Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 33

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

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 32
32 be directly calculated by Mohr's circle of stress, as shown in f pc Vcw = ft 1 + bw d + Vp Figures B-4 and F-2 in the appendixes; see Equation 18. ft (Eq. 15) Av f y d Vs = cot where d is suggested to be not less than 0.8h. s (Eq. 17) The tensile strength of the concrete, ft, can be taken as somewhere between 2 fc and 4 fc where f c is in psi units. f pc where: cot = 1 + (Eq. 18) In the proposed provisions, stress is expressed in ksi but it is ft considered more useful to present the proposal with the stress given in psi units. A tensile cracking strength close When fpc = 0, the axial stress is zero, or if flexure-shear to 4 fc is believed to provide a more accurate estimate of cracking governs, then cot = 1 ( = 45 degrees). the diagonal cracking strength in the design of the end regions of a fully prestressed member in which there is no effect of flexure while a value of 2 fc is a better estimate of 3.1.2 Proposed Simplified Provisions the diagonal cracking load in a reinforced concrete member The proposed simplified provisions are given here in both or a prestressed member with a low level of prestressing. A ksi and psi units. In order not to imply a greater level of pre- transition between those two levels is a function of the level cision in the procedure than can be justified, the coefficients of the prestress and the axial load. for the expressions in ksi units, as currently used in the LRFD specifications, are rounded off. Flexure-Shear Cracking Strength,Vci For flexure-shear cracking of prestressed beams, the Web-Shear Cracking Strength expression used in the AASHTO Standard Specification is Vcw = (0.06 fc + 0.30 f pc )bv d v + Vp Vi Mcr (where stress is in ksi units) (Eq. 19) Vci = 0.6 fcbw d + Vd + 1.7 fcbw d M max (Eq. 16) which is equivalent to where the sum of the second and third terms is an estimate Vcw = (1.9 fc + 0.30 f pc ) bv d v + Vp of the shear force at the time of flexural cracking while the first term is the increase in shear that has been observed (where stress is in psi units) (Eq. 20) in experiments for a flexural crack to propagate into a diag- onal crack. Flexure-Shear Cracking Strength Although the concrete contribution, Vc, is taken as an esti- mate of the diagonal cracking load, it must also be a lower Vi Mcr Vci = 0.02 fcbv d v + Vd + 0.06 fcbv d v bound estimate of the concrete contribution to shear resis- M max tance at the ultimate limit state. At the ultimate limit state, the concrete contribution is the sum of the shear carried in the (where stress is in ksi units) (Eq. 21) compression zone, the shear carried across diagonal crack which is equivalent to due to shear-friction (aggregate interlock), direct tension across diagonal cracks, dowel action, and arch action. Many Vi Mcr Vci = 0.632 fcbv d v + Vd + 1.9 fcbv d v factors influence the contributions of each of these mecha- M max nisms and attempts to reasonably account for them lead to (where stress is in psi units) (Eq. 22) complicated expressions for Vc. Thus, the approach taken by the research team in developing this simplified approach has The 0.06 coefficient in Equation 19 establishes a uniform been to use a lower bound estimate of the diagonal cracking minimum Vc contribution over the length of the member load that, when added to the calculated stirrup contribution independent of whether a web-shear or flexure-shear region to shear resistance, is shown to provide a conservative esti- is being designed. The coefficient of 0.06 (ksi units) is also mate of the capacity of test beams presented and discussed in Section 3.3. very close to the traditional coefficient of 1.7 (psi units) when it is considered that dv = 0.9d. Contribution of Shear Reinforcement, Vs Vci ,min = 1.7 fcbv d v (psi units) 1.7 fc / 1000 / 0.9 0.0597 fcbv d v ( ksi units) The contribution of the shear reinforcement to shear resis- tance is given in Equation 17. The angle of shear cracking can (Eq. 23)