maximum allowable settlement\differential settlement

[h-badawy]

Dear Experts

i am structural engineer and i am working now on a EPC contract project (the contractor is the client)and he gave me the bearing capacity around (130 to 180) KN/m2 with allowable settlement 50mm , the thing is that he want to squeeze the geo-technical specialist to increase the allowable settlement around (75 to 100)mm to increase the bearing capacity and he need my approval, my question is, there is any reference in any codes controlling the maximum value of the allowable settlement or it's depend on the geo-technical specialist evaluation .

note :the building is precast bearing walls with hollow core slabs .

Thank you

 

[Ron]

Most codes do not address settlement limitations. That depends on the type of structural frame you have and what you can tolerate structurally. Differential settlement is also influenced by the type of cladding the building will have. Keep in mind that differential settlement causes connection rotation,thus increasing moments at the connection and in the member if the connection is sufficiently robust.

Also, just because the contractor is your client, don't yield to his non-engineering opinion for an engineering issue. He might be placing cost saving ahead of structural safety. You can't do that!

 

[h-badawy]

thnx Ron

the design of supper structural precast wall and slabs by vendor and he does not have any problem to increase the settlement may be because all of the connections will be hinged (not monolithic) , so if there is no limitations in any codes or standards control the settlement i can't reject his suggested.

Thank you

 

[Mad Mike]

You've posted this thread under Soil Mechanics too. Not cool.

Mike

 

[h-badawy]

thnx Mad

the thing is that , i don't know under which forum i can put this thread, and sorry for repeating and interruption .

Thank you

 

[fattdad]

I've practiced geotechnical engineering in Central Virginia (USA) for the last 30 years. I have had a lot of different clients, most of whom don't want any settlement. They are ignorant on how geotechnical engineering works and it's my job to help them understand. That said, they will typically go with the standard 1-in and 1/4-in for differential.

These are made up numbers; however. We use the loads from the structural engineer to inform our settlement calculations. These loading numbers have all sorts of live load assumptions and the live load does not represent a sustained condition that will lead to settlement, in all honesty. We (some that is) often just take that load and run the numbers!

Along the way, I did pick up an industrial client. My client liaison was an architect that actually wore his thinking cap. He also wanted cost-effective design. In that instance, he just allowed us to deliver a design that showed a paper settlement of less than 3 inches.

There was never a problem.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

Similar to f-d, USACE manuals recommends 25 mm for total settlement and 12 mm for differential. This is typical. For mat foundations, typically 50 mm is recommended.

Also, what are the size of your footings for the given allowable bearing pressure of 130 to 180 kPa? I assume that the soils are medium dense sandy soils but depending of the size of the footings you can replace some soils with engineered granular fill to get higher bearing pressures. Check also the groundwater levels if you go with the soil overexcavation/replacement option...

 

[h-badawy]

thnx Okiryu

the type of footings are strip footing (inverted beam) and i main soil is silty sand , also i have about 5 meter fill and the ground water table is very far from the surface .

Thank you

 

[dik]

Has the 5m of fill been properly consolidated? and for the full depth? You could have issues if there is a difference in the soil type across the building plan area.

Shooting for 3" of settlement is not a good idea, IMHO...

Dik

 

[h-badawy]

not the contractor will fill when he will start construction , i know what you mean , you mean the soil will take the full settlement from the over burden pressure of fill before the foundation loads so it will not effect on the soil capacity .

Thank you

 

[Okiryu]

What type of soil is below the 5 m of fill? I guess that the geotech has already considered the settlement due to the fill load. Perhaps , you can also consider to use well graded gravel instead silty sand for the fill so you may have higher bearing pressures. This is something that you need to discuss with your client and the geotech as the gravel fill is more expensive than the silty sand. As dik mentioned, 5 m of fill is quite large and assure good compaction is critical.

 

[Ron]

Shooting for 3" of settlement is not a good idea, IMHO...

dik....that's not a humble opinion....that's a correct one! Three inches of settlement is ludicrous. If you have 3 inches of settlement, you're bound to have an inch of differential, just from imbalance of loading! I've seen structures fail with less!

 

[dik]

Ron... look at Millennium Tower in 'Frisco... last I heard it was 16" or 18"

Being opinionated has been a failing of mine since I was a kid... and my wife keeps telling me I have no tact... but, what does she know <G>.

Dik

 

[Ron]

dik....yep...been following that one! They designed and built that with no geotechnical involvement!

Opinionated? Yep...have the same failing....and the same spousal opinion!

By the way SWMBO is obviously not reading this!

 

[GSI2722]

Depends on structure and response to subsurface conditions. Differential settlement of 1 inch is fine if columns are far apart and structure is resilient. I've investigated many buildings which have settled without damage, you must also consider the angular distortion, sagging, hogging, tilt and racking. Also, is settlement elastic or time-dependent? What percentage of load is live load? How much elastic, dead load settlement will have occurred during construction?

The oft-cited maximum settlement limit of 1-inch total and 3/4-inch differential in most geotechnical reports goes back to Peck about 50 years ago and has little basis in reality.

 

[ATSE]

GSI2722: "Differential settlement of 1 inch is fine..." and "...little basis in reality."
You get the unhappy face of the day.
Most dissatisfied clients and lawsuits are not due to structural resilience, or lack thereof, but concrete cracking.

 

[fattdad]

Shooting for 3" of settlement is not a good idea, IMHO...

I just can't take this at face value.

If generally-accepted practice is to design for 1-in, then your statement is correct. If the client is knowledgeable and elects to evaluate risk-reward, then it's neither good or bad. It's an informed decision that may be just fine (i.e., a good idea).

So, your statement only makes sense if you don't have a discussion that includes the geotechnical engineer, the owner and the structural engineer.

f-d

ípapß gordo ainÆt no madre flaca!

 

[BigH]

For differential settlement, Tomlinson's foundation book has a good few pages about it. Distortion angles rule. Permitting vertical settlements in excess of say 50 mm is atypical.