1、COMMENTARIES 1996 Commentary to Standard Specifications for Highway Bridges INTRODUCTION Note: The 16th Edition of Standard SpeciJications of Highway Bridges includes a Commentary to offer further explanation of the revisions provided in 1996. DIVISION I C5.2.1.4 MSE Walls The existing specification
2、 is restrictive because it refers only to modular precast facing. The proposed wording al- lows the use of other kinds of facings which are available today. C5.2.2.3 Overall Stability The existing specification implies that it is acceptable to proceed with a wall design without soillrock data by usi
3、ng a slightly higher factor of safety. It is clearly un- acceptable and dangerous to proceed with a wall design without adequate data; and, it conflicts with minimum standards of safety for site investigations already con- tained in AASHTO Bridge Specifications. The proposed revision requires that s
4、ite data be obtained for all wall designs, but still distinguishes between normal wall in- stallations and those supporting bridge abutments, build- ings or critical utilities. C5.5.5 Structure Dimensions and External Stability Existing Article 5.5.5 requires the same factor of safety for seismic lo
5、ads as for static loads. However, Article 5.8.10.1 allows a reduced factor of safety for seismic loads. It is reasonable to use a lower factor of safety for seismic loads because it is an infrequent and temporary load. For static loads, we reserve some capacity for unknown loads, fabrication, and wo
6、rkmanship. The proposed revision al- lows the designer to use judgment for the specific site and also brings this article in line with MSE wall criteria. (3.6.2 Earth Pressure and Surcharge Loading This revision is to correct an error in the formula for embedment in rock in Figure 5.6.2A. C5.8.1 Str
7、ucture Dimensions The existing specifications regarding embedment depth are based on latent physical characteristics of the ground. Because of this, most cases are overly conservative, but extreme cases could be equally unconservative. Embed- ment depths should be based on engineering calculations f
8、or stability, bearing capacity, and settlement. Frost heave, scour and proximity to slopes are special considerations. C5.8.2 External Stability and Figure 5.8.4.1A The existing specification requires the designer to use Equation (5.8.2.1) to determine the lateral earth pressure coefficient needed f
9、or external stability calculations for MSE walls. However, for all other gravity walls, the de- signer is required to use Figure 5.5.2B. Since the lateral earth pressure coefficient is not dependent on wall type, there should not be two methods in the specification. In addition, for current practice
10、, it is generally assumed that no wall friction is generated at the back of the wall for overturning and sliding calculations for MSE walls. This can be easily accommodated by setting 6 = . This pro- posal eliminates Equation (5.8.2.1) and requires the use of Figure 5.5.2.B. Additional revisions in
11、this Article include the elimi- nation of the reference to 0.7 as the minimum reinforce- ment ratio in the fifth paragraph and in Figures 5.8.2A3, 5.8.2B, and 5.8.2C. Also revised is Figure 5.8.4.1Afor the same reason. In Figure 5.8.2A, the term V2, which is the weight of the traffic surcharge above
12、 the reinforced soil mass, con- flicts with V2, as defined in the Notations Section, which is the weight of the sloping soil surcharge on top of the re- inforced soil mass. Rather than introduce another V term, it is believed that the “q” load symbol above the rein- forced soil mass is adequate to g
13、ive direction to the de- signers. Also revised is the formula for factor of safety against sliding, which should not include the traffic sur- charge above the reinforced soil mass since this would provide a higher factor of safety than is realistic. It should include the traffic surcharge behind the
14、 soil mass. See also C5.8.2 (1998). c-3 c-4 HIGHWAY BRIDGES C5.8.3 Bearing Capacity and Foundation Stability The existing specification is conservative for locations in rock and is not consistent with Article 4.4.8. The pro- posed revision to Article 5.5.5 covers this issue ade- quately, so this rev
15、ision to Article 5.8.3 is to eliminate the sentence and refer to Article 5.5.5 for guidance on loca- tion of the resultant force. (25.8.7.1 Allowable Stresses, Steel Reinforcements The existing specification requires transverse and lon- gitudinal grid members to be the same size. Since welded wire i
16、s generally not manufactured with these bars being the same size, the revision allows the bars to be sized properly and refers to ASTM A- 185, the most widely ac- cepted standard for welded wire. C8.15.5.5.5, C8.27.1, C8.16.6.5.5, and C9.20.4.5 Since the implementation of reinforced concrete and pre
17、stressed concrete into the AASHTO Specification, a provision in both respective design sections calls for all Section 17 General Section 17 has been revised to incorporate new Standard Installations for concrete pipe, replacing the historical B, C, and D beddings as explained later in this Commentar
18、y. The earth loads and pressure distribution associated with the new beddings are also incorporated as described in the appropriate commentary articles. Direct design for pipe installed in the new Stan- dard Installations, using the design equations that have been a part of Section 17 since 1983, is
19、 facilitated using the Federal Highway Administration Computer pro- gram PIPECAR. This program has recently been updated to include analysis and design procedures for the earth loads and pressure distribution associated with the new Standard Installation types. A version of this program known as SID
20、D is also available for direct design of concrete pipe using only the earth loads and pressure distribution associated with the new Standard Installations. An alternate indirect design procedure for pipe in- stalled in one of the new Standard Installations is also in- cluded in this revision of Sect
21、ion 17 to facilitate the use of these installations for indirect pipe design procedures that related field strength requirements to equivalent three- edge bearing strengths. vertical shear reinforcement in the girders, to be extended into the cast-in-place deck. This extended reinforcement is often
22、shaped in the configuration of a bent stirrup. The purpose of this reinforcement is to provide addi- tional composite action between the girder and the deck. The primary design mechanism for the horizontal shear at the interface, is the shear friction theory. Other design criteria include the contro
23、l of slippage at service load and fatigue strength. The parameters for shear friction design are outlined in the AASHTO Specifications. The amount of steel crossing the interface using cur- rent provisions, may in some cases be much larger than that required by the shear friction theory. In regards
24、to bridge construction, this provision has been shown to increase the amount of time required to re- move the bridge deck from the top of the girders. Clean- ing the concrete deck from around the extended shear stirrups is a labor intensive process, and includes the pos- sibility of damage to the to
25、p flange of the girder, especially when small stirrups at narrow spacing are used. This revision is intended to permit decreasing the num- ber of extended shear stirrups into the deck slab provided the beam shear reinforcement is adequately anchored to provide full beam design shear capacity. The Co
26、ncrete Pipe Technology Handbook presents historical and current state-of-the-art design and method- ologies from the development of the Marston-Spangler theories, through the Olander and Paris methods to the development of the Standard Installations, the associated earth loads and pressure distribut
27、ion (named Heger distri- bution), and the direct design method. The handbook also presents example design calculations done by hand and by using SIDD. The new Standard Installations and associated direct design method were extensively considered by a new ASCE Standards Committee comprised of consult
28、ing en- gineers and technical representatives of user agencies and the pipe industry. The proposed Standard Installations and direct design procedure that is essentially the same as that proposed for direct design of buried concrete pipe in Sec- tion 17 were accepted in 1993 as ASCE 15-93, Standard
29、Practice for Direct Design of Buried Precast Concrete Pipe Using Standard Installations (SIDD). See also C Section 17 (1997). C17.1.2 Notations Seven new parameters are defined as required for de- sign using the new Standard Installations. 1996 COMMENTARY c-5 C17.4.3 (existing) has been renumbered C
30、17.4.2.3 Concrete Cover for Reinforcement-as a subsection under 17.4.2 Materials C17.4.3 Installations (new) C17.4.3.1 Standard Installations This new section defines the four new Standard Instal- lations, Types l, 2, 3, and 4, for trench and embankment installations. See Figure 17.4Afor schematic d
31、rawings for various kinds of installation. Specific soil and dimensional requirements for the four Standard Installation types in trench and embankment configurations are given in Fig- ures 17.4B and 17.4C and in Tables 17.4A and 17.4B. The four new Standard Installations are recommended to replace
32、the historic standard installation or bedding classes A, B, and C. This recommendation is based on an extensive research program performed by Simpson, Gumpertz and Heger, Inc. under the direction of Dr. Frank J. Heger. Dr. Ernest Selig, Professor of Geotechnical En- gineering at the University of Ma
33、ssachusetts, Amherst, Massachusetts, was geotechnical consultant for the re- search team. A comprehensive soil-structure interaction analysis and design program named SPIDA was devel- oped and used to perform many soil-structure interaction analyses for the various soil and installation parameters i
34、nvestigated by the research team. Based on these results, and numerous consultations with engineers having exten- sive experience with design, construction, and perfor- mance of concrete pipe, the research team recommended the four new Standard Installations for concrete pipe to the Technical Commit
35、tee of the American Concrete Pipe Association. The SPIDA studies used to develop the Standard In- stallations were conducted for positive projection em- bankment conditions, which are the worst-case vertical load conditions for pipe and which provide conservative results for other embankment and tre
36、nch conditions. These studies also conservatively assumed a hard foun- dation and bedding existed beneath the invert of the pipe, plus void and/or poorly compacted material in the haunch areas, 15“ to 40“ each side of the invert, resulting in a load concentration such that calculated moments, thrust
37、s and shears were increased. The modeling of the soil pres- sure distribution presented in Figure 17.4A, while an ac- curate presentation, is additionally conservative by 10-20 percent as compared to the exact SPIDA results. The new Standard Installations offer the following ad- vantages for design
38、of concrete pipe: Specify more quantitative requirements for soil type and level of compaction than the historic B, C, and D beddings. Thus, design is more rational using them. Provide a quantitative and rational basis for direct design of concrete pipe for the installed condition, based on state-of
39、-the-art soil-structure interaction analyses. Do not preclude the use of the more traditional indi- rect design procedure for engineers who wish to re- late field strength requirements to equivalent three- edge bearing test requirements. 0 Allow the use of both select embedment soils (which may have
40、 to be imported), or potentially less expen- sive soils from the site excavations, with proper ac- count of relative properties for supporting the pipe. The cost-benefit relationship of pipe strength versus installation quality can take into consideration more easily the use of better quality instal
41、lations for high fill heights. 0 Recognize the benefit of maintaining a lower com- paction level below the invert region (middle third of diameter) relative to the outer third. 0 After review by the Technical Committee of the ACPA and the AASHTO Rigid Culvert Committee, the Rigid Culvert Committee r
42、ecommended accep- tance of these new Standard Installations and their associated direct and indirect design procedures by the AASHTO Bridge Committee for inclusion in Section 17 of the AASHTO Bridge Specification. Specific earth loads and earth pressure distributions are associated with these new St
43、andard Installations. These are discussed in later sections of this Com- mentary. C17.4.3.2 Soils The soil classifications used to define the minimum re- quirements for soil type are given in Table 17.4C. C17.4.4.2.1 Earth Load and Pressure Distribution The earth load for designing pipe in a Standar
44、d Instal- lation is obtained by multiplying the weight of the col- umn of earth above the outside diameter of the pipe by the soil-structure interaction factor, Fe, for the design in- stallation type. F, accounts for the transfer of some of the overburden soil above the regions at the sides of the p
45、ipe because the pipe is more rigid than the soil at the side of the pipe for pipe in embankment and wide trench instal- C-6 HIGHWAY BRIDGES lations. Because of the difficulty of controlling maximum trench width in the field with the widespread use of trench boxes or sloped walls for construction saf
46、ety, the potential reduction in earth load for pipe in trenches of moderate to narrow width is not taken into account in the determination of earth load and earth pressure distri- bution on the pipe. Both trench and embankment in- stallations are to be designed for embankment (positive projecting) l
47、oads and pressure distribution in direct design, or bedding factors in indirect design. The soil structure interaction factor, Fe, is the vertical arching factor VAF given for the Heger Pressure Distribution in Figure 17.4A. For direct design, the earth pressure distribution and lateral earth force
48、for a unit vertical load is the Heger pres- sure distribution and horizontal arching factor, HAF, given in Figure 17.4A. The normalized pressure distribu- tion and HAF values were obtained for each Standard In- stallation type from the results of soil-structure interaction analyses using SPIDA toget
49、her with the minimum soil properties for the soil types and compaction levels speci- fied in various parts of the installations, as shown in Fig- ures 17.4B and C and Tables 17.4A and B. Equation (17-2) for Fe, with maximum Fe = 1.2 for compacted sidefills for embankment installations in the previous edition of Section 17, was not found to be con- sistent with the research results that are the basis of design with the new Standard Installations. Research has indicated values for Fe in the range of 1.35 to 1.45, as a function of sidefill compaction, are appropriate for embankment installations
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