LRFD Design and Construction of Shallow Foundations for Highway Bridge Structures (2010) / Chapter Skim
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From page 57... ...
. In contrast, out of the 39 responding states, 6 states do not use shallow foundations for bridges at all, and an additional 8 states use shallow foundations in 5% or less of highway bridge foundations.
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From page 58... ...
2. About 19% of the states using foundations on rock use presumptive values alone, 22% use engineering analyses alone, and 59% use both when evaluating bearing capacity.
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From page 59... ...
A major concern expressed by Michigan was written by a bridge designer referring to the difficulties in using effective width for bearing capacity calculations as it requires iterations for each load case for service and strength. Moreover, the division of responsibilities between the geotechnical section (providing allowable pressure)
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From page 60... ...
, and the use of ground improvement techniques was found to be less attractive than the use of deep foundations in such cases. The design process of the shallow foundations mostly includes SPT, internal friction angle, bearing capacity analysis without inclination factors, and then settlement evaluation that controls the foundation size.
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From page 61... ...
A summary of the major attributes of each database is presented below. Additional statistics are presented for relevant analyses (e.g., see Section 3.5 for centric vertical loading on shallow foundations in/on granular materials)
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From page 62... ...
, (2) Settlement of Shallow Foundations on Granular Soils, a report to the Massachusetts Highway Department by Lutenegger and DeGroot (1995)
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From page 63... ...
63 SiteConditionID 40103 SiteConditionID 40104 SiteConditionID 40101 SiteConditionID 40102 SiteConditionID 40105 Figure 47. Footing dimensions and site details along with the associated SiteConditionID employed in database UML-GTR ShalFound07.
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From page 64... ...
All the rock sockets in the database are circular for which the end bearing capacity (tip resistance) could be isolated, separating it from the shaft resistance of the rock sockets.
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From page 65... ...
65 0 2 4 6 8 10 12 14 16 18 20 22 24 Footing width, B (ft)
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From page 66... ...
3.3.2 Failure (Ultimate Load) Criteria 3.3.2.1 Overview -- Shallow Foundations on Soils The strength limit state is a "failure" load or the ultimate capacity of the foundation.
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From page 67... ...
in logarithmic scale can be used as an alternative to the linear scale plot if it facilitates the identification of the starting of minimum slope and hence the failure load. 3.3.2.3 The Uncertainty in the Minimum Slope Failure Criterion Interpretation In order to examine the uncertainty in the method selected for defining the bearing capacity of shallow foundations on soils, the following failure criteria (described in detail in Appendix F)
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From page 68... ...
Histogram for the ratio of representative measured capacity to interpreted capacity using the minimum slope criterion for 196 footing cases in granular soils under centric vertical loading. Figure 55.
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From page 69... ...
; dc, dγ, dq = depth correction factors to account for the shearing resistance along the failure surface passing through the soil above the bearing elevation as specified in Table 28 (dim.) ; and ic, iγ, iq = load inclination factors as specified in Table 29 (dim.)
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From page 70... ...
f = 0 L B2.01 1.0 1.0 Shape Factors s c, s , sq f > 0 c q N N L B1 L B4.01 ftanL B1 Table 27. Shape correction factors sc, s, sq (Vesic´, 1975)
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From page 71... ...
Comparison of various correlations between granular soil friction angle and corrected SPT blow counts using the overburden correction proposed by Liao and Whitman (1986)
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From page 72... ...
The soil friction angle used in these laboratory tests was extensively tested, and Figure 59 shows the results of 52 direct shear tests carried out on dry Essen sand with a dry unit weight in the range of 15.46 ≤ γ ≤ 17.54 kN/m3 (98.5 ≤ γ ≤ 111.75 pcf )
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From page 73... ...
Figure 59. Revised correlation for angle of internal friction and dry unit weight of Essen sand.
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From page 74... ...
relationship between measured and calculated bearing capacity for all cases of shallow foundations under vertical-centric loading.
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From page 75... ...
relationship between measured and calculated bearing capacity for vertical, centrically loaded shallow foundations on controlled soil conditions. 1 10 100 Calculated bearing capacity, qu,calc (Vesic, 1975 and modified AASHTO)
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From page 76... ...
The analysis shows that in the case of a test with a radial load path it is sufficient to consider only the vertical load versus vertical displacement curve. This curve already includes the unfavorable effect that a horizontal load or a bending moment has on the bearing capacity of a shallow foundation, leading to smaller vertical failure loads compared to the case of centric vertical loading.
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From page 77... ...
vertical load versus vertical displacement and (b) horizontal load versus horizontal displacement.
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From page 78... ...
The vertical failure loads, F1,ult, are the ones corresponding to the horizontal failure loads, F3,ult, and coincide with the constant vertical load in each test. As the load inclination is increased during the test, the maximum load inclination reached is the load inclination at failure, tan δult = F3,ult/F1,ult.
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From page 79... ...
Figure 67 presents a histogram and PDF of the bias as well as the relationship between measured and calculated bearing capacity for all inclined, centrically loaded shallow foundations. There are no differences in the biases obtained from the two-slope and the minimum slope failure criteria for the cases of step-like load paths.
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From page 80... ...
Figure 68 presents a histogram and PDF of the bias as well as the relationship between measured and calculated bearing capacity for all inclined, eccentrically loaded shallow foundation cases. As in the inclined-centric loading cases, there is no significant difference in the tests results between the radial and the step-like load paths.
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From page 81... ...
relationship between measured and calculated bearing capacity for all inclined, eccentrically loaded shallow foundations. Minimum slope criterion Two-slope criterion Tests No.
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From page 82... ...
The induced rotations counteract the displacements forced by the horizontal load, leading to a higher resistance of the footing compared with the inclined-centric load case and, thus, to higher failure loads. In contrast, the footing in the lower part of Figure 69 is loaded by an eccentric vertical load with "positive" eccentricity.
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From page 83... ...
relationship between measured and calculated bearing capacity for all inclined, eccentrically loaded shallow foundations under positive moment. 1.2 1.8 2.4 3 3.6 4.2 4.8 5.4 6 6.6 7.2 Bias, λ = qu,meas / qu,calc 0 1 2 3 N um be r o f o bs er va tio ns 0 0.1 0.2 0.3 0.4 0.5 Fr eq ue nc y Inclined-eccentric loading Negative eccentricity n = 7 mean = 3.43 COV = 0.523 normal distribution lognormal distribution (a)
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From page 84... ...
In reference to Figure 64, F2 is the horizontal component of the inclined load and b2 is the footing length in the same direction. Different loading directions and different load levels have been analyzed in Figure 74, resulting in distorted trend lines due to the existence of a higher capacity if horizontal load and moment act in the opposite direction (i.e., both load components are positive and the loading eccentricity is negative)
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From page 85... ...
was used to assess the uncertainty of the selected design methods for the 119 case histories of database GTR-UML RockFound07. Section 1.7 details the methods of analysis selected for the bearing capacity calculations.
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From page 86... ...
method is described in Section 1.7.6 and its application is demonstrated in Section G.7 in Appendix G Table E-2 of Appendix E presents the calculated bearing capacity values and the associated bias for each of the 119 case histories of database UML-GTR RockFound07 (Table E-2 includes all 122 original cases and the excluded 3 cases as noted)
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From page 87... ...
and the effect of the friction angle (φf) of the rock on the calculated bearing capacity.
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From page 88... ...
mean = 8.00 COV = 1.240 lognormal distribution normal distribution 4 Figure 76. Distribution of the ratio of the interpreted bearing capacity (qL2)
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From page 89... ...
Table 42 is a summary of the statistics for the ratio of the measured bearing capacity (qL2) to the calculated bearing capacity (qult)
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From page 90... ...
Summary of the statistics for the ratio of measured (qL2) to calculated bearing capacity (qult)
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From page 91... ...
mean = 1.35 COV = 0.535 lognormal distribution normal distribution Figure 79. Distribution of the ratio of the interpreted bearing capacity (qL2)
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From page 92... ...
A practical summary and appropriate resistance factors are further discussed and presented in Chapter 4. 3.9.2 Experimental Results Using a Dual Interface Apparatus (DIA)
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From page 93... ...
° from the direct shear tests of 17 samples. As a result, the ratio of the friction coefficients, tan(δcenter)
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From page 94... ...
MDS (= center m) Converted friction coefficient ratio Zone I 6.0 0.8 0.17 1.50 9.0 0.25 Zone II 8.0 to 25.0 0.17 to 0.90 1.20 9.5 to 30.0 0.25 to 1.00 Zone III 28.7 0.90 1.10 31.5 1.00 Note: Material friction angle obtained from direct shear test = 31.6° (±1.0°)
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From page 95... ...
, the interface roughness in Zone III is relevant and, further, that the uncertainties in the sliding friction coefficient ratio (tan δs/tan φf) directly correspond to those existing in the method by which the soil friction angle is being defined (i.e., lab test, SPT, and so forth)
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From page 96... ...
Uncertainties in interface friction coefficient ratio according to interface roughness and the determination of the soil friction angle. Interface Materials ta n δ s Friction (degrees)
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From page 97... ...
Figure 86 presents the relationship between the soil's unit weight and the internal friction angle. Figure 87 presents the relationship between the soil's unit weight and the measured friction coefficient ratios of the footings.
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From page 98... ...
Ratio of measured footing friction coefficient ratios to the soil's internal friction coefficient versus soil unit weight (Foik, 1984)
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From page 99... ...
2. Figures 88 and 89, which show the interface friction coefficient as a function of the soil's internal friction coefficient and the vertical applied stress (respectively)
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