a program they barely understand is doing this? Are the programs set up for this type of analysis? I highly doubt it. They are probably doing MWFRS as this would overall result in a "lighter" truss, thus saving them money. Since another engineer is designing it, and it is his judgment, as the EOR even if I do not agree completely should I allow it? If I don't like their reactions, I can always upsize the connections on my own.
 
Conrad, in Florida, much like in Australia from what I know, we get hurricanes/cyclones during the summer and fall (opposite times of year from you I think). (The middle part of the US gets LOTS of tornadoes, from Texas and the Gulf States up through Wisconsin). However, we also get freak thunderstorms and tornadoes year round that can result in close to code mandated wind speeds. Doing forensic work I have seen what wind pressure does to wood frame structures and the roofs are the first to fail. This was my conclusion while doing forensic investigations of many Gulf homes in Mississippi after Katrina. My point is strengthening of the roofing system and connections may save many more structures than strengthening the CMU or foundations, at a relatively low cost %... Some custom roofs have more money in the roof tile or the cabinets then the entire roofing system.
 
Often CMU walls are left intact with extensive roof damage, and then extensive water damage to the interior due to the compromised roof structure. This water damage is often enough to cause the entire structure to be demolished and start over... I advocate if we are going to be conservative in a portion of structural design, and as a result a very small economic price is to be paid, but the potential life and money savings is huge, we do so. This is my opinion and "style" in design, and I often slightly oversize what I consider "critical connections". Now when it comes to huge steel beams, moment frames, foundations, shear walls, etc., I use every LL reduction and other reduction specified in code, and MWFRS as applicable. Keep in mind the worse disaster in the last 30 years as a result of a structural engineering mistake was a simple all-thread connection supporting a walkway at the Hyatt Regency (also highlighting shop drawing importance).... Connections connections connections....
 
So I am not arguing that Chris and Conrad do not have great theoretical points, and may very well be spot on with their arguments or at least opened up a great discussion, but I would like to focus on what you would do with real world designs. Also, none of us have two days to do wind loading on a 3000sf single story building, and what is to really be gained by such precision? Thank you so much for your insight and look forward to more discussion on this subject....
 
Andrew