Week 4 Challenge

3) How a shallow foundation in terms of stability,designing,construction different in flyover and in river bridge?

Design Process:

Step 1: Initial Evaluation of Shallow Foundation:

• Verify that the proposed bottom of footing elevation is appropriate. If, for example,
  the requested bottom of footing elevation is in a layer of unsuitable material (e.g.,
  loose sand), inform the Structure Designer and provide a revised bottom of footing
  elevation or discuss options for improving the bearing layer by removing unsuitable
  soils and replacing with engineered fill or lean concrete.
• Verify that the shallow foundation is acceptable considering known geological
  hazards (e.g., faulting, scour).

Step 2: Calculate the Permissible Net Contact Stress (Service Limit State):

Calculate the Permissible Net Contact Stress (qpn) that produces the permissible settlement (usually 1” or 2”) for the footing dimensions provided by the Structure Designer in the Foundation Data table, and footing dimensions with varying length to width (L/B)
ratios as shown in MTD 4-1, Attachment 2. This is an indirect calculation that produces a permissible settlement for an initial guess at the net contact stress. Further guesses are used until the chosen net contact stress produces the required permissible settlement

The footing dimensions provided in these tables are labeled as “Effective” with an assumed eccentricity of zero (i.e., L=L’ and B=B’) because the support loads are unknown at this stage of the design process. The Structure Designer will use the data as input for
the software that calculates the structural loads and the effective footing sizes. The Permissible Net Contact Stress results are presented differently for abutment and bent supports:

• For abutment design determine the Permissible Net Contact Stress (qpn) for one effective footing length (fixed by the bridge dimensions) and at least five effective footing widths. Determine the range of effective footing widths and physical
  constraints with the Structure Designer. Present the agreed upon footing size range results in the End Supports (Abutments) table (after MTD 4-1 Attachment 2,Table 1).
• For bent design determine the permissible net contact stress (qpn) for at least five effective footing widths and five L’/B’ ratios. Determine the desired range of L’/B’ ratios and physical constraints (e.g. maximum widths) with the Structure Designer. Present the effective footing widths and L’/B’ ratio range results in the Intermediate Supports (Bents and Piers) table (after MTD 4-1 Attachment 2, Table 2).

Step 3: Calculate the Gross Nominal Bearing Resistance:

Calculate the Gross Nominal Bearing Resistance (qn) for the effective footing sizes in the End Supports (Abutments) table and the Intermediate Supports (Bents and Piers) table. As discussed in step 2, the effective footing widths have an assumed eccentricity of zero at this stage of the design process (i.e., L=L’ and B=B’). Enter the results in the appropriate columns of each table

Step 4: Calculate the Factored Gross Nominal Bearing Resistance (Strength Limit State):

Calculate the Factored Gross Nominal Bearing Resistance (qR) for the effective footing sizes in the End Supports (Abutments) table and the Intermediate Supports (Bents and Piers) table. Enter the results in the appropriate columns of each table.

Step 5: Send Preliminary Design Data to Structure Design:

End Supports (Abutments) table (MTD 4-1 Attachment 2, Table 1)
Intermediate Supports (Bents and Piers) table (MTD 4-1 Attachment 2, Table 2) 

Step 6: Request Design Data from Structure Design:
Request that SD provide the following tables with the appropriate load information:
        * Foundation Data (MTD 4-1 Attachment 4, Table 1)
        *Scour Data (MTD 4-1 Attachment 4, Table 2), if applicable
         *Caltrans Geotechnical Manual
LRFD Service-I Limit State Loads for Controlling Load Combination (MTD 4-1Attachment 4, Table 3)
LRFD Strength/Construction and Extreme Event Loads for Controlling Load Combinations (MTD 4-1 Attachment 4, Table 4)
The Structure Designer will use the information from the End Supports (Abutments) and the Intermediate Supports (Bents and Piers) tables to design the footings using the CTAbut and WinFoot programs, and will return the design details (dimensions, elevations, loads, and moments). If the range of footing sizes and resistances supplied by GS in Step 5 are not acceptable, the Structure Designer will request the entire procedure beginagain with a new preliminary footing size.

Step 7: Evaluate Inclination Factors (if omitted, then go to Step 8)
After receiving the tables from Step 6, calculate the settlement and bearing capacity to verify the footing design and calculate inclination factors to determine if the gross nominal bearing resistance is affected. In general, inclination factors should be omitted when
calculating the gross nominal bearing resistance (AASHTO 10.6.3.1.2a-1), however there are some cases when it may be necessary to consider these factors. Unusual column geometry or loading configurations may require consideration be given
to evaluating load inclination factors. A column that is not aligned normal to the footing bearing surface would be one example where inclination factors would be given consideration.
In the rare case where inclination factors are to be evaluated, use the lower value of the bearing resistance using either inclination factors or shape factors. Applying both shape and inclination factors will result in overly conservative design. If the inclination factor produces a lower bearing resistance than the shape factor, the nominal bearing resistances determined in Steps 3 and 4 must be recalculated using the inclination factors in place of the shape factors and provided to the Structure Designer.

Step 8: Verify the Shallow Foundation Design:
Use the updated information (MTD 4-1, Attachment 4) to calculate the eccentricities and determine the effective footing dimensions. Check the footing design for all limit states:

For the Service Limit State, verify that the Permissible Net Contact Stress is greater than or equal to the Net Uniform Bearing Stress (soil) or Net Maximum Bearing Stress (rock).
For soil foundations, verify if the differential settlement between adjacent supports, using the Net Uniform Bearing Stresses, is acceptable.
For the Strength and Extreme Event Limit States, verify that the calculated
Factored Gross Nominal Bearing Resistance is greater than or equal to the Gross
Uniform Bearing Stress (soil) and Gross Maximum Bearing Stress (rock).
If any of the above criteria are not met, inform the Structure Designer.

Step 9: Global Stability Check:
The calculated resistance factors for global stability must be less than or equal to the AASHTO LRFD criteria for both the Service-I (static) Limit State (φ = 0.65), as well as the Extreme Event (pseudo-static) Limit State (φ = 0.9). If the stability analysis resistance factors are greater than the resistance factors listed in AASHTO 10.5.2.3 and 11.6.2.3, inform the Structure Designer and discuss design alternatives or site improvements to address the issue.

Step 10: Reporting :
Report foundation recommendations as required by the Foundation Reports for Bridges module.