Service Load combination for Tank design - ACI 350. 2

[ak.t]

Dear All,

Section 8 of ACI 350 states the ultimate load combinations for watertight structures design as below :
1.4D+1.7L
1.4D+1.7L+1.7H
1.4D+1.7L+1.7F
0.9D+1.7H
0.9D+1.7F

Section 9 of ACI 350 states the ultimate load combinations for watertight structures design as below :
U = 1.4(D + F) (9-1)
U = 1.2(D + F + T) + 1.6(L + H) (9-2)
+ 0.5(Lr or S or R)
U = 1.2D + 1.6(Lr or S or R) + (1.0L or 0.8W) (9-3)
U = 1.2D + 1.6W + 1.0L + 0.5(Lr or S or R) (9-4)
U = 1.2D + 1.2F + 1.0E + 1.6H + 1.0L + 0.2S (9-5)
U = 0.9D + 1.2F + 1.6W + 1.6H (9-6)
U = 0.9D + 1.2F + 1.0E + 1.6H

what service load combinations should i consider? Could not find in ACI 350.
Can I consider the ultimate combinations that are listed in section 9 instead 8 for underground tank design.?

Thank you,

 

[RJ Design]

Section 9 incorporates Wind, Snow, and Seismic; if these are part of your design parameters/requirements then use table 9.

 

[r13]

The load combination set was used in the past issues of ACI 318. It has switched to the load combination set in section 9 since 200?. I think for new design you shall use current requirement (section 9), and use section 8 combinations for evaluating past works.

 

[JoelTXCive]

I'm confused about what document you are getting the "Section 8" load factors from. Is that from ACI 350-89?

I looked at this a while ago, and as best I can tell the ACI 350 load factors and the ACI 318 load factors have matched up since the 2006 version of ACI 350 (350-06). The only difference now is the sanitary coefficient, which is applied to the ACI 350 designs.

Below is a table I made to keep it straight in my head.

 

[RJ Design]

I found the following in ACI 350-06:

ACI 350-06 [Appendix C-ALTERNATIVE LOAD FACTORS, STRENGTH REDUCTION FACTORS, AND DISTRIBUTION OF FLEXURAL REINFORCEMENT]
1.4D+1.7L (C-1)
1.4D+1.7L+1.7H (C-6)
1.4D+1.7L+1.7F (in RC 9.2.5)
0.9D+1.7H (in RC 9.2.4)
0.9D+1.7F (in RC 9.2.5)

ACI 350-06 [R9.2.1]
U = 1.4(D + F) (9-1)
U = 1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or S or R) (9-2)
U = 1.2D + 1.6(Lr or S or R) + (1.0L or 0.8W) (9-3)
U = 1.2D + 1.6W + 1.0L + 0.5(Lr or S or R) (9-4)
U = 1.2D + 1.2F + 1.0E + 1.6H + 1.0L + 0.2S (9-5)
U = 0.9D + 1.2F + 1.6W + 1.6H (9-6)
U = 0.9D + 1.2F + 1.0E + 1.6H (9-7)

I would go through the trouble of deciphering Appendix C equations/conditions, run the calcs, and see which would be the most conservative. it has been a few years since I delved into these, maybe someone more current with ACI 350 could shed some light.

 

[JoelTXCive]

I could be wrong on this, but the ACI 350-06 Appendix items are design methodologies left over from older versions of ACI 350. They are 'grandfathered in'

Basically, ACI says "if you like the old load factors and method of calculating the sanitary coefficient, then you can keep using it"

If you are going to use Appendix C though, make sure you read Section C.1.1. It says that if you use the methods in the Appendix, then you have to use ALL the provisions in the appendix. You can't mix and match current rules with the older methodology in the Appendix.

 

[EDub24]

ACI uses ultimate strength design. I assume you want allowable loads for soil related checks (stability, bearing etc). The load combos in Ch 9 line up pretty well with the IBC and ASCE. Off the top of my head the main difference is that ACI includes a Fluid Load, F, that is combined with Dead Load. Because of that you can reasonably concert the load combos to allowable and use IBC or ASCE as a guide. Just make sure to include the Fluid Load with the Dead Load.

 

[ak.t]

Thank u all.

RJ DesignWorks, design includes only earth, water, dead and live loads.

retired13, Yeah. agreed. The new version ACI 350-06 does not have those combinations. Thanks for this

JoelTXCive, Thank you for the snapshot that compares the load combination from older to new version. Section 10.6.5 of ACI 318-14 says that it does not sufficient for water tight structures So, can we rely only on ACI 350?

EDub24, I use the following load combination. Could be wrong. Kindly lemme know. My part of design includes only Dead, Live, Earth & Water pressure.


Service:
D + F (IBC)
D +F + L + H (IBC)
D +F + L (IBC)
0.6D + H (IBC)

Ultimate:
1.4D + 1.4F (Aci 350, Eq 9-1)
1.2D + 1.2F + 1.6L + 1.6H (Aci 350, Eq 9-2)
0.9D + 1.6H (ACI 350, (Aci 350, Eq 9-7)
Also Can we use Eq 9-7 for Fluid pressure also?
why dont we use 0.6D + H and 0.9D + 1.6H for fluid pressure

Also what is about the d) & e) here?

 

[EDub24]

You keep referencing ACI 350 section 8 but I think it's been cleared up that you should be referencing Chapter 9 unless you mean to use Appendix C? Chapter 8 in ACI 350-06 is general considerations for analysis and design. The actual load combinations are in Chapter 9. Either way what you list looks fine to me. Sub-section (d) in basically says that if the soil load is opposite the fluid/dead/live load then you have to use a 0.6 factor instead of 1.6. So say you have an buried tank, the hydrostatic fluid load puts a positive moment on the wall while the soil pushes against it creating a negative moment and thereby reducing the overall design moment. This sub-section basically says if the soil reduces the design load then you have to reduce the design soil load. The IBC says that if the soil load resists the main loads then you have to use a 0 factor instead of 1.0. This is over-conservative in my opinion and there's a case to be made to use a 0.375 factor (0.6/1.6 = 0.375). That's really up to you and whoever is reviewing the design. Sub-section (e) says you have to include load combinations where the live load and fluid load are zero (separately and at the same time). Basically you need to check all the potential scenarios for the tank (tank is empty vs full, soil backfill is there vs not etc).

I also assume you're in a region that doesn't have seismic concerns but if so you should add that to your load combinations. You can check out ACI 350.3 for seismic loading in tanks.

 

[ak.t]

Thank u EDub24. Yes, As u and retired13 said, It is removed in latest version of ACI 350. I agree and will follow section 9.

Also I have question on this following. I could not see this in ACI 350-06 whereas ACI 350-01 has mentioned this.

Is this still valid?

 

[r13]

Akshiks,

It (1.3) was called hydraulic factors to further increase the load intensity after load combinations. If you are working on water way works, you should make sure it is lo longer apply (US Corp of Engineers for instance).

 

[JoelTXCive]

@AkshikaS

JoelTXCive, Thank you for the snapshot that compares the load combination from older to new version. Section 10.6.5 of ACI 318-14 says that it does not sufficient for water tight structures So, can we rely only on ACI 350?

ACI 350-06 and ACI 318-14/19's basic load factors are in agreement. The only difference is that ACI 350-06 will apply an additional Sanitary Coefficient (Sd) on top of the basic load factors. The Sanitary coefficient will always be greater than or equal to 1. This ensures that a design done with ACI 350 will always meets the strength design requirements of ACI 318.

 

[EDub24]

I haven't used ACI 350-01 but I assume the 'S' factor is equivalent to the 'Sd' factor in ACI 350-06 §9.2.6 which is the durability factor. You should read the commentary but in summary this factor is used to reduce the overall stresses in a concrete member to better control cracking.

The Sd factor is technically applied to the design load to increase it (as JoelTXCive sayid it has to be > 1.0) but I use it to reduce the design strength (same thing but putting it on the other side of the equation capacity > demand). This is because for shear only the shear carried by the shear reinforcement and not the shear carried by concrete is affected by it. So for a wall for instance you wouldn't include the factor for the shear design since usually you just thicken the wall to get the required shear capacity from the concrete but you would include it for the flexural design of the wall.

 

[ak.t]

Thank you all. i got it.