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RE: Another ASCE 7-05 Wind Load Question

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Interesting, using letters for exposure classification. I guess you cannot interpolate between classifications and multipliers, or start with roughness length (z0) and calculate an exposure classification. Here we have separate multipliers for Terrain category M[z,cat], shielding Ms, topography Mt, and direction Md. I believe the ISO guidelines combines several of these into a single factor, our code writers aligned by placing brackets around the combined multipliers.


The Terrain Category is probably the closest to the exposure class, and is numbered 1 to 4. With TC1 being ice field or similar, TC2 is the reference at airports or rural fields, TC3 being suburban or woodlands, and TC4 being big city. Derivative codes often use TC2.5 not quite suburban, not quite open rural fields, the developing fringes of suburbs or country towns. Working from descriptions of terrain and association to roughness length z0, it is possible to calculate and assign other values for terrain category, z0=0.04 gives TC2.3 which is more representative of serial crops ready for harvest. Then get into arguments about the seasonal variation of terrain category. It is preferable to stick to the basic categories of 1,2,3 or 4, however the detail is helpful when assessing existing structures.


The problem with our approach however is when is a forest part of the surface roughness (M[z,cat]) and when is it providing shielding (Ms). Like wise is topography: shielding a building (Ms), creating surface roughness (M[z,cat]), or escalating the wind speed (Mt)? Also topographical only covers wind flowing up over a hill, not flowing along the bottom of a valley, nor flowing over one hill down into a valley and then up over the next hill. The topographical factor is purely for wind flowing off a plain up the side of a hill or escarpment. So AS1170.2 is fine for wind flowing off the Adelaide plains up the Adelaide Hills, but not realistic within the hills themselves.


How does ASCE7-05 treat topography?




Conrad Harrison

B.Tech (mfg & mech), MIIE, gradTIEAust



South Australia

From: Scott Maxwell [mailto:smaxwell(--nospam--at)]
Sent: Thursday, 21 February 2008 04:45
To: seaint(--nospam--at)
Subject: RE: Another ASCE 7-05 Wind Load Question


In terms of intent (i.e. suburban/urban), yes, they are the same.  Since they are worded differently, however, you might have some code official (or engineer) interpret them differently.  I can see some "issues" with the way the UBC describes it (not that ASCE 7 is perfect).  I use the general intent...B is suburban/urban and rural in a forest (i.e. lots of trees), C is rural with fields (i.e. not rural in forests) and hurricane coasts, and D is non-hurricane coasts (i.e. West Coast and Great Lakes).





Adrian, MI

-----Original Message-----
From: Bill Allen [mailto:T.W.Allen(--nospam--at)]
Sent: Wednesday, February 20, 2008 11:52 AM
To: Seaint
Subject: Another ASCE 7-05 Wind Load Question

Is Exposure B the same as Exposure B in the 1997 UBC? It sounds like Exposure B in ASCE 7 is applicable in typical suburban/urban areas. OTOH, I’ve never been able to get UBC Exposure B to fly here without rigorous hoop jumping.




T. William (Bill) Allen, S.E.


Consulting Structural Engineers
V (949) 248-8588 F(949) 209-2509