ASCE 7-05 question (tank supported by tower)

[cessna98j]

Hi all, I've recently come across something in ASCE 7-05 that concerns me with regard to non-building structures. Specifically, I'm having trouble with the following section:

15.7.10.4 Transfer of Lateral forces into support tower.

d. The connection of the post or leg with the foundation shall be designed to resist both the vertical and lateral resultant from the yield load in the bracing assuming the directions of the lateral load is oreinted to produce the maximum lateral shear at the post to foundation interface. Where multiple rods are connected to the same location, the anchorage shall be dsigned to resist the concurrent tensile loads in the braces.

We design foundations & anchorage for quite a few cement silos, aggregate bins, etc. that are supported by braced towers. Designing the anchorage for the full yield strength of the braces at the base of the tower would result in significantly more anchorage than we've used in the past - and we're already using appendix D for anchorage design so anchorage is significantly larger than it used to be.

One thing in this paragraph that confuses me is that it talks about "rods" as bracing. This leads me to believe they are talking about tension only bracing. Our bracing is typically designed for both tension and compression - does this mean we don't have to abide by this rule?

Any help with this is greatly appreciated, if we are stuck using this our anchorage is going to need to be greatly increased, shear keys added, etc.

Thanks!

 

[JedClampett]

I suspect there were observed cases where the anchorage failed, even when it was designed for the calculated loads. As the anchorage is likely to fail in a brittle manner, they would rather "push" the failure to a more energy absorbing member.
If you have to design for this, I would recommend using load factors of 1.0 x Fy for your anchor design to avoid being overly conservative.

 

[mudflaps]

Something else to consider - it's easier to replace a "stretched" brace than repair a failed anchorage. Especially if there has been concrete damage.

Mudflaps

 

[JStephen]

I'd have to look up the context here. But AWWA D100 water towers will always use rod bracing in tension only, never compression members. If that section is dealing with water towers, then that is certainly the assumption being made.

As a general observation, ASCE 7 covers certain aspects of tank/ silo design, but seems excedingly poorly written in this regard, even through several editions, and it isn't at all surprising to see unreasonable, contradictory, or unclear provisions for tanks or silos.

 

[JStephen]

See if paragraphs 15.7.3(c) and (d) help any.

 

[cessna98j]

Thanks for your replies. I was kind of thinking that this was more applicable to tension only bracing as well. I guess I'll just design the anchorage for additional uplift & shear based on the seismic overstrength factor in the lower bracing, and not for the full yield strength of the brace.