Introduction to QuikBeamAASHTO

Preliminary Design Aids : Introduction to QuikBeamAASHTO

INTRODUCTION TO QuikBeamAASHTO

A full preliminary analysis of prestressed concrete beams is highly desirable but seldom performed in practice due to the time consuming requirements in determining the reinforcement and initial concrete strengths. Design engineers usually face the situation where several initial estimates of the beam size, amount of reinforcement, concrete release strength and material quantities are needed before final framing and layout decisions are made. In the final design stage they will proceed to run detailed calculations and check conformance to the design requirements. Commonly used bridge beam types are shown in Figure 1.

Figure 1

These tedious design steps usually take a substantial amount of time. Even with the use of current software available to the designer, the program input and iterations are still time consuming. Furthermore, a previous design of a 98-feet-span structure, for example, cannot simply be extrapolated to a subsequent 118-feet-span structure because of other variables such as girder spacing, concrete strength and beam size which are associated with a specific project.

In the Type, Size and Locations (T S & L) phase of a bridge selection process, designers are often faced with the following questions for a bridge with a given width:

(1) Given an alternative solution with a span, L1, a number of beams, Nb (or a girder spacing S1), is the solution feasible? The answer is yes, if:

(a) the sections require a number of 1/2 inch strands, N, that is less than the maximum number physically allowed by the form design, and

(b) if the required release strength for concrete does not exceed a maximum regional, structural, or producer limit, f'ci.

(2) If one pier is eliminated a new alternate design is possible with a deeper section and a longer span, L2. Is a new spacing S2<S1 required? Is the solution in (2) more economical than (1)?

(3) If one girder line is eliminated, would the new solution with a spacing S3> S1 be feasible and more economical?

A fast prediction of N and f'ci is essential at the conceptual T S & L or estimating stages since producers have limitations on concrete strengths and the amount of reinforcement they can place in a beam. In order to answer these questions in a systematic way, a relationship between N (or f'ci), the girder spacing, S, and the span, L, was first developed in the early 1990s.

Figure 2

In this study, it was assumed that L and S are the dominant variables. Based on more than 1,100 exact solutions, it was found that the following equations are excellent predictor formulas (also see Fig. 2 above) where N is the number of standard 1/2 inch diameter, low-relaxation strand, and f'ci is the required concrete release strength:

N = A(L)x (S)y [1]

f'ci = B(L)u (S)v [2]

QuikBeamAASHTO, a macro driven spreadsheet using sections available in the Central Atlantic states, was also developed using these equations. For 95% of the cases the average relative prediction error using Equations [1] and [2] was found to be 4% only, with a range of 0 to 8%. It requires inputting a limited number of data such as bridge type (I-beam, spread box or adjacent box), bridge width, and simple span length. Additional variables, such as slab overhang, the minimum number of girders, smallest desirable beam depth and maximum permissible release strength are optional. The program then generates up to 9 solutions with prestressing and strength requirements in addition to summarizing the material quantities (concrete volume and number of strands).

Preliminary Design Aids : Introduction to QuikBeamAASHTO