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2005 AASHTO BRIDGE COMMITTEE AGENDA ITEM: 40 (REVISION 2)
2005 AASHTO BRIDGE COMMITTEE AGENDA ITEM: 40 (REVISION 2)SUBJECT: LRFD Bridge Design Specifications: Section 10 ReplacementTECHNICAL COMMITTEE: T-15 FoundationsREVISION ADDITION NEW DOCUMENTDESIGN SPEC CONSTRUCTION SPEC MOVABLE SPECLRFR MANUAL OTHERUS VERSION SI VERSION BOTHDATE PREPARED: 3/15/05DATE REVISED: 6/30/05AGENDA ITEM:Replace all of Section 10 of the AASHTO LRFD Design Specifications with the attached new Section 10.OTHER AFFECTED ARTICLES:Article 3.11.8 (proposed as a separate agenda item).Articles 5.7.4 and 5.13.4.5 (proposed agenda items by the T-10 committee).There are references to the existing articles in Section 10 throughout other sections in the Third Edition. Each ofthese references must be changed to a reference to the revised Section 10 which will require identifying the properarticle number with the proper engineering content. Attached is a list of affected articles in the Third Edition.BACKGROUND:States who have attempted to implement Section 10 of the AASHTO LRFD Design Specifications have had greatdifficulty in doing so for one or more of the following reasons: The recommended procedures in the current Section 10 are not consistent with local and nationalgeotechnical design practice, including design practice as recommended in widely used FHWA manuals. The current LRFD Section 10 results in foundation designs that are considerably more conservative thanwhat would be required based on AASHTO Load Factor Design (i.e., the Standard Specifications). The current Section 10 specifies the use of out of date design methods. Resistance factors are not provided for certain aspects of foundation design. The current Section 10 is not clear on how to implement the procedures provided.This rewrite of Section 10 is the result of an FHWA funded project that includes both specification updating andthe development of an updated NHI course on LRFD foundation and wall design. The rewrite is the result of acollaborative effort between the consultant team hired by the FHWA and a Technical Working Group (TWG)assembled to review and help develop the specifications and NHI course. This TWG consists of seniorgeotechnical and structural engineers from several state DOT’s (WADOT, PENNDOT, VADOT, FLDOT,OHDOT, CALTRANS – both the AASHTO T-15 Chair and Vice Chair were among these state representatives)and the FHWA.A draft rewrite of Section 10 was completed and sent out for a nationwide review in August 2004 for a 3 month period. Over 700 comments were received from 15 states and 3 industry organizations, the proposed Section 10amended, and responses provided to each agency or organization who provided comments. A second opportunityfor review of the amended version of Section 10 was provided to those who commented, during February 2005, andthe proposed Section 10 was further amended to address the comments received, to produce the final draft agendaitem.Key changes to Section 10 in the 2004 AASHTO LRFD Design Specifications resulting from this rewrite aresummarized as follows:Article 10.4 – This article has been rewritten to reflect the guidance provided in FHWA Geotechnical EngineeringCircular GEC-5. More definitive guidance is provided in the setting up of the geotechnical field investigation todefine the subsurface conditions for foundations and walls (note that Section 11 does not address geotechnicalinvestigation requirements for walls – therefore, all of the geotechnical investigation requirements for foundationsand walls are proposed to be included in Article 10.4), but some flexibility in the required number and depth ofborings to adapt to the specific site conditions, based on local experience, has been retained in this article. Therequired objectives of the geotechnical investigation are more clearly laid out. The role of geophysical testing tobetter define the subsurface conditions and to allow the number of borings to be reduced as appropriate is nowdescribed. The objectives of the laboratory and in-situ field testing programs are now more clearly laid out, and thedetailed lists of test procedures provided in the current specifications have been removed, but included in thespecification instead by reference to CEG-5, which contains a very thorough treatment of laboratory and in-situtesting. Minimum requirements and detailed guidance on the selection of soil and rock design properties from thelaboratory and in-situ field testing programs has been added to this article, again based on the information providedin GEC-5. For common in-situ tests such as the Standard Penetration Test (SPT), the correlations used todetermine soil properties (e.g., shear strength) that were used as part of the calibration of resistance factors (forthose factors in which reliability theory was used) are provided. A new article on the determination of rockproperties has been added to this article.Article 10.5.2.2 “Tolerable Movements and Movement Criteria” – The key change to this article is to eliminate“Design horizontal movements at the service limit state should not exceed 1.5 in.,” and replace it with text thatprovides criteria for determining the movement criterion to be used and where to apply it. The magnitude ofdisplacement that should be used will depend on the structure geometry, for example, column height, bridge seatwidth, etc., as a “one size fits all” approach does not work. Furthermore, movement criteria for other foundationtypes have been consolidated here rather than being split up among other articles for specific foundation types.Article 10.5.2.2 “Overall Stability” – This article has been modified to reference Article 11.6.3.4, to eliminateredundancy in the specifications.Article 10.5.3 “Strength Limit State” – As previously written, this article applied fairly well to footing foundations,but did not apply to pile or shaft foundations. This article has been broken out into three subarticles to more clearlyaddress each foundation type.Article 10.5.5 “Resistance Factors” – As previously written, this article was written to address only the strengthlimit state. This article is now broken up into sub-articles to provide clear guidance on resistance factors for serviceand extreme event limit states. Explanation as to the basis of the resistance factor values provided was lacking inthe previous specification. Commentary has been added to help the user of the specifications better understand howthe current resistance factors were derived, how they relate to current LFD or ASD practice, and if they are to beadjusted for local or regional practices or site specific considerations, how to go about doing that. The resistancefactors for footings are in general the same as they were in the previous specifications, but the resistance factortable for footings has been simplified. For piles, the resistance factors have been widely recognized as being tooconservative, and recommendations in the previous specifications to combine the resistance factors for static pileanalysis methods with dynamic pile analysis methods is not consistent with the approach in current AASHTOStandard Specifications and national practice in general. To be consistent with the approach in the AASHTOStandard Specifications regarding pile design, it is proposed to tie the resistance factor selection directly to themethod used to estimate pile resistance in the field. Furthermore, the results provided in NCHRP Report 507,which contain the results of a detailed calibration based on an extensive database for deep foundations, have beenused to provide more accurate resistance factors, also including more commonly used methods for estimating pile capacity than what was previously included in this article. The relationship between resistance factor selection andthe various aspects of strength limit state pile design has been clarified, especially with regarding to pile lengthestimation versus estimating the size of the pile foundation needed to support the factored loads. For shafts, severalof the methods listed are out of date or are earlier versions of the Reese and O’Neill (1988) method. Outdated shaftdesign methods have been removed from the specifications, and new resistance factors consistent with pastLFD/ASD practice have been included. Work is on-going to determine reliability based resistance factors forshafts. Regarding the new sub-article on resistance factors for extreme event limit states, some clarificationregarding how to apply resistance factors for various scour events (e.g., refer to Article 2.6) has been provided. Ingeneral, however, resistance factors for service and extreme event limit states are unchanged from previousspecifications. As part of the rewrite effort, an FHWA report has been produced that summarizes the data and logicused to establish the foundation design resistance factors proposed in this agenda item (see Allen, T. M., 2005,Development of Geotechnical Resistance Factors and Downdrag Load Factors for LRFD Foundation StrengthLimit State Design, FHWA, Publication No. FHWA-NHI-05-052, February 2005).Article 10.6.1.3 “Effective Footing Dimensions” – This article has been moved to much farther up front in thefooting design articles, since the effective footing dimensions affect settlement and bearing resistance calculationsin addition to the determination of whether or not the footing meets eccentricity requirements.Article 10.6.1.4 “Bearing Stress Distributions” – The technical requirements have in general not changed, but theguidance provided is clearer and better tied into Article 11.6.3.2, where more specific guidance on how to calculatethe stress distribution is given.Article 10.6.2.4.1 “Settlement of Cohesionless Soils” – The Hough Method, used widely nationwide andrecommended in FHWA manuals, has been added as an acceptable method for estimating settlement. Detailedprocedures are provided.Article 10.6.3.1.2 “Theoretical Estimation” – The theoretical method for estimating strength limit state bearingresistance of footings currently in the specifications (the Rational Method – Barker, et al., 1991) is proposed to bereplaced with the general bearing capacity method as presented by Munfakh, et al (2001). The Rational Methoddoes not allow soils with both cohesion and friction, a common occurrence, to be analyzed regarding bearingresistance. Some states have found that this inability has caused the Rational Method to produce overlyconservative bearing resistance values. The general method as presented by Munfakh, et al. (2001) is consistentwith traditional bearing capacity theory, and does allow soils with both cohesion and friction to be evaluated. Inaddition, there is been a great deal of concern and confusion regarding the use of load inclination factors regardingtheir applicability to traditional bearing resistance determination. The proposed specifications and commentaryattempt to clarify when inclination factors should be used and clarify what is current national practice regarding theuse of inclination factors. Clearer guidance is also provided regarding bearing resistance of footings where twodistinct soil layers are present.Article 10.6.3.1.4 “Plate Load Tests” – More detailed guidance on plate load tests and how they should be used hasbeen added.Article 10.6.4 “Extreme Event Limit State Design” – This is a new article that provides general considerations fordesign for this limit state, in anticipation that more detailed guidance will be provided later once new LRFDseismic provisions are developed. Furthermore, this article ties into Article 11.6.5 so that design footings in generalis consistent with design of footings that support walls. This consistency is missing in the current specifications.Article 10.7 “Driven Piles” – This portion of Section 10 has been completely rewritten to be more consistent withnational design practice in its organization, flow, and technical content based on review of the AASHTO StandardSpecifications (2002), FHWA pile design manuals, and discussions with representatives from various statetransportation departments.Article 10.7.1.5 “Determination of Pile Loads” – This article (including sub-articles) has been pulled together fromother existing articles, and expanded to address forces due to expansive soils, and conceptually how to address theeffect on or by nearly structures. In the current (2004) specifications, while downdrag loads are addressed, they arenot specifically addressed regarding how they are to be handled with respect to service, strength, and extreme event limit states. More detailed guidance is provided in the proposed specifications for the service (Article 10.7.2.5) andstrength (Article 10.7.3.6) limit states regarding application of downdrag forces. For extreme events, onlyconceptual guidance is given until such time that LRFD seismic design specifications are more fully developed.The specific guidance on estimating downdrag loads in Article 10.7 in the current specifications has been moved toArticle 3.11.8. Therefore, Article 10.7.1.5 refers to Article 3.11.8 for specific guidance on determining downdragloads.Article 10.7.2. “Service Limit State Design” – This article (and sub-articles) has been expanded to address overallstability, to be consistent with changes to Sections 3 and 11 made in 2003 to make overall stability a service limitdesign consideration, and to add lateral squeeze as a potential service limit state consideration.Article 10.7.2.3 “Settlement” – The pile group stress distributions available have been expanded to address variouslayered soil profile situations. The method to estimate the group settlement magnitude has not been changed,however.Article 10.7.2.4 “Horizontal Pile Foundation Movement” – This article combines together several existing articlesthat deal with this subject, and has been expanded somewhat to provide clearer explanation on how to conductlateral load/deflection analyses, focusing primarily on the P-y curve approach. Strain wedge theory has been addedas an alternative approach for lateral analysis of short stiff piles. Group reduction factors have been updated toreflect current FHWA manual guidance (Hannigan, et al., 2005).Article 10.7.3 “Strength Limit State Design” – This article has been expanded and rewritten to identify all elementsof the strength limit state pile design process.Article 10.7.3.1 “Point Bearing Piles on Rock” – This article has been rewritten to be consistent with the practice ofseveral DOT’s who routinely drive piles to bedrock, and to include practical considerations of design andinstallation.Article 10.7.3.2 “Pile Length Estimates for Contract Documents” – This has been expanded from the originalarticle in the current specifications. The current specifications only briefly mention this key aspect of pilefoundation design. This article now explains how to estimate the pile penetration depth required to obtain thedesired nominal resistance, and how to relate the compression resistance determined using a dynamic methodduring driving to the results from a static analysis method, attempting to maintain a consistent level of reliability forboth the dynamic and static determination of pile resistance. The proposed provisions were developed throughconsultation with 15 state DOT’s who were actively involved in the development or review of this agenda item, andrepresents a consensus of those states.Article 10.7.3.3 “Nominal Axial Resistance Change after Pile Driving” – This is a new article. The currentspecifications do not address this issue. The guidance provided in conceptual in nature and emphasizes the need forrestrike testing if significant soil setup or relaxation is anticipated. Time dependent axial resistance changes are acommon occurrence, and can affect interpretation of testing and pile driving results.Article 10.7.3.5 “Scour” – Scour is not addressed in the current specifications. This article was added to addressthis key failure mechanism.Article 10.7.3.6 “Downdrag” – This article provides significantly expanded guidance on how to deal withdowndrag loads for strength limit state design. Handling downdrag in design has been an area of confusion in thepast. The guidance provided in this article attempts to provide clarity on how to account for downdrag in thestrength limit state.Article 10.7.3.7 “Determination of Nominal Axial Pile resistance in Compression” – This article providessignificantly expanded guidance on the assessment of axial pile resistance in compression, providing detailedguidance on the use of pile load tests for design, dynamic testing with signal matching, wave equation analysis, anddriving formulae. The resistance factors recommended in Article 10.5 for each of these methods were developedwith certain assumptions as to how such methods are used. The guidance provided in this article provides neededdescription of how each of these methods should be used to justify the use of the specified resistance factors, and to be consistent with how they were calibrated. Various state DOT’s were consulted to make sure the use of themethods described in this article is consistent with widespread practice.Article 10.7.3.7.5 “Static Analysis” – The Nordlund/Thurman method for piles in sand was added within thisarticle. This method is in widespread use nationally and is recommended in FHWA manuals.Article 10.7.3.11 “Nominal Horizontal Resistance of Pile Foundations” – This article has been rewritten andupdated to be consistent with Article 10.7.2.4 (see previous comment on Article 10.7.2.4), and to conceptuallyaddress the issue of pile foundation fixity. Detailed recommendations on how to address pile fixity are notavailable at this time.Article 10.7.3.12 “Pile Structural Resistance” – This article has been rewritten to provide clearer reference toarticles in other sections of the LRFD specifications where structural design recommendations can be found.Furthermore, in the current specifications, an approximate empirical approach was recommended as the primarymethod to determine the effective pile length and the depth of fixity. Routine computer modeling using P-yanalyses demonstrates that the empirical method provided in the current specifications does not produce results thatare consistent with the P-y approach. Therefore, to be consistent with Article 10.7.3.11 and related articles, theempirical approach has been identified for use in preliminary design only, and that the approach provided in Article10.7.3.11 should be used for final design.Article 10.7.4 “Extreme Event Limit State” – This is a new article that provides conceptual guidance to address piledesign for seismic and other extreme event limit states. Detailed guidance cannot be provided, however, until anew LRFD seismic specification is developed. Scour as an extreme event is also addressed in this article and is tiedto the check flood.Article 10.7.6 “Determination of Minimum Pile Penetration” – This article has been expanded from the originalarticle to more fully address all the design issues that affect the selection of a minimum pile penetrationrequirement. This article also addresses what pile design requirements should be included in the constructioncontract documents.Article 10.7.7 “Determination of RndrUsed to Establish Contract Driving Criteria for Bearing” – This is a newarticle that specifies how to select the nominal driving resistance required to meet all the applicable limit states.How to make this selection is not clear in the current specifications.Article 10.7.8 “Drivability Analysis” – The current specifications only address driving stress criteria, andfurthermore are not fully written as a LRFD criteria. This article presents these driving stresses more clearly inLRFD format, provides more detailed reference to other articles where the appropriate resistance factors can befound, provides more detailed guidance on how to apply these criteria as part of a drivability analysis, and how thedrivability analysis should be used as a part of the pile foundation design process.Article 10.8.1.2 “Shaft Spacing, Clearance, and Embedment into Cap” – The spacing where interaction effectsshould be evaluated has been increased based on experience and the guidance regarding this issue clarified.Article 10.8.1.3 “Shaft Diameter and Enlarged Bases” – A portion of this article that was originally in thespecifications (in particular rock socket diameter) has been moved into the commentary.Article 10.8.1.5 “Drilled Shaft Resistance” – Specific considerations and limit states for determining the shaftresistance required have been added (the current specification refers to the parallel pile design article, which,because Article 10.7 has been rewritten, no longer applies). In addition, specific shaft construction issues andtechniques that affect the shaft design have been added (the current article provides only a general mention ofconstructability issues).Article 10.8.1.6 “Determination of Shaft Loads” – In the current specifications, this article only addressesdowndrag loads. This article has been expanded somewhat to address other load issues that are applicable to shafts,specifically application of the dynamic load allowance article in Section 3. For downdrag, this article has beenrevised to refer to Article 3.11.8 and parallel provisions in Article 10.7.1.5.1. Therefore, the changes made regarding the estimation of downdrag and how it is applied as discussed in those articles now apply to drilled shaftsas well.Article 10.8.2.2 “Settlement” – This article has been updated to be consistent with the recommendations providedby O’Neill and Reese (1999), as the current article is based on Reese and O’Neill (1988). In general, theprocedures are the same, except the 1999 procedures include additional load transfer curves and procedures forsoils classified as a gravel, and for a new class of materials termed Intermediate Geo Materials (IGM’s), which aretransitional materials between soil and rock.Article 10.8.3.5 “Nominal Axial Compression Resistance of Single Drilled Shafts” – Commentary has been addedto provide specific guidance on the use of design methods other than those specified in this article, as allowed byArticle 10.1. The specific procedures recommended in this article have been updated from the Reese and O’Neill(1988) to be consistent with O’Neill and Reese (1999). Several older methods for shafts in sand were alsoeliminated from this article. The biggest change is the method for estimating resistance of shafts in rock. Asubarticle has also been added that addresses Intermediate Geo Materials (IGM’s). See Allen (2005) for a moredetailed summary of the differences between the 1988 and 1999 drilled shaft resistance methods. Additionalguidance has also been provided in this article on the effect of permanent casing on shaft side resistance.Article 10.8.3.5.3 “Shafts in Strong Soil Overlying Weak Compressible Soil” – This is a new article that addressesthe case of a strong bearing stratum of limited thickness overlying weaker soil.Article 10.8.3.5.6 “Shaft Load Test” – This article has been expanded to address load testing of smaller scale shaftsand the use of the Osterberg load cell for load testing. In addition, more detailed guidance has been added toaddress how load test results should be applied to design.Article 10.8.3.6.3 “Cohesionless Soil” – The factors for shaft group resistance in this article have been modifiedfrom the current specifications to be consistent with the shaft group efficiency results summarized by O’Neill andReese (1999) – the main change is that now equals 1.0 at a shaft spacing of 4.0 diameters rather than 6.0diameters. Commentary has also been added to explain this.ANTICIPATED EFFECT ON BRIDGES:Changes to the resistance factors could make foundation designs more or less conservative, which could affect theoverall bridge design. However, the changes are not anticipated to be large. These changes were necessary tomake the reliability of the foundations, as much as possible, to be more consistent. The reliability of current, andpast, foundation design was considered in the final selection of resistance factors. See FHWA NHI report by Allen(2005) for specific justification for the changes.REFERENCES:Allen, T. M., 2005, Development of Geotechnical Resistance Factors and Downdrag Load Factors for LRFDFoundation Strength Limit State Design, FHWA, Publication No. FHWA-NHI-05-052, February 2005.Barker, R. M., Duncan, J. M., Rojiani, K. B., Ooi, P. S. K., Tan, C. K. and Kim. S. G. (1991). Manuals for theDesign of Bridge Foundations. NCHRP Report 343, TRB, National Research Council, Washington, DC.Hannigan, P.J., G.G.Goble, G. Thendean, G.E. Likins and F. Rausche 2005. "Design and Construction of DrivenPile Foundations" - Vol. I and II, Federal Highway Administration Report No. FHWA-HI-05, Federal HighwayAdministration, Washington, D.C.Munfakh, G., Arman, A., Collin, J. G., Hung, J. C.-J., and Brouillette, R. P. (2001). Shallow FoundationsReference Manual. Publication No. FHWA-NHI-01-023, Federal Highway Administration, Washington, D.C.O’Neill, M. W. and Reese, L. C. (1999). Drilled Shafts: Construction Procedures and Design Methods. Report No.FHWA-IF-99-025, Federal Highway Administration, Washington, D.C. Paikowsky, S. G., with contributions from Birgisson, B., McVay, M., Nguyen, T., Kuo, C., Baecher, G., Ayyab, B.,Stenersen, K., O’Malley, K., Chernauskas, L., and O’Neill, M. (2004). Load and Resistance Factor Design (LRFD)for Deep Foundations. NCHRP (Final) Report 507, Transportation Research Board, Washington, DC, 126 pp.Reese, L. C., and O’Neill, M. W. (1988). Drilled Shafts: Construction Procedures and Design Methods. FHWAPublication No. FHWA-HI-88-042, 564 pp.Sabatini, P. J., Bachus, R. C., Mayne, P. W., Schneider, T. E., Zettler, T. E., FHWA-IF-02-034, 2002, Evaluation ofsoil and rock properties, Geotechnical Engineering Circular No. 5.OTHER:None . 2005 AASHTO BRIDGE COMMITTEE AGENDA ITEM: 40 (REVISION 2)SUBJECT: LRFD Bridge Design Specifications: Section 10 ReplacementTECHNICAL COMMITTEE: . VERSION BOTHDATE PREPARED: 3/15/05DATE REVISED: 6/30/0 5AGENDA ITEM:Replace all of Section 10 of the AASHTO LRFD Design Specifications with the attached new