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OCR for page 28
28 50 Limit on Vn for PC members 40 in AASHTO STD (fpc/sqrt(f'c)=10, f'c=10000 psi) 30 Vu test f'c 20 10 Limit on Vn for RC members (f'c=10000 psi) in AASHTO STD 0 0 500 1000 1500 2000 2500 Stirrup Strength, v fy (psi ) Figure 20. Test data and maximum shear stress limit. member. In many situations in practice, the ends of beams quently, many organizations are actively beginning the are built integrally at their ends into piers, columns, or conversion process, and thus are on, or just beginning, the diaphragms. The results of the beam tests conducted for learning curve with the LRFD Sectional Design Model NCHRP Project 12-56 demonstrate that higher maximum (Section of the LRFD specification). This is relevant shear stresses can be achieved for that situation than for because two camps of designers seem to exist: those that have beams simply supported on bearings positioned underneath become reasonably comfortable with the production of LRFD the member. shear designs and those who view it as a significant hurdle yet to be surmounted. Although some users have become famil- iar with the mechanics of the method, almost universally 2.4 RESULTS OF SURVEY OF PRACTICE designers report that the method is not easily executed by hand and that one often loses sight of the relative mechanics A survey of the design practices of 26 different state DOTs of what is happening in the structure. All agree that the LRFD and federal lands bridge design agencies was conducted. shear design provisions must be automated with software to This survey included both a written questionnaire and either be used in production design. This fact naturally leads to loss a telephone briefing on the response to the questionnaire or a of comfort with respect to checking designs, because the written response. Of the 26 agencies polled, 21 responded; method cannot be readily executed by hand. Most designers these are listed alphabetically at the end of this section. The also agree that the Standard Specification method for pre- specific questions and the responses are included in Appen- stressed design (Section 9.20) that includes Vci and Vcw must dix E, which is part of NCHRP Web-Only Document 78. The also be automated to be effective in production work, even questionnaire was to determine the status of conversion to though that method is executable by hand. Thus, with the LRFD, identify specific problems and practices with respect existing AASHTO LRFD provisions one of the most preva- to concrete element shear design, ascertain preferences for lent comments was that designers are losing their physical shear design methodologies, and provide a vehicle for orga- "feel" for shear design because of the increasing complexity nizations to express their opinion of the current LRFD shear of the design provisions and the resulting automation. design methodology. Most agencies using the LRFD shear provisions make no Some recurring themes and trends emerged. First, many of modifications or simplifications to the process before adop- the organizations have not yet converted the bulk of their tion into their design procedures, although many respondents practice to LRFD, although most have undertaken serious in- indicated that they would if the simplifications were reason- house evaluation of the likely effects of conversion. In most able. The primary simplification that is being used by a few cases, the in-house evaluators were interviewed and were the organizations is the elimination of the iterative process primary respondents to the Questionnaire. Only 7 of 21 had required to determine the crack angle, . converted to LRFD for most of their bridges, even though a Almost all respondents indicated that there had been some deadline for conversion has been set nationally. Conse- difficulty in applying the MCFT provisions and that often

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29 difficulties also arose when applying the provisions to bent Most of the LRFD users thought that the engineering time cap beams, columns, and footings. Difficulties in applying for shear design is not significantly increased over that for the the strut-and-tie method were also commonly reported, par- Standard Specifications, provided that shear design is auto- ticularly for indeterminate beams. Furthermore, the potential mated using spreadsheets, MathCAD, or other software. How- for more than one shear design solution was perceived as a ever, if designs are attempted by hand, the design time is often problem by about one-third of the respondents, although this significantly increased over that required by the Standard Spec- was not seen as a significant problem by most. ifications. This is often a source of discontent when the effort When asked whether the LRFD shear provisions produce is increased, but the results change very little. The discontent is significantly different designs than the Standard Specifica- reinforced by the fact that shear steel is not typically a signifi- tion method, the responses were mixed. Many were not sure cant cost relative to the entire bridge cost and by the fact that yet, but about one-third indicated that more shear steel is many designers prefer to be conservative for shear design. often required, although the amounts of the increase were For the relatively common case of girders made continu- quite variable, with the high response being about 40 to 50 per- ous for live load, it is widely thought that the current LRFD cent increases for bridges with large skews. This particular provisions do not adequately explain the application of the respondent also indicated that the demand side had as much method to this case. Problems seem to be particularly com- to do with the increase as the resistance side, because the mon in the negative moment region. The problem of appro- LRFD load combinations often produce larger design forces. priate use of simultaneously acting internal forces in the Several respondents indicated that the revisions to the and resistance equations is, however, not new to LRFD, although values incorporated into the second edition of LRFD confusion seems to be worse with the LRFD shear design helped bring the steel contents into parity with the Standard procedure. This confusion stems from the fact that cases arise Specifications method. in practice that are more complex than those envisaged dur- Regarding the use of the simpler 1979 AASHTO shear ing the development of the specifications, thereby producing design procedure allowed by the Standard Specifications in confusion among designers. This problem is at least in part a footnote to Section 9.20, about one-third indicated that they related to definitions, where common cases are not always still used this method. Furthermore, they report that no prob- clearly explained. Definition problems are compounded lems have apparently arisen from the continued use of the when confusion also arises over appropriate signs to be used 1979 method. for internal forces. Nearly all the respondents indicated that a relative simple The common themes in terms of the most important design design method would be highly desirable, even if it was used issues can be synthesized into the following: (1) There for checking only. In fact, a simple checking method is one of should be a simple, logical method for performing shear the most requested items. Also, many designers recognize design or alternately, checks of designs; (2) The method the advantages of the LRFD provisions for a more accurate should provide a feel for the mechanics and should help evaluation of shear resistance. The LRFD shear provisions designers develop a comfort level with the results; (3) The thus can often offset the increases on the demand side that are simplified method need not supplant the MCFT theory, but common with LRFD, though this tends toward less conserv- need only supplement the method and provide a logical alter- ative designs for shear. Most designers seem to prefer a nate; and (4) The method does not need to be highly accu- palette of design approaches. If simple methods provide sim- rate, provided it is conservative. ilar designs to more complex methods, the need to switch to Regarding field problems that may be related to shear the complex method is not felt warranted. Some would even design, most indicated that few, if any, problems have prefer to include some of the older, tried and proven shear occurred with the more modern design procedures. Several design methods as alternates. This would ease the `transition designers outlined problems that have arisen in older bridges trauma' associated with the LRFD learning curve. whose design predates the current procedures in the Standard The most common types of concrete bridges appear to be Specifications. However, one respondent did indicate that prestressed concrete girder bridges, including those designed potential problems in segmental construction have arisen as simple spans for both dead and live loads and those made when using the LRFD methods because of the lack of a prin- continuous for live load. Only a few respondents indicated a cipal tension check for the webs. With respect to fabrication routine use of box girders and segmental construction. Most of precast elements, many designers indicated that conges- did not know of any cases where the LRFD shear design had tion at the ends of beams is quite common, although this eliminated a bridge type from consideration for a given proj- stems more from the confinement steel requirements than ect. However, one case of deeper girders being required from shear steel requirements. when high skews were present was cited. For PC girder Finally, the issue of bridge rating using LRFR (the LRFD bridges, standard designs have been common in the past and rating approach) versus the current LFR rating is a source of are still desirable. However, standard designs have been ongoing concern, although most have not had any experience difficult, if not impossible to achieve, with the LRFD shear with the LRFR method yet. Whether this is a potential problem procedures. depends primarily on how the method is phased in and whether