Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 48
48
design codes. The most typical laboratory test considered useful for evaluating the safety and econ-
structures are small (less than 15 inches deep), have omy of design provisions, particularly for the types of
rectangular cross sections, do not contain shear rein- structures and regions for which there is little experi-
forcement, are simply supported, are stocky, are loaded mental test data.
by point loads over short shear spans, and are supported 2. Although these comparisons were useful, the dataset
on bearings positioned underneath the member. In selected by the research team may not well represent
addition, nearly all members are designed so that shear the types and frequency of structures to be designed by
failures occur near supports. By contrast, a large frac- provisions.
tion of the bridge members in the field are large, con- 3. The assessment of the effect of the proposed changes
tinuous, have top flanges, are subjected to uniformly on bridge design practice would also have been more
distributed loads and are built integrally at their ends reliable if the design database well represented the
into diaphragms or piers. In addition, members in the types and frequencies of structures to be designed by
field are designed for shear over their entire length and these provisions.
away from simple supports where there can be sub- 4. The results would also have been more useful if there
stantial effects of flexure on shear capacity. were more computational tools (other than Response
2. Because most code provisions are ultimately validated 2000) for predicting the required strength of shear rein-
by test data, and because most members in the experi- forcement in these design cases.
mental database do not represent what is built with pro- 5. The NCHRP Process 12-50 helped establish a frame-
visions, there is great uncertainty about the safety, work for addressing the three foregoing shortcomings,
economy, and validity of these provisions for most but the design example database has yet to be populated
shear design regions in most structures. A particular with representative types and frequencies of members
case in point is the region of contraflexure in a contin- designed with provisions.
uous beam. The wide spread in the shear requirements
found in Example 5 of Appendix J for this region for
4.1.4 Differences in Shear Design Provisions
different provisions is a direct reflection of the uncer-
tainty of the safety of those provisions for that region. 1. There is a wide variation in the forms of shear design
3. Most experimental researchers fail to collect or report specifications used in different influential codes of
detailed information about the performance of the test practice such that the amount of shear reinforcement
members before failure. This information consists of required by one code may be two to three times that
material properties, member or test set-up geometry, required by another code for the same section and fac-
crack patterns and widths, stirrups strains, measured tored sectional forces.
diagonal compressive stresses, and shear deformations. 2. There remains considerable disagreement in codes of
Thus, most tests are not useful for assessing the condi- practice on the minimum required amount of shear
tion of members under service loads or for evaluating the reinforcement and when this minimum reinforcement
accuracy of complete behavioral models for resistance. is required. There is a factor of about 2 in the minimum
4. It is difficult to judge the overall safety of design code required amounts of shear reinforcement. Some codes
provisions from field experience because most struc- required minimum shear reinforcement when the fac-
tures in the field have redundant load paths, additional tored design shear force exceeds one half of the design
load resisting elements not accounted for in design, and strength provided by concrete alone while others do not
are unlikely to be subjected to loads approaching their require minimum shear reinforcement until the fac-
factored design loads. Further, many of the difficulties tored design shear force exceeds this design strength.
observed in the field are dominated by an interaction of The types of members exempt from more stringent
deterioration, environmental and repeated loading minimum shear reinforcement requirements also vary.
effects. 3. There is a large variation in the maximum allowable
shear stress by different codes of practice. The differ-
4.1.3 Role of Design Database and Numerical ence can be a factor of two and one-half between the
Tools AASHTO Standard Specifications and LRFD Sec-
tional Design Model.
1. In this project, a comparison was made of the required 4. The depth effect in shear that has been strongly
strength of the shear reinforcement (vfy) by four dif- observed in members without shear reinforcement is
ferent design approaches and by Response 2000 for captured in some codes of practice by making the
about 500 design cases. The research team chose these allowable design stress a function of the overall depth
design cases in an effort to capture the range in design of the member. The depth effect can change the allow-
cases for which shear design specifications would able shear design stress by more than a factor of two for
be applied. The results of these comparisons were different sized members.
