Apparent Conflict between Different Codes

[msquared48]

I am looking at an existing 40' tall steel lattice tower mounted on an existing building.

The building design falls under the IBC, but the tower under ANSI TIA 222-G, and the IBC in section 3108.1 specifically states that the loadings specific in TIA 222-G shall govern for the Tower design, not the IBC.

That's all well and good, but the building seeks the Kzt factor for wind and ASCE 7 gives the equation to calculate it to apply to the building forces, and TIA 222-G magnifies the wind loads by a "topographic Category", applying unknown factors to the wind pressures per this category that are not quantified externally in the output. I have no way of determining what value of the factor was used... other than calling TNX Tower, which I will.

My problem is that the jurisdiction wants me to apply the Kzt to the tower in spite of the code exclusion, but I can see the logic of their position too in that the tower is sitting on the building, not the ground. From Google Earth, I calculate 1.19, and they calculate 1.38, which may be a local cover-it-all factor, not site specific. I see no way of applying the Kzt factor exactly within the confines of program input other than the topo factor, which will not give the actual case, but something far more restrictive, ending up being way too conservative in my opinion.

I guess that I just have to call the reviewer and TNX tower and see what I can work out...



Mike McCann
MMC Engineering

 

[SAIL3]

this brings up an interesting question that I never thought of before....I was always under the impression that the Kzt factor is used to incorporate the local abnomilies in the wind velocity boundary layer and it's affect on the local wind loading caused by unusual characteristics of the terrain at the site in question.....so why would one not apply it to the addition of a structure to an existing structure that mimics some of the abnomilies of a terrain.....first off, I hardly ever get any info on a project that would activate the calculation of the Kzt factor and usually the most info I get is in regards to choosing wheather it is exposure B or C, etc.....ASCE has already addressed the the addition of incidental items on the roofs of bldgs.....based on my own engineering judgement, I would not have a problem with using 1.19 and aggree that 1.38 is "over-the-top"...what I take away from all of this is that the trend nowadays encouraged by recent codes of beating every little wind load out of a structure is counter-productive when one has major decisions like this and many others that can blow away a 5% wind load savings that one has sctatched out of a structure...coming back to your OP...say boundary layer is normally 30 to 40 ft...if the existing bldg is of this height or greater, then, that would also give me a greater degree of confidence.....

 

[GlobeTrekker]

>
Section 102.1, from the 2006 IBC, requires "the most restrictive"application when
different materials, methods of construction or other requirements are
being considered.

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