AASHTO LRFD: Structural Foundations and Earth Retaining Structures. Specification Background What’s Happening Now! Limit States, Soil and Rock Properties Deep Foundations Shallow Foundations Earth Retaining Structures Jerry DiMaggio, P. E., Principal Bridge Engineer (Geotechnical)
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Jerry DiMaggio, P. E., Principal Bridge Engineer (Geotechnical)
Federal Highway Administration
Office of Bridge Technology
New Legal Load
* TRB/ NCHRP Activities (A LOT!)
* Geotechnical Engineering does NOT have a broad based presence on AASHTO SubCommittees and Task Forces as do other technical specialties.
* SubCommittee on Construction (guide construction specs)
* SubCommittee on Materials (specs on materials and testing standards)
* SubCommittee on Bridges and Structures (specs on materials/ systems, design, and construction)
* First structural “Guideline Specification” early 1930s
(A code yet NOT A code!).
* First “significant” Geotechnical content 1989.
* First LRFD specification 1994 (Current – 2004, 3rd edition).
* First REAL Geotechnical involvement in Bridge SubCommittee activities @ 1996. (Focus on mse walls).
* Technical advances to Standard Specifications STOPPED in 1998 to encourage LRFD use (secret).
* Major rewrites needed to walls and foundations sections (NOW COMPLETE).
* Topics Included: Subsurface Investigations, soil and rock properties, shallow foundations, driven piles, drilled shafts, rigid and flexible culverts, abutments, WALLS (cantilever, mse, crib, bin, anchor).
* Topics NOT addressed: integral abutments, micropiles, augercast piles, soil nails, reinforced slopes, and ALL SOIL and ROCK EARTHWORK FEATURES.
* Currently AASHTO has 2 separate specifications: Standard specs 17th edition and LRFD, 2004 3rd edition.
* Standard Specifications use a combination of working stress and load factor design platform.
* LRFD uses a limit states design platform with different load and resistance factors (than LFD).
Geotechnically, most States still use a working stress approach for earthworks, structural foundations, and earth retaining structures. Several States have totally adopted LRFD.
Many State Geo/Structural personnel and consultants ARE NOT FAMILAR with the content of LRFD 3rd edition.
“AASHTO and FHWA have agreed that all state DOTs will use LRFD for NEW structure design by 10/07.”
* Strong influence of construction on design.
* GEOTECHs strong bias toward performance based specifications.
* Natural variability of GEO materials.
* Variability in the type, and frequency of tests, and method to determine design property values of soil and rock.
* Differences between earthwork and structural foundation design model approaches.
* Influence of regional and local factors.
* General lack of data on limit state conditions.
* FHWA sponsored a complete rewrite of Section 10 during 2004. The rewrite was prepared by National subject matter experts and had broad input from a number of Key State Dots, (including T-15 member States), and the Geotechnical community (ASCE - GI, DFI, ADSC, PDCA).
* During the Proposed spec development @ 2000 comments were addressed. The Proposed spec was then distributed to all States for review. An additional @ 1000 comments were addressed.
* The revised Proposed Specification was advanced and approved by the AASHTO’s Bridge and Structures Sub-Committeee in June 2005.
The revised Proposed Specification is used in the NHI LRFD Substructure course which currently available.
Fundamentals of LRFD
Principles of Limit State Designs
* Define the term “Limit State”
* Define the term “Resistance”
* Identify the applicability of each of the four primary limit states.
* Understand the components of the fundamental LRFD equation.
A Limit State is a defined condition beyond which a structural component, ceases to satisfy the provisions for which it is designed.
Resistance is a quantifiable value that defines the point beyond which the particular limit state under investigation for a particular component will be exceeded.
* Load/Force (static/ dynamic, dead/ live)
* Stress (normal, shear, torsional)
* Number of cycles
* Strength Limit State
* Extreme Event Limit State
* Service Limit State
* Fatigue Limit State
Strength Limit State
Extreme Event Limit State
Service Limit State
Load modifier (eta)
Load factor (gamma)
Resistance factor (phi)
Probability of Occurrence
Q or R
10.4SOIL AND ROCK PROPERTIES
10.4.6Selection of Design Properties
10.4.6.1.1Undrained strength of Cohesive Soils
10.4.6.1.2Drained Strength of Cohesive Soils
10.4.6.1.3Drained strength of Granular Soils
10.4.6.3Rock Mass Strength
10.4.6.4Rock Mass Deformation
10.4.6.5erodibility of rock
* Composed of individual grains of rock
* Relatively low strength
* Coarse grained (+ #200)
* High permeability
* Fine grained (- #200)
* Low permeability
* Time dependant effects
* Intermediate geomaterials,qu = 50-1500 psi
* Hard rock, qu > 1500 psi
* Rock mass properties
Vane Shear Test
su = qu/2
su = 250 - 4000 psf
c’ = 100 - 500 psf
f’f = 20o - 35o
Guided Walk Through
For N160 = 10, select ’f = 30o
Initial elastic settlement (all soils)
Fine-grained (cohesive) soils
p’ = Preconsolidation Stress
Void Ratio (e)
Stress Range, 40 – 80 kPa
One log cycle
Void ratio (e)
Elapsed Time (min)
Unconfined Compression, qu
qu = 1500 - 50000 psi
Point Load Test
Not sound, highly weathered
Not sound, centerline pieces < 4 inches, highly weathered
Total = 4 ft
CR = 95%RQD = 53%
Shear stress, t
Effective Normal Stress, s’
f’i = tan-1(4 h cos2[30+0.33sin-1(h-3/2)]-1)-1/2
t = (cot f’i – cos f’i)mqu/8
h = 1 + 16(ms’n+squ)/(3m2qu)
Poisson’s Ratio, u
(psi x 106)
In situ modulus of deformation, EM (GPa)
Ea = 2 RMR - 100
Rock mass rating RMR
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Jerry A. DiMaggio P. E.Principal Bridge Engineer TEL: (202) 366-1569FAX: (202) 366-3077
The best Geotechnical web site in town! www.fhwa.dot.gov/bridge
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