4.3 Pretensioned Beams

4.3.1 General (01/09) 2009 Jan: D. Inserted note “2.) Distribute debonded strands evenly…”; 2007 July: Revised D.5 to “pretensioned” members for the use of transformed section properties and clarified when to use the “Approximate Estimate of Time-Dependent Losses.“;  2006 July: Per TDB C06-01, Replaced 4.3.1.D.5; 2006 Jan: Per TDB C05-13, added F. requiring bearing plates in beam ends.

A. The use of ASTM A416, Grade 270, low-relaxation, straight, prestressing strands is preferred for the design of prestressed beams. However, the following requirements apply to simply supported, fully pretensioned beams, whether of straight or depressed (draped) strand profile, except where specifically noted otherwise.

B. Bridges with varying span lengths, skew angles, beam spacing, beam loads, or other design criteria may result in very similar individual designs. Consider the individual beam designs as a first trial subject to modifications by combining similar designs into groups of common materials and stranding based upon the following priorities:

1.) 28-Day Compressive Concrete Strength (f ’c)

2.) Stranding (size, number, and location)

3.) Compressive Concrete Strength at Release (f ’ci)

4.) Full Length Shielding (Debonding) of prestressing strands is prohibited.

Commentary: Grouping beam designs in accordance with the priority list maximizes casting bed usage and minimizes variations in materials and stranding.

C. In analyzing stresses and/or determining the required length of debonding, stresses must be limited to the following values:

1.) Tension (psi) at top of beam at release (straight strand only):

Outer 15 percent of design span: 12√f'ci

Center 70 percent of design span: 6√f'ci

2.) Tension at top of beam at release (depressed strands only): 6√f'ci

D. In order to achieve uniformity and consistency in designing strand patterns, the following parameters apply:

1.) Strand patterns utilizing an odd number of strands per row (a strand located on the centerline of beam) and a minimum side cover (centerline of strand to face of concrete) of 3-inches are required for all AASHTO and Florida Bulb-Tee beam sections except AASHTO Type V and VI beams for which a strand pattern with an even number of strands per row must be utilized.

2.) Distribute debonded strands evenly throughout strand pattern.  Whenever possible, separate debonded strands in all directions by at least one fully bonded strand.

3.) Use “L-shaped” longitudinal bars in the webs and flanges in end zone areas.

4.) The minimum compressive concrete strength at release will be the greater of 4.0 ksi or 0.6 f ’c. Higher release strengths may be used on a case by case basis but must not exceed the lesser of 0.8  f ’c  or 6.0 ksi.

5.) Design and specify prestressed beams to conform to classes and related strengths of concrete as shown in Table 4.1.

6.) When calculating the Service Limit State capacity for pretensioned concrete flat slabs and girders, use the transformed section properties as follows: at strand transfer; for calculation of prestress losses; for live load application.  For precast, pretensioned, normal weight concrete members designed as simply supported beams, use LRFD 5.9.5.3, Approximate Estimate of Time-Dependent Losses. For all other members use LRFD 5.9.5.4 with a 180-day differential between girder concrete casting and placement of the deck concrete.

Commentary: The FDOT cannot practically control, nor require the Contractor to control, the construction sequence and materials for simple span precast, prestressed beams.  To benefit from the use of refined time-dependent analysis, literally every prestressed beam design would have to be re-analyzed using the proper construction times, temperature, humidity, material properties, etc. of both the beam and the yet-to-be-cast composite slab.

7.) Stress and camber calculations for the design of simple span, pretensioned components must be based upon the use of transformed section properties.

8.) When wide-top beams such as bulb-tees and AASHTO Types V and VI beams are used in conjunction with stay-in-place metal forms, evaluate the edges of flanges of those beams to safely and adequately support the self-weight of the forms, concrete, and construction load specified in Section 400 of the FDOT Standard Specifications for Road and Bridge Construction.

9.) The design thickness of the composite slab must be provided from the top of the stay-in-place metal form to the finished slab surface, and the superstructure concrete quantity will not include the concrete required to fill the form flutes.

 

Table 4.1 Concrete Classes and Strengths

Class of Concrete

28-Day Compressive Strength (f’c) KSI

Class III*

5.0

Class IV

5.5

Class V (special)

6.0

Class V

6.5

Class VI

8.5

*Class III concrete may be used only when the superstructure environment is classified as Slightly Aggressive in accordance with the criteria in Chapter 2.

E. The maximum prestressing force from fully bonded strands at the ends of prestressed beams must be limited to the values shown on the Standard Drawings. Do not apply losses to the calculated prestressing force. The minimum length of debonding from the ends of the beams is half the depth of the beam. Do not modify the reinforcing in the ends of the beams shown in the Standard Drawings without the approval of the State Structures Design Engineer.

Commentary:  To minimize horizontal and diagonal web cracks and accommodate the longer bursting force distribution length (h/4) adopted by LRFD in 2002, the maximum bonded prestressing force in the ends of prestressed beams has been limited. The maximum prestressing force is based on 13 ksi bursting steel stress for AASHTO and Florida Bulb-Tee beams, 18 ksi bursting steel stress for Florida U-Beams, and 20 ksi bursting steel stress for inverted-T beams.

F. Provide embedded bearing plates in all prestressed I-Girder beams deeper than 60 inches.  This includes standard AASHTO Type V, VI and Florida Bulb-T prestressed concrete beams and any project specific designs meeting this criteria.

Commentary:  Bearing plates add strength to the ends of the concrete beams to resist the temporary loadings created in the bearing area by the release of prestressing forces and subsequent camber and elastic shortening.

4.3.2 Beam Camber/Build-Up over Beams (01/09) 2009 Jan: Added 4.3.2 C.

A. Unless otherwise required as a design parameter, beam camber for computing the build-up shown on the plans must be based on 120-day old beam concrete.

B. On the build-up detail, show the age of beam concrete used for camber calculations as well as the value of camber due to prestressing minus the dead load deflection of the beam.

C. Consider the effects of horizontal curvature with bridge deck cross slope when determining the minimum buildup over the tip of the inside flange.

Commentary: In the past, the FDOT has experienced significant slab construction problems associated with excessive prestressed, pretensioned beam camber.  The use of straight strand beam designs, higher strength materials permitting longer spans, stage construction, long storage periods, improperly placed dunnage, and construction delays are some of the factors that have contributed to camber growth.  Actual camber at the time of casting the slab equal to 2 to 3 times the initial camber at release is not uncommon.

4.3.3 Florida Bulb-Tee Beams [5.14.1.2.2]

The minimum web thicknesses for Florida Bulb-Tee beams are:

Pretensioned Beams

6½-inches

Post-Tensioned Beams

See Table 4.5