Looking for Info ANSI A58.1-72 1

[GJBrown]

I'm trying to compare the current wind loads to the design wind loads for some methane gas spheres. They were designed in 1978 so I'm assuming that the Building Code would have referenced the then-current standard, ANSI A58.1-72. If someone has a copy I'd like to get some information from it. Mainly the wind speed, formulas, and coefficients. Thanks in advance for your time.

Gloria

 

[chichuck]

GJBrown,

That goes back a long way. I don't have ANSI A58.1-1972, but I do have ASCE 7-88, which was the first one that ASCE sponsored. I can glean a little information from that.

In 1988, and in 1972 they used fastest-mile windspeeds, now we use 3-sec gusts. The 3-sec gusts are faster, but do not represent a different or faster wind than the old speeds.

The windspeed maps from ASCE 7-88 show speeds at a ref. h of 33' (10m), but I recall that in 1972 ANSI A58.1 had ref. speeds at 30'. I believe those maps in 1972 were slightly different. ASCE 7-88 has contours in the interior of US down to speed of 75mph, I think the ANSI maps went down to lower speeds. There were also some differences in the coastline areas and hurricane prone regions.

The basic equation for qz was the same in 1988 as in 1972:

qz = .00256Kz * (IV)^2


In 1988, Kz was as follows:

Kz = 2.58*(z/zg)^2/alpha z >= 15'

Kz = 2.58*(15/zg)^2/alpha z < 15'

alpha and zg were defined in a table in the code comentary. I don't know if the value of alpha, zg or the 2.58 were unchanged from 1972 or not, since they had the ref. height change, there might have been changes here.

now currently we use

Kz = 2.01*(z/zg)^2/alpha z>= 15'

Kz = 2.01 * (33/zg)^2/alpha z < 15'

alpha has a new definition, the 2.58 changed to 2.01 and zg is unchanged from 1988. This is due to the change from fastest-mile to 3-sec gusts. Now the maps go only down to 90mph, that is roughly corresponding with the earlier lower limit contour of 75 mph. The current maps have some differences in coastal areas from the maps circa-1988. The differences are more than those coming from the change to 3-sec gusts, there was also some change in the stattistical basis for the contours that was made on the current maps.

Gust factors are much different now than they were then. Importance factors are better defined than they were back then as well.

Coefficients for internal pressures are different now than they were in 1988, the coefficients are a little lower for "typical" buildings (now GCpi = +/- 0.18, in 1988 it was +/- 0.25.) I don't recall what the value was in 1972.


Coefficients for external pressures are essentially the same now as they were in 1988 and also in 1972, these haven't changed, except to refine some of the pressure coefficients for gable roofs both + and - pressures.

The coefficients for calculating wall pressures in buildings is unchanged as far back as 1988, and I think that in 1972 they were also the same.

In 1972 they didn't have methods for handling low rise buildings like they do today, and that code didn't address spheres. Now, they don't either.

So, even if you find someone who has a copy of ANSI A589.1-1972, it may not be any help. I can say, in general, your speeds will look lower from 1972 but that's because then it was fastest-mile speed. The basic theory is the same, and some of the coefficients have been adjusted to cover that change. There were some refinements in knowledge and therefore in some of the coded defined coefficients. Are the wind loads now higher or lower than in 1972? Depends on the location. If in a hurricane area, that's possible. If away from a coast, then maybe not. I would even venture a guess, not much higher at all.

Without being specific about anything here, have I helped?


Regards,

chichuck

 

[GJBrown]

Thansk for the info. What I was trying to do was run the original loads as the structure was designed. Then we reran it under the new loading conditions. They wanted to make sure it was designed right originally and how it would fair under todays loading conditions. When all is said and done it 'seems' that the new loads and the original loads are very similar.

 

[chichuck]

GJBrown,

That's what I would expect, unless maybe the spheres are located on say, the Gulf Coast. Even though the current wind speed looks higher than the one from 1972, its not really a different or faster wind.

But since the current code doesn't give any help on how to figure wind loads on a shere, and neither did the 1972 code, how did you figure the correct Cf values for wind pressures?

Regards,

chichuck

 

[GJBrown]

I used a Cf of 1.0. I looked at figure 6-19 for a round tank, very rough you have 0.8, 1.0 and 1.2 so I use a 1.0 as being conservative for the sphere (probably too much). But even with this very conservative value the overall loads ended up being very similar (within 10%)