Elevated Tank Leg Bracing and R value

[CaliEng]

ASCE Table 15.4-2 specifies two different R values for elevated tanks based on leg bracing. R of 2 for unbraced legs and R of 3 for symmetrically braced legs. I am wondering if the bay bracing must be 2-way to be considered symmetrically braced or if 1-way bracing satisfies this.

 

[JStephen]

AWWA D100-21 addresses welded steel municipal storage tanks, including flat-bottom and leg-type elevated tanks.
They use the 3.0 value for "cross-braced column-supported elevated tank", Table 24.
And per Section 13.1.3.1, that standard includes tanks that use tension-only diagonal bracing. "Tanks that use tension-compression diagonal bracing are beyond the scope of this standard."
There are requirements about yielding of diagonal bracing in that standard, and I assume they more or less mirror the overstrength requirements of ASCE 7, but haven't looked into them.

 

[HTURKAK]

AWWA D 100;

Table 28 Response modification factors Ri and Rc

- Cross-braced column-supported elevated tank Ri= 3.0

ASCE 7-16 ;

Table 15.4-2 Seismic Coefficients for Nonbuilding Structures Not Similar to Buildings

Elevated tanks, vessels, bins, or hoppers:
-On symmetrically braced legs (not similar to buildings) R= 3
-On unbraced legs or asymmetrically braced legs (not similar to buildings) R= 2

ASCE 7-16 Commentary C15.4.1 Design Basis;

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No comment !









Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[CaliEng]

Thanks for the comments. Interresting commentary there regarding the values "may seem to be conflicting or confusing". Maybe more context would be helpful.
Image below shows 3 variation of bay and bracing (A) no Bracing (B) one-way diagonal and (C) two-way diagonal.
My question is whether (B) and (C) can both be considered 'Symetrically braced". Is there a clear cut definition of what symetrically braced is?

 

[SWComposites]

I don't see any "symmetry" in B.

 

[JStephen]

In B, is there just the once brace, or are there actually braces like in C but you're only showing the one acting in tension?
I would say C, with tension-only braces, meets the "symmetrically braced". You'd have to look at the definitions more closely on B (you might have symmetry about a digaonal, for example, and I'm not sure if that would count.)

 

[CaliEng]

JStephen
B is just one brace. Seems SWCOmposite comment is pretty apparent, no symetry in X or Y.
And these wouldnt be tention or compression braces, say round pipe as shown

 

[HTURKAK]

The picture below depicts the symmetrically , X braced , tension only braced tower.. Notice that , steel rods are used for X bracing which can buckle easily if experience compression force.

Another picture , ASCE 7-16 FIGURE C15.1-1 Steel multilegged water tower







Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )