wind on signs - asce 7-05

huangyhg

wind on signs - asce 7-05
i'm new to this site, so please forgive me if this has been the topic of another thread.  i searched for other articles, but did not find any.
i design signs and billboards on a daily basis and i'm disturbed at the increase in the wind loads from asce 7-02 to asce 7-05.  does anyone share my concern?
i design signs and billboards as well, but i haven't seen asce 7-05 yet (our copy is coming any day now).  what changed that caused the loads to increase?
the force coefficients (cf) in figure 6-20 increased from 1.2 for most typical sign sizes to about 1.8 for many typical sign sizes.  the format of figure 6-20 is very different than previously.
it is interest to note that although since at least 1982 the pressure on open signs has been higher than that on solid signs, this is no longer the case.  the pressure has gone up for solid signs but remains unchanged for open signs.
read the asce commentary - the new sign provisions including all of the new eccentric loading cases are based on wind-tunnel research completed since the previous versions.  
if the wind tunnel test data have been interpreted correctly and have also been applied to the code correctly (i.e. with appropriate wind speeds, gust effect factors, etc.), then the current building codes are significantly unconservative.  the implications of this are huge.
the difference between this standard and those from a few years ago approach the factor of safety level.  this implies that we should be fearful of all existing signs!  even those designed according to the 2003 ibc (asce 7-02) contain only 2/3 of the strength that they should.
if this is true and adequately supported, shouldn't building officials start making emergency provisions (pre-2006 ibc adoption) that require all new signs to be designed according to this new standard and that requires all existing signs to be reinforced for these loads?
the building official is only authorized to ensure the current code adopted. if the jurisdiction is under 7-02, that is what has to be enforced. additionally, chapter 34 of the ibc allows structures, including signs, to remain as previously approved. it does not give authority to the building official to go back to a structure previously approved. otherwise, every 3 years, all structures would be required to be altered to meet the new codes.
as an example, with the 2000 ibc, when assembly use groups in certain types of construction of a certain size are built, the buildings are required to be sprinklered. we have a whole of of unsprinklered churches that could not be built today without them.
similary, with the 2002 nec, arc-fault protection is required in sleeping rooms but there are still many houses built years ago without ground-fault protection in bathrooms, kitchens, and outdoors.
in both cases, the existing is allowed to remain.
don phillips
sorry, first sentance should say "the building official is only authorized to enforce the current adopted code."
don phillips
my point is only that if the new wind loads (50% higher than current) are correct, then it would be dangerous to leave existing signs without retrofit.  
there are two issues involved
1 - results of wind tunnel tests
2 - probability of failure
i have found a similar issue in the australian wind code. some of the designs that come out using the code figures for signage seem crazy when compared with what has been standing around town for the last 50 years. sometimes the research stuff seems right until you dig deeper into the assumptions.
i don't have enough data to hand to call their bluff, but if i don't design to current code practise i open myself to professional liability suits - be they ever so remote.
usually three things have to happen for a failure
1 - a design fault
2 - a materials fault
3 - a construction fault
often structures are fairly forgiving for one or two items, but if all three coincide the fur flies everywhere and you're up before the magistrate.
i think the old russian method was pretty good - if you design the bridge, you get to stand under it while they apply the test loads.
johnp.rz
i wouldn't dream of designing to less than the required building code.  we are bound by law to adhere to the code.  this is why the large increase is disturbing to me.
does anyone know how the wind tunnel tests are performed?  what type of wind loading do they use?  can a wind tunnel model the difference between a 3-sec gust and a fastest-mile wind speed?
if you look at table 1609.3.1 in ibc 2003 the equivalent wind speeds provide a difference in wind pressure of about a factor of 1.4-1.5 for many of the speeds...  oddly enough, this is about the same as the increase in force coefficients.
compare 0.00256*v^2 for yourself.  
(85^2)/(70^2) = 1.47
(90^2)/(75^2) = 1.44
(100^2)/(80^2) = 1.56
(105^2)/(85^2) = 1.53
(110^2)/(90^2) = 1.49
(120^2)/(100^2) = 1.44
(125^2)/(105^2) = 1.42
and so on...
any thoughts?
another factor to take into account is the height of the sign. some of the modelling is fine above 10 m - but below that people lose interest on the basis that it doesn't really matter.
that's when you have to look closely at the fudge factors for terrain and approach velocities.
johnp.rz