INTRODUCTION TO QuikBeamPennDOT
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 (by PENNDOT's DM-4, Publication 15M). Commonly used bridge
beam types are shown in Figure 1.
Figure 1
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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.
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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]
QuikBeamPennDOT, a macro driven spreadsheet
using PENNDOT sections, 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).
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Preliminary Design Aids : Introduction to QuikBeamPennDOT
