TBacon said:
I regret that I find the current wind design to be so complicated and confusing.
TBacon said:
I truly do want to master wind design per the ASCE, but honestly miss the simplicity and efficiency of the old UBC wind design equation which was maybe not that sophisticated but it was also pretty fool proof.
TBacon said:
I've always had a lack of resources available to me for continuing education and have struggled to keep up with the constantly evolving code.
You're not alone on any of these. This is a huge complaint by many, myself included.
TBacon said:
is there a simple way to explain the whole concept of internal pressure?
Think of a structure as a aircraft wing. As the air is forced to move around the structure it causes a low pressure inside the structure (essentially it's sucking air out of the structure as the structure is not airtight). This is represented as a negative internal pressure, this increases the forces on the windward wall (it's being pushed on from the exterior wind pressure while simultaneously being pulled on by the negative interior pressure).
However, if we open up a bunch of windows and doors on the windward wall then the building turns into a parachute and not a wing. You're blowing it up like a balloon. This causes a positive internal pressure.
For the MWFRS this often wont matter much as the end result is it all cancels out (assuming both the windward and leeward walls provide similar areas for the pressures to act on). However, the roof structure external pressure will likely be controlled by the suction external pressure; adding a large positive internal pressure onto it adds to the net pressure and may overload the roof hold-downs. This is why people board up windows for hurricanes, they're trying to prevent large, windward openings forming in the building envelope, generating a large positive pressure. This is also why partially enclosed structures have higher internal pressures in ASCE 7.
Simplified designs like the IBC essentially combine internal and external pressure into a unified "net" pressure via the tables, so one way or another you're always considering it. Where differentiating between internal and external pressures matter is when your building is not symmetrical, then the internal pressures do not cancel out nicely.
I could go on and on but realistically you'll want to read up a bit on this on your own, don't forget the ASCE 7-10 commentary and having an example or two that you can follow will likely make everything much clearer. I'd personally recommend buying a book or two on your own dime if your work can't/won't provide them. Loads are the most basic part of structural engineering and the most important design step in my mind. While a pain, it's worth learning how to calculate them correctly for modern codes and to make the effort to keep up with things.
All said, ASCE 7-70 does have one advantage. You don't need to know all the methods, just learn the most applicable one for your needs and design everything with that one method. If you find the IBC method or simplified method of ch 28 part 2 the easiest and most applicable then there is nothing incorrect about that.
P.S. Don't forget components and cladding wind loads and the minimum wind load cases.
P.P.S. The minimum wind load cases for MWFRS design are just that, minimum
load cases. Not minimum wind pressures.
However, for components and cladding the minimum pressures are just that, minimum pressures.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries