Comments for NCS Consultants, LLC http://www.ncsconsultants.com Geotechnical Engineers Sat, 21 Jan 2012 02:13:31 +0000 hourly 1 http://wordpress.org/?v=3.2.1 Comment on LRFD for Deep Foundations – Shafts by NCSamtani http://www.ncsconsultants.com/2012/01/lrfd-for-deep-foundations-shafts/#comment-108 NCSamtani Sat, 21 Jan 2012 02:13:31 +0000 http://www.ncsconsultants.com/?p=2135#comment-108 Discussion of Webinar Homework Problem: First let us try to figure out the answers given at the bottom of Slide 80. Step 1. From Slide 60, Strength I-max vertical load = 3,736 kips, say V Step 2. From Slide 34, for single (mono)shaft foundation system, reduce the resistance factor by 20%. Therefore, compute the factored adjusted vertical load, Vadj = 4,670 kips. [Leave a comment on this blog if you have trouble calculating this value] Step 3. In the chart shown in Slide 61 (or Slide 56), draw a vertical line corresponding to a factored axial resistance of 4,760 kips. Step 4. Read the diameter load combinations where the vertical line drawn in Step 3 intersects the various curves below line CD and above line AB. You will find the following solutions (as noted on Slide 80): {D=6 ft, L=118 ft}, or {D=7 ft, L=95 ft}, or {D=10 ft, L=55 ft}. Other combinations are not valid because they fall between lines AB and CD. Now let us talk about the choice from these three possible combinations. The choice would depend on local contractor expertise in drilling deep large diameter shafts. As the shaft diameter increases, the torque required to excavate the shaft increases significantly. This requires larger equipment which increases project costs. Longer shafts on the other hand mean longer drilling times and longer cages. Shorter shafts may result may not develop "fixity" from a lateral loading perspective. You should also be concerned about the settlement of the shaft that is obtained from service limit state analyses similar that done in Slide 65. So, the answer is not clear cut and depends on a consideration of all these factors. Which one would you select and why? I would be happy to discuss your thoughts. Discussion of Webinar Homework Problem:

First let us try to figure out the answers given at the bottom of Slide 80.
Step 1.
From Slide 60, Strength I-max vertical load = 3,736 kips, say V

Step 2.
From Slide 34, for single (mono)shaft foundation system, reduce the resistance factor by 20%. Therefore, compute the factored adjusted vertical load, Vadj = 4,670 kips. [Leave a comment on this blog if you have trouble calculating this value]

Step 3.
In the chart shown in Slide 61 (or Slide 56), draw a vertical line corresponding to a factored axial resistance of 4,760 kips.

Step 4.
Read the diameter load combinations where the vertical line drawn in Step 3 intersects the various curves below line CD and above line AB. You will find the following solutions (as noted on Slide 80): {D=6 ft, L=118 ft}, or {D=7 ft, L=95 ft}, or {D=10 ft, L=55 ft}. Other combinations are not valid because they fall between lines AB and CD.

Now let us talk about the choice from these three possible combinations. The choice would depend on local contractor expertise in drilling deep large diameter shafts. As the shaft diameter increases, the torque required to excavate the shaft increases significantly. This requires larger equipment which increases project costs. Longer shafts on the other hand mean longer drilling times and longer cages. Shorter shafts may result may not develop “fixity” from a lateral loading perspective. You should also be concerned about the settlement of the shaft that is obtained from service limit state analyses similar that done in Slide 65. So, the answer is not clear cut and depends on a consideration of all these factors. Which one would you select and why? I would be happy to discuss your thoughts.

]]> Comment on ADOT SF-3 (Spread Footing – Sliding and Bearing Resistance Factors) by NCSamtani http://www.ncsconsultants.com/2011/09/adot-sf-3-spread-footing-%e2%80%93-sliding-and-bearing-resistance-factors/#comment-106 NCSamtani Mon, 05 Sep 2011 18:19:29 +0000 http://www.ncsconsultants.com/?p=4126#comment-106 Memorandum ADOT SF-3 attempts to clarify the confusion between the resistance factors for sliding presented in Section 11 (Abutments, Piers, and Walls) and Section 10 (Foundations). In Section 11, a single resistance factor of 0.55 for bearing resistance of gravity and semi-gravity walls and a resistance factor of 1.0 for sliding resistance of all walls. In Section 10, the resistance factors are a function of the method of computation of resistance, interface between the structural element and the foundation soil, and the type of soil. Memorandum ADOT SF-3 presents clear guidelines on the use of appropriate resistance factors in this regard. Memorandum ADOT SF-3 attempts to clarify the confusion between the resistance factors for sliding presented in Section 11 (Abutments, Piers, and Walls) and Section 10 (Foundations). In Section 11, a single resistance factor of 0.55 for bearing resistance of gravity and semi-gravity walls and a resistance factor of 1.0 for sliding resistance of all walls. In Section 10, the resistance factors are a function of the method of computation of resistance, interface between the structural element and the foundation soil, and the type of soil. Memorandum ADOT SF-3 presents clear guidelines on the use of appropriate resistance factors in this regard.

]]> Comment on ADOT SF-2 (Spread Footing – Limiting Eccentricity) by NCSamtani http://www.ncsconsultants.com/2011/09/adot-sf-2-spread-footing-%e2%80%93-limiting-eccentricity/#comment-105 NCSamtani Mon, 05 Sep 2011 18:13:03 +0000 http://www.ncsconsultants.com/?p=4124#comment-105 The purpose of Memorandum ADOT SF-2 is to address inconsistencies related to limiting eccentricity criteria between allowable stress design and and current LRFD practice. Based on comprehensive parametric analyses it was found that the limiting eccentricity criteria in AASHTO (2010 and previous LRFD editions) were based on load factors from Load Factor Design (LFD) and not load factors from LRFD. Based on the parametric analyses, limiting eccentricity criteria used in LRFD with respect to eccentric loads and overturning have been modified to match successful past ASD practice. The recommended limiting eccentricity criteria for eccentric loads were reviewed by AASHTO and the next version of AASHTO will include modified criteria that are based on the work reported in Memorandum SF-2. The purpose of Memorandum ADOT SF-2 is to address inconsistencies related to limiting eccentricity criteria between allowable stress design and and current LRFD practice. Based on comprehensive parametric analyses it was found that the limiting eccentricity criteria in AASHTO (2010 and previous LRFD editions) were based on load factors from Load Factor Design (LFD) and not load factors from LRFD. Based on the parametric analyses, limiting eccentricity criteria used in LRFD with respect to eccentric loads and overturning have been modified to match successful past ASD practice. The recommended limiting eccentricity criteria for eccentric loads were reviewed by AASHTO and the next version of AASHTO will include modified criteria that are based on the work reported in Memorandum SF-2.

]]> Comment on ADOT SF-1 (Spread Footing – Bearing Resistance and Settlement) by NCSamtani http://www.ncsconsultants.com/2011/09/adot-sf-1-spread-footing-%e2%80%93-bearing-resistance-and-settlement/#comment-104 NCSamtani Mon, 05 Sep 2011 18:05:59 +0000 http://www.ncsconsultants.com/?p=4122#comment-104 Memorandum SF-1 presents guidance for the bearing and settlement analysis of spread footings. The intent of this policy is to present a general overview of the development of the information needed by the bridge designer to design substructure elements consisting of spread footings. The concept of bearing resistance chart is introduced which permits a combined evaluation of strength and service limit states. The concept is illustrated by an example subsurface profile and a step-by-step solution. Evaluation of the computed settlements in terms of effect of deformations on a bridge structure and staged construciton analysis is also presented. Memorandum SF-1 presents guidance for the bearing and settlement analysis of spread footings. The intent of this policy is to present a general overview of the development of the information needed by the bridge designer to design substructure elements consisting of spread footings. The concept of bearing resistance chart is introduced which permits a combined evaluation of strength and service limit states. The concept is illustrated by an example subsurface profile and a step-by-step solution. Evaluation of the computed settlements in terms of effect of deformations on a bridge structure and staged construciton analysis is also presented.

]]> Comment on ADOT DS-3 (Drilled Shaft – Lateral Analysis) by NCSamtani http://www.ncsconsultants.com/2011/09/adot-ds-3-drilled-shaft-%e2%80%93-lateral-analysis/#comment-103 NCSamtani Mon, 05 Sep 2011 18:00:33 +0000 http://www.ncsconsultants.com/?p=4120#comment-103 Memorandum DS-3 presents guidance for analysis of drilled shafts under lateral loads for the following aspects: I.Shaft Length II.Minimum requirements for analytical models III.Depth to fixity IV.Considerations for collapse-susceptible soils The intent of the memorandum is to present step-by-step guidance for latetal load analysis of drilled shafts. The guidance is also applicable for other deep foundations. Particular emphasis is placed on specific considerations for geotechnical stability, strength limit state and service limit state. Guidance is also provided for proper selection of analytical methods. Depth to fixity concept is discussed in detail. Finally, an approach for modeling collapse-susceptible soils while evaluating deep foundations in such soils is presented. Memorandum DS-3 presents guidance for analysis of drilled shafts under lateral loads for the following aspects:
I.Shaft Length
II.Minimum requirements for analytical models
III.Depth to fixity
IV.Considerations for collapse-susceptible soils

The intent of the memorandum is to present step-by-step guidance for latetal load analysis of drilled shafts. The guidance is also applicable for other deep foundations. Particular emphasis is placed on specific considerations for geotechnical stability, strength limit state and service limit state. Guidance is also provided for proper selection of analytical methods. Depth to fixity concept is discussed in detail. Finally, an approach for modeling collapse-susceptible soils while evaluating deep foundations in such soils is presented.

]]> Comment on ADOT DS-2 (Drilled Shaft – Gravels and Gravelly Soils) by NCSamtani http://www.ncsconsultants.com/2011/09/adot-ds-2-drilled-shaft-%e2%80%93-gravels-and-gravelly-soils/#comment-102 NCSamtani Mon, 05 Sep 2011 17:56:26 +0000 http://www.ncsconsultants.com/?p=4118#comment-102 Memorandum DS-2 presents guidance specifically for the design of drilled shafts in gravels and gravelly soils that exhibit drained behavior. Guidance regarding both side resistance and tip resistance is provided. Resistance mobilization curves are presented for gravels and gravelly sands by on work performed by Rollins in Utah and by Arizona Department of Transportation (ADOT). The criterion for “sands” in AASHTO (2010) is also presented to aid in a clear determination of gravels, gravelly soils, and sands within the overall framework of coarse-grained soils exhibiting drained behavior. Clear direction is provided in terms of use of equations to predict nominal resistance in terms of sands, gravels, gravelly sands, and intermediate geomaterials. The guidance in this memorandum was developed with respect to soils in Arizona but most concepts may be generally valid for other regions as well, e.g., definitions for gravels and gravelly sands. Memorandum DS-2 presents guidance specifically for the design of drilled shafts in gravels and gravelly soils that exhibit drained behavior. Guidance regarding both side resistance and tip resistance is provided. Resistance mobilization curves are presented for gravels and gravelly sands by on work performed by Rollins in Utah and by Arizona Department of Transportation (ADOT). The criterion for “sands” in AASHTO (2010) is also presented to aid in a clear determination of gravels, gravelly soils, and sands within the overall framework of coarse-grained soils exhibiting drained behavior. Clear direction is provided in terms of use of equations to predict nominal resistance in terms of sands, gravels, gravelly sands, and intermediate geomaterials. The guidance in this memorandum was developed with respect to soils in Arizona but most concepts may be generally valid for other regions as well, e.g., definitions for gravels and gravelly sands.

]]> Comment on ADOT DS-1 (Drilled Shaft – Axial Resistance) by NCSamtani http://www.ncsconsultants.com/2011/09/adot-ds-1-drilled-shaft-%e2%80%93-axial-resistance/#comment-101 NCSamtani Mon, 05 Sep 2011 17:48:56 +0000 http://www.ncsconsultants.com/?p=4116#comment-101 Memorandum ADOT DS-1 outlines the development of drilled shaft axial resistance charts based on methods specified in AASHTO (2010). The intent of this memorandum is to present a general overview of the development of the information needed by the bridge designer to design substructure elements consisting of drilled shafts. The memorandum presents detailed explanation for the development of axial resistance charts that show variation of axial resistance with depth. Procedures for developing axial resistance charts for strength limit state and service limit states are discussed. Development of a site and shaft specific load-deformation relationship is demonstrated which is important for consideration of serviceability of the structure supported by drilled shaft foundations. Memorandum ADOT DS-1 outlines the development of drilled shaft axial resistance charts based on methods specified in AASHTO (2010). The intent of this memorandum is to present a general overview of the development of the information needed by the bridge designer to design substructure elements consisting of drilled shafts. The memorandum presents detailed explanation for the development of axial resistance charts that show variation of axial resistance with depth. Procedures for developing axial resistance charts for strength limit state and service limit states are discussed. Development of a site and shaft specific load-deformation relationship is demonstrated which is important for consideration of serviceability of the structure supported by drilled shaft foundations.

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