STRUCTURAL ENGINEERING FORUM OF INDIA [SEFI]

Rabinder Shekher · Posted: Tue May 09, 2023 3:29 pm Post subject: Substructure Design of Bridge

Kindly advise the best option for using Gross Bearing capacity or Net Bearing capacity while designing the abutments of the bridge. Specific reason for considering the either of two with codal provisions. Kindly guide as I am in fix to decide as some of the designers use GSB while other use NBC
Regards

vikram.jeet · General Sponsor Joined: 26 Jan 2003 Posts: 3706

Bridge foundations - Sbc to be taken

Gross safe bearing capacity need to be taken as while working various loads full soil weight comming on abutment  is taken in calculations

But it must be seen that soil report must provide value of GSbc value at founding level,,
If only net sbc is given then,,
GSbc = NSbc + ( Vsat - 1.0) *H'  + Vdry *H

Vsat   - Saturated unit wt of soil
H'  - Depth of foundation below HFL*
H - ht above HFL upto original GL at abutment as taken in soil report*
* In case HFL is higher than original ground profile level,, then  H= 0 and H' = foundation depth below OGL

But please inform Proof Checker in advance

amitavabh · SEFI Member Joined: 26 Jan 2003 Posts: 19

Calculate the actual bearing pressure below the footing = P1

Calculate pressure created due to weight of footing = P2
Total actual pressure = P1+P2 = P
Compare this calculated pressure P with Net bearing capacity given in Geotech Report at a specific elevation in the ground.

-
OR
Net bearing capacity given in Geotech Report at specified elevation + pressure due to weight of soil = Gross bearing capacity.

Calculate P + pressure of soil above the top of footing = Actual gross pressure.
Compare this Actual gross pressure with Gross bearing capacity.

Hope this clears everything.
Thank you
Amitava

If you use gross bearing capacity then use gross

On Tuesday, May 9, 2023 at 11:35:46 PM GMT-6, Rabinder Shekher <forum@sefindia.org> wrote:

Kindly advise the best option for using Gross Bearing capacity or Net Bearing capacity while designing the abutments of the bridge. Specific reason for considering the either of two with codal provisions. Kindly guide as I am in fix to decide as some of the designers use GSB while other use NBC
Regards

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Dipendra Bhowmick · SEFI Member Joined: 03 Oct 2010 Posts: 1

Thank you Mr. Amitava, for your useful explanation. I have one more question. For abutment, do we take the weight of backfill in the calculation of P1? Secondly, is this P1, comprises all the weights upto the top of foundation?
Regards,
D. Bhowmick

On Thu, May 11, 2023, 2:11 AM amitavabh <forum@sefindia.org (forum@sefindia.org)> wrote:

mpradyumna · SEFI Member Joined: 31 Dec 2014 Posts: 11

Horizontal elements, like footings, are subjected to net pressure. Hence, design these for net pressure. Failure of soil is due to total pressure, hence for checking bearing capacity use gross pressure.

Thanks,

M. Pradyumna

On 10-May-2023, at 7:22 PM, amitavabh <forum@sefindia.org> wrote:
Calculate the actual bearing pressure below the footing = P1

Calculate pressure created due to weight of footing = P2
Total actual pressure = P1+P2 = P
Compare this calculated pressure P with Net bearing capacity given in Geotech Report at a specific elevation in the ground.

-
OR
Net bearing capacity given in Geotech Report at specified elevation + pressure due to weight of soil = Gross bearing capacity.

Calculate P + pressure of soil above the top of footing = Actual gross pressure.
Compare this Actual gross pressure with Gross bearing capacity.

Hope this clears everything.
Thank you
Amitava

If you use gross bearing capacity then use gross

On Tuesday, May 9, 2023 at 11:35:46 PM GMT-6, Rabinder Shekher wrote:

Kindly advise the best option for using Gross Bearing capacity or Net Bearing capacity while designing the abutments of the bridge. Specific reason for considering the either of two with codal provisions. Kindly guide as I am in fix to decide as some of the designers use GSB while other use NBC
Regards

Posted via Email

saibai40 · Silver Sponsor Joined: 16 Feb 2014 Posts: 6

Sir,
The pressure diagram below the bridge footing on Soil cannot be as simple as Pier/Column Load/Footing Area. Schematic details are available on URL https://www.researchgate.net/figure/Schematic-diagram-of-pressure-bulb-beneath-a-footing-of-width-B-1-a-and-beneath-a_fig1_260788979
The IS Codes: IS-456, IS 1904 (1986), IS 2950-1 (1981) etc, provide details.
Raghvendra Govind Kulkarni

On Fri, May 12, 2023 at 5:51â€¯PM Dipendra Bhowmick <forum@sefindia.org (forum@sefindia.org)> wrote:

[quote] Thank you Mr. Amitava, for your useful explanation. I have one more question. For abutment, do we take the weight of backfill in the calculation of P1? Secondly, is this P1, comprises all the weights upto the top of foundation?
Regards,
D. Bhowmick

On Thu, May 11, 2023, 2:11 AM amitavabh forum@sefindia.org (forum@sefindia.org))> wrote:

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anandads · SEFI Member Joined: 20 Oct 2015 Posts: 13

Hi friends,
The total positive base pressure computed from stability analysis ought to be compared with safe bearing capacity of soil.

If high water table is there uplift force should also be considered in which case tension should be restricted.

In addition to above if it is bridge across river/stream scour depth plays important level deciding founding level.

Anand

On Sat, 13 May 2023, 14:56 saibai40, <forum@sefindia.org (forum@sefindia.org)> wrote: