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# IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

## IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

(OP)
Does this sentence mean that whenever the load H resists the primary variable load effect, its load factor "H" shall be changed to 0.9 where the value of H is permanent? And throughout all other equations within this load combination, will the value of H be set to 0?

According to the exception on IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design #2 "Where the effect of H resists the primary variable load effect, a load factor of 0.9 shall be included with H where H is permanent and H shall be set to zero for all other conditions."

For instance, in a tank, you have load combinations such as:

1.4(D +F)(Equation 16-1)
1.2 (D + F) + 1.6(L + H) + 0.5 (Lr or S or R)( Equation 16-2)
1.2(D + F) + 1.6 (Lr or S or R) + 1.6H + (f1L or 0.5 W)(Equation 16-3)
1.2(D + F) + 1.0W + f1L + 1.6H + 0.5 (Lr or S or R) (Equation 16-4)
1.2 (D + F) + 1.0E + f1L + 1.6H + f2S ( Equation 16-5)
0.9D+ 1.0W+ 1.6H(Equation 16-6)
0.9(D + F) + 1.0E+ 1.6H (Equation 16-7)

And given that the pressure of the fluid (F) inside the tank is being resisted by the lateral pressure (H) acting outside the tank. Will the primary load H be set to 0.9H and all other combinations including H be turned to 0 and would the load combinations then be like these?

1.4(D +F)(Equation 16-1)
1.2 (D + F) + 1.6(L) + 0.5 (Lr or S or R)( Equation 16-2)
1.2(D + F) + 1.6 (Lr or S or R) + (f1L or 0.5 W)(Equation 16-3)
1.2(D + F) + 1.0W + f1L + 0.5 (Lr or S or R) (Equation 16-4)
1.2 (D + F) + 1.0E + f1L + f2S ( Equation 16-5)
0.9D+ 1.0W (Equation 16-6)
0.9(D + F) + 1.0E (Equation 16-7)

Thank you in advance for your thoughts with regards to this matter.

### RE: IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

#### Quote (OP)

Will the primary load H be set to 0.9H and all other combinations including H be turned to 0 and would the load combinations then be like these?

Correct.

Also, it's worth noting that tanks are often leak tested prior to backfilling which will result in the same load case. That said, pretty much all my buried tank designs are controlled by inward soil pressure with no internal fluid pressure. The exterior wall rebar is usually just the minimum reinforcement required.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

### RE: IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

(OP)
@TehMightyEngineer
So when you design tanks and have to consider these load combinations, taking into account that I am using Staad, should I still consider the conditions met on the PCA Rectangular Tank?

i.e.
Condition 1: Soil is absent, fluid is present.
Condition 2: Soil is present, fluid is absent.
Condition 3: Uplift
Condition 4: Earthquake

Thanks

### RE: IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

Correct.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

### RE: IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

(OP)
Hi, hope you would be able to read this.. Is it still advisable to use the Sanitary Coefficient of 1.3 1.65 1.3 or use the new Sd formula of phi*fy / gamma*fs

Thanks.

### RE: IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

I don't know anyone still designing tanks to the sanitary coefficients any more; every environmental concrete design I've run into in the past 5 years or so uses ACI 350-06 which utilizes the Sd formula as you described.

What kind of tank is it?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

### RE: IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

(OP)
@TehMightyEngineer

Hi, just read through your reply. It is a multi-cell tank mostly for storing liquid. We are to have an investigation with regards to the structure and most of the process that I am seeing in the internet and from the PCA guidelines use the values of 1.3 1.65 and 1.3. As to checking ACI 350-06, it states that these values are now changed with consideration to the value of fs and the equation I stated before. Well the sanitary coefficients or Sanitary Durability Factor still applies because you still have to multiply it with the Load Combination and such. The one thing I am concerned about though is with regards to the γ, since I am not particularly sure if I would use 1.4 or 1.7. PCA states that using the value of 1.7 as a multiplier i.e. Mu = Sd x 1.7 x F is a much more conservative approach than using the value of 1.4. But as of recent from the ACI 350-06 articles and post and practices state that the use of 1.4 is being considered more.

Sorry for my english, I am quite confuse how to go about with this approach.

### RE: IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

If I understand you correctly you're following the load factors from PCA rectangular concrete tanks which are old. They're based on ACI 318-95 if I recall correctly. The usual factors used in modern LRFD codes are 1.2Dead + 1.6Live. ACI 350 considers soil & hydrostatic pressures to be at the live load level so I'd use a 1.6 factor on them.

Let me know if I misunderstood you.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

### RE: IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

(OP)
Thanks and no, you got what I mean to say. I was not clearly sure what to do about the matter. I recently came across ACI 350.03-06 13.6.1.5 "All liquid loads, whether considered live or dead, shall be multiplied by the appropriate load factor of 1.7, per Chapter 9. If fluid pressure do not act simultaneously on all panels, live load, including that resulting from fluid pressures, shall not exceed three times the dead load."

Does it mean to say I would use the load factor of 1.7 when considering liquid loads ? I'm confused whether to use the values of 1.7 or 1.6 and since on the Appendix C, it also state that an alternative way determining the load combinations is of the use of values of 1.7 on both F and H.

Thanks,

### RE: IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

I'm confused; my copy of ACI 350-06 doesn't use the load factor of 1.7 in chapter 9; it uses the usual LRFD factor of 1.6. Unless of course you're using the alternative load combinations of Appendix C.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

### RE: IBC 2015 1605.2 Load Combinations Using Strength Design or Load and Resistance Factor Design

(OP)
It was indicated in chapter 13 of the ACI 350.03-06. Not on chapter 9. That is why I am confused as to what I should use.

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