huangyhg

help on stress analysis of lattice tower
i have a customer who wants me to sign off on a tower design. i have detailed drawings of the proposed tower design and engineering calculations that show the tower will withstand a 70 mph wind. however, my customer needs to show that the tower will meet the requirements of tia-222 rev g, which calculate wind forces differently.
what is the easiest way of doing this? i believe i could model the entire tower in ansys and apply the loads given in tia-222. i can then look at the maximum stress intensity and as long as it is safely below the yield strength of steel, i can show it is safe. however, this will take some time to model the lattice structure.
is there an easier way of performing the calculations to show this design is safe? thanks
perhaps you could set up a ratio of the new and old loads and multiply the old stresses by this ratio. this assumes that the load profile is somwhat similiar.
another approach might be to determine the ratio between the old stresses and the allowable stresses and then multiply the old loads by the inverse of this ratio and determine if the result is loads larger than the new loads would be.
obviously this is an engineering judgement.
a couple of comments...
i don't know of any area in the us where the design wind speed is less than 90 mph. check that.
if you are going to accept someone else's design as your own, most states require validation of the design (in your case, do enough calcs to validate or refute). in the case of a tower, with many load paths and distributions, the easier way is to model the whole thing. it doesn't take that long and you'll have your answer at the end....fully.
assuming you have their printout, perhaps you can get their input file and convert it to your software package. i've done a lot of tower analysis in several different packages and by hand....it is a lot easier to model it.
"i can then look at the maximum stress intensity and as long as it is safely below the yield strength of steel, i can show it is safe."
is there not any kind of building code that applies to the tower?
yielding is not normally the only mode of failure considered.
yeah, you'd think column allowables would feature somewhere ... but, ok, maybe the op meant that.
just as a point of query, from someone who knows nothink about your field, how do you convert windspeed into load ? of course, you could assume cd = 1, so that the load is dynamic pressure on the surface of the building (sounds way conservative). but wind on a lattice tower would (reasonably possibly) create lift forces as well (obviously acting in a completely different direction to drag forces.
geezee66...one other point to consider is that some municipalities have "fall or collapse zone" restrictions on towers. this means that if failure occurs, it must initiate in a progressive manner so that the tower collapses within a certain perimeter, not just falling over onto anything in its path.
rb1957,
it's fairly simple to do the calculations. there is a standard document called tia-222 that has been developed and includes all of the equations to calculate loads in the structure. the calculations have inputs of wind speed, icing, and a a few other factors, and the output is the load that is applied to the structure.
the hard thing about the calculations is to make sure that no part of the tower receives a load that will exceed the capability of the tower. there have been some good ideas here and i'll keep looking at them.
well then, how do these new loads compare with what you've applied (for 70mph) ? maybe it's enough to show that the new loads increase by a smaller factor than your reserve factor (= ms+1).
presumably you've checked compression as well as tension.
there is a huge difference between the f version of the tia code and the g version. therefore, do not take for granted that it will automatically work. nor can you assume just a factor on the loading.
some comments:
f was based on asd and g is based on lrfd.
f was based on fastest mile winds and g is based on 3 second gusts.
icing requirments in g are significantly changed (and more conservative).
the way feed lines should treated has also changed a lot.
there are more stringent analysis requirments in the g standard than were present in the f standard.
tower design is something of a specialty field. most of the steel design concepts and analysis concepts remain similar to regular building design. however, the loading is very different and challenging... at least if you've never worked in the field.
if you're interested in getting into this field then by all means jump into it. but, i wouldn't recommend it if you're never planning on doing it again.
sincerely,
josh

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