Hi everyone, Good day! I'm now d 1

[Lord Ets]

Hi everyone, Good day!

I'm now designing the foundation of storage tanks. Due to weak soil, mat foundation is necessary. Rebars and allowable soil bearing capacity is now ok, the settlement is not. The foundations settlement was ranging from 75mm-90mm. I doubt if it safe or not. For the soil bearing capacity, I only assume 96 kPa since no data. Only dimensions and location of project. Here are the dimensions.

11.56m dia x 10.10m height - 2pcs.
12.4m dia x 16.245m height
8.4m dia x 13.52m height
8m dia x 10.48m height
3.95m dia x 5.52m height - with frame.

Based on your experience, can you give me some ideas or advice regarding to the allowable settlement for tanks foundation. Thank you so much. God bless.

 

[RVAmeche]

Sounds like you need to consult a geotechnical engineer and have them tell you the foundation requirements based on your situation.

 

[r13]

The allowable settlement is determined by the effect on the external piping, and stability due to tilting (uneven/differential settlement). For the latter, yes, consult with the geotechnical engineer.

 

[HTURKAK]

Apparently you do not have a soil report. How did you decide for allowable bearing capacity and how did you predict the settlement is ranging from 75mm-90mm without soil data?... You should look for a soil investigation and geotechnical report..
Is the settlement ranging from 75mm-90mm at center of the tank ?

The use of mat foundation is OK for the tank 3.95m dia x 5.52m height however, is not preferable for the others..

Regarding the allowable settlement pls look API 650 7.5 DIMENSIONAL TOLERANCES and plumbness and look 7.3.6.6 ( If settlement measurements are specified by the Purchaser, any differential settlement greater than 13 mm per 10 m (1/2 in. per 32 ft) of circumference or a uniform settlement over 50 mm (2 in.) shall be reported to the Purchaser for evaluation.
Filling of the tank shall be stopped until cleared by the Purchaser.


API 650 is silent for the allowable settlement at center of the tank. IMO , (based on experience ), the center settlement of the tank should not compromise the free flow at the bottom of the tank for the bottom slopes cone up to center and total settlement should be limited to 50 mm.

If the soil is 'weak' , it is better to improve the soil ( soil exchange for the upper most 2-3 m) than mat foundation.

If you want to get better responds, provide soil profile a sketch for the piping.

 

[Lord Ets]

The problem here in my country, there's a lot of company designing a structure without soil report. Especially my boss, he always compare my work to the previous designs of their past structural engineer. Even without soil data he design the structure. But that's him, not me. Yes, I always ask for soil report. But my boss keep telling me just assume. To be honest, after this pandemic I will look for a good company.

When I assume 96 kPa and input it to CSI SAFE, it gives me a 75mm-90mm settlement. Here's the catch,

If used I this formula to compute the subgrade modulus. Ks = qa/allowable settlement, it gives me a result with actual settlement exceeds the allowable. But the allowable bearing capacity is safe.

When I used this, Ks = 40(FS)(qa). It gives belows 15mm settlement, but the allowable bearing capacity exceeded.

Factor of safety (FS) = 2 for granular, and 3 for cohessive.

But since I don't have data, here's what happen when I assume for factor of safety.

When FS = 2, both settlement and allowable bearing cap is safe.

When FS = 3, settlement is ok but allowable bearing cap is exceeded.

 

[r13]

Questions:

What is the allowable settlement limit, adopted from what source? What value of qa is used for Ks calculation? There is a conflict/confusion on your findings, please clarify:

- When I used this, Ks = 40(FS)(qa). It gives belows 15mm settlement, but the allowable bearing capacity exceeded.
- When FS = 2, both settlement and allowable bearing cap is safe.
- When FS = 3, settlement is ok but allowable bearing cap is exceeded.

Lastly, what are the total settlement and differential settlement for each case?

 

[Lord Ets]

qa = 96 kPa, based on previous designs of our past structural engineer. My boss don't know how to design, all he want is low value design. My boss don't care about our professional license.

Ks = qu/s
qu = FS x qa
Ks = (FS X qa) / s

I used allowable settlement of 50mm for mat foundation.

Ks = 20 x FS x qa

Ref: Foundation analysis and design
By: J.E. Bowles

Now the settlements are 16mm to 18mm.
Actural soil pressure 94kPa.

I don't know if it's ok to assume 50mm allowable settlement but when I searched at google, some says 50mm for mat foundation is allowed.

 

[r13]

It seems you have magically solved your problems to bring both bearing & settlement within allowable. See HTURKAK's comments above for settlement and relevant code provisions.

 

[Lord Ets]

Yes. When the time I started to design the foundations, all of those computations are just magic because no data. I know that. Every professional got my respect. That's why I always ask my boss for soil test report, we need geothecnical engineer for this. And he always answer me (just assume, why Engr. _ _ _ (past designer) can design even if without soil report. And now the projects are finished). So unprofessional.

Can you guys advice me how to convince my boss that every project/design we need a soil test report.

 

[HTURKAK]

It makes me very sad that we are witnessing the last days of engineering and an oncoming plague software engineering without knowing the range of application ...

1- This is copy and paste from API 650 Annex B ;

(B.2.1 At any tank site, the subsurface conditions must be known to estimate the soil bearing capacity and settlement that will be
experienced. This information is generally obtained from soil borings, load tests, sampling, laboratory testing, and analysis by an
experienced geotechnical engineer familiar with the history of similar structures in the vicinity. The subgrade must be capable of
supporting the load of the tank and its contents. The total settlement must not strain connecting piping or produce gauging inaccuracies,and the settlement should not continue to a point at which the tank bottom is below the surrounding ground surface....)

2- Foundation analysis and design ( J.E. Bowles ) one of the best reference book . Ks = 20 x FS x qa is a formula to estimate soil spring constant for analysis of slab, foundation , beam on elastic foundation .. based on the WINKLER method. The output deformations are elastic deflections of the subject plate. NOT the total SETTLEMENT OF THE SOİL and in order to find the total settlement, you are expected to perform settlement analysis look to the Chapter 5 FOUNDATION SETTLEMENTS ( same book ) ...and you still need the properties of soil layers..

3- I will suggest you to look Process Industry Practices Structural PIP STE03020 Guidelines for Tank Foundation Designs in order to understand and choose the correct foundation type . The use of software SAFE is only a tool for analyzing the slab foundation . The following is a snap from PIP STE03020 showing the ring foundation calculation concept.

 

[Lord Ets]

Yes, I already design the ringwall with PIP STE03020, SABP-Q-005 (Saudi code), and even ACI 318-05. I got same results of rebars numbers and actual soil pressure.

The soil pressure below the ringwall is exceeded the allowable bearing capacity even if I increased it up to the point it reaches the maximum width of 460mm. API 650 Annex B says, if the width of ringwall exceeds 460mm a footing beneath the wall should be considered. But the soil pressure below the tank exceeds also the allowable.

So, ringwall or ringwall with footing is not applicable for the tank/s.

That's why mat footing is one of my option, and since our past structural engineer leave some foundation design reference specially foundatiom for tanks. So I used it, and as my boss told me to make that as a reference. His design is from STAAD to SAFE. Since I don't have a choice so I used it. I'm only 1year experience and my specialty is structural analysis and design of commercial, residential, and steel frame. Not for tanks.

All of my computation for tank/s are manual and my references are API 650, PIPSTE03020, SABP-Q-005,and ACI 318-05. I'm not just rely on softwares.

I already read the Annex B of API 650. The problem is no soil report and my boss without know about design.

I'm not just design a structure without basis.

I'm just also asking if in your country make an assumption of soil bearing capacity or not. Because here, if 2 storey residential it is ok to assume. But these are heavily loaded tanks and not a hollow structure.

And I'm asking how to convince my boss that we need a soil report before designing it. Because he always told to just assume. So unprofessional.

Sometimes there's soil report, and sometimes nothing.

To be honest, if we are not experiencing a pandemic right now. I will resign and look for another company.

Since ringwall or ringwall with footing are not the good choice, can you guys give me idea of what type of foundation can I use. And I hope a reference with example design. Thank you.

 

[LittleInch]

And I'm asking how to convince my boss that we need a soil report before designing it.

You won't be able to.

There's only two things that will count here to him (I assume it's a him). Money and Not paying fines or going to jail.

On the money front, only if you can show that if you had a soil report which was better than "assumed", you could save money on the foundations. Otherwise it will only be because the client has said he needs to do it and won't pay him if he didn't do it or will sue him if and when the tank settles too much.

The latter is from governmental authorities.

In reality, there are some quite rightly large FoS in these calcs and hence probably 99 times out of 100 he will get away with it. When you combine it with the fact that the tanks are rarely completely full then you can understand why at a superficial level why spend money on something that you don't need to. Also your tanks are relatively small. When you get into 50, 60 or bigger diameter tanks then you can get some serious failures.

Now the only way is possibly to use very low strength / bearing assumptions and then say the design can only be made better with proper geotechnical information.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[r13]

Another way to explore - raise the ground with selected fill, compact to required bearing to handle immediate settlement, and set the tank directly on the compact fill. Long term settlement will occur, but it is visible/measurable and easier to handle.

 

[HTURKAK]

Dear Lord Ets (Civil/Environmental)(OP),
I got disappointed to your last respond.. it is up to you to pack up , resign and look for another company. But , you should be persistent and try to convince for soil exploration..

Find below my answers to your querries:



First You estimated the safe bearing pressure 96 kPa ( which corresponds to 8-10 m tank height ) without any factual report and now you do NOT have the right to say ringwall or ringwall with footing is not applicable for the tanks. Your claim , the only option is the raft foundation with enlarged dia. ( e.g. for tank 12.4m dia x 16.245m height , the use of mat with dia. 17.0 m) that is , the mat with cantilever to reduce the bearing stress is not correct approach..
I do not remember the no. of tanks i have designed .. but i can say , the cost of enlarged mat found vs ring type will compensate the cost of geotec. several times...


Making assumption for this case is not possible..This is not guessing game... Consider the tank 12.4m dia x 16.245m height, the soil pressure is almost equivalent to 16 storey apartment bldg.. these are not simple barracks..

These are stainless steel tanks ( from your previous thread ) and the budget of tanks should be 10++ million Dollars and your boss trying to save the cost of geotech. report which could be in the range of a few thousand Dollars ..

Do you want to convince your boss ? Just tell him to look this thread and my responds.. If unexpected settlement occurs and the tanks tilts which may endanger the integrity and service what will be his option?

Always Remember this quote ;

 

[r13]

Even a tree can be transplanted The influence depth of a tank is much greater than the typical building on shallow foundation. The cost comparison makes good sense.

 

[Lord Ets]

Thank you so much for your response guys.

96kPa is not mine, that's from my boss hahaha.
I forgot to tell, he always said. "Just assume, use the allowable bearing capacity on our previous projects" hahahaha. (He is civil engineer too, but I don't know why he can't understand my situation hahaha)

These are no soil report
11.56m dia x 10.10m height - 2pcs.


These have soil report (but it's not exact location, almost 100m away between the tanks and the bore holes. Is it ok?)
12.4m dia x 16.245m height
8.4m dia x 13.52m height
8m dia x 10.48m height
3.95m dia x 5.52m height - with frame

Those two tanks with no soil report needs to design first. But I already designed the thickness of other tanks.

I'm very thankful to have you guys. Thanks to the quote Sir HTURKAK.

Yes, I really want to move now. But I need money, my family needs me.

Our country hits hard by the pandemic. Huge number of cases, that's why I request to my boss for work from home and he allowed me. I think it's not good idea to resign for now.