HOME USE OF DOUBLE TEE SPAN LOAD TABLES

ATTENTION:   The Span-Load Tables included herein were derived from computer-calculated data, are intended as aids to preliminary sizing, and must be interpreted using sound engineering judgement.

INTRODUCTION - These Span-Load Tables present three distinct types of Double Tee flexural members: the "standard" thin-flange Double Tee, with and without composite topping (pages 8-21 ); and the thick-flange Double Tee (pages 22-28).

The "standard" thin-flange Double Tee without composite topping is commonly used for lightly loaded applications (not exceeding 80 psf), such as roof structures.

The "standard" thin-flange Double Tee with composite topping, and the thick-flange Double Tee, are used for moderately loaded applications, such as floors for most warehouses, office buildings, schools, shopping malls and parking garages. Cast-in-place concrete topping provides a smooth, level floor surface that serves as the horizontal diaphragm. The alternative untopped thick-flange Double Tee provides a plant-produced wearing surface that exhibits superior durability when compared to typical field-placed concrete topping. In this case, the horizontal diaphragm is provided by welded flange connectors.

Flanges thicker than four inches are also available for heavily loaded surfaces, such as bridges and piers. Contact CTC's Marketing Department for capacities of thicker flanged Double Tees.

DESIGN CRITERIA FOR DEVELOPMENT OF THE SPAN-LOAD TABLES

The Span-Load Tables were developed in accordance with the provisions of the "Building Code Requirements for Reinforced Concrete", ACI 318-89, as described below:

COMPRESSION - The extreme fiber stress in both the Double Tee flange and topping slab under the application of full service load is limited to 0.45f'c [ACI 318-89, Section 18.4.2(a)].

TENSION - The extreme fiber stress in the precompressed tensile zone under the application of full service load is limited to 6c [ACI 318-89, Section 18.4.2(b)]. If service is to be in a corrosive environment (salt water, certain industrial chemicals, etc.), or if full design loads are likely to be sustained for extended periods (such as in warehouses), designers should consider using an allowable tensile stress of less than 6c.

ULTIMATE STRENGTH - The nominal flexural strength, øMn, exceeds the required ultimate strength Mu - 1.4Md + 1.7Ml [ACI 318-89, Section 9.2.1]. The stress in the prestressed reinforcement at nominal strength (fps) was calculated as set forth in ACI 318-89, Section 18.7.2(a), and all superimposed loads were considered as live loads. When ultimate strength governs the design, a more rigorous analysis based on strain compatability will increase the span-load capacity, as will superimposed loads comprised of dead and live load combinations.


SHEAR - The nominal shear strength, øVn, exceeds the required ultimate shear Vu = 1.4Vd + 1.7Vl [ACI 318-89, Sections 9.2.1 and 11.1.1]. Web and flexure shear strengths were calculated as set forth in ACI 318-89, Section 11.4.2. Additional web shear capacity can be obtained by computing the shear force corresponding to dead and live loads that result in a principal tensile stress of 4c at the centroidal axis of the member.

Values above and to the right of the bold type in all Span-Load Tables are based on Double Tees without shear reinforcement. For all of these values, the shear strength of the concrete section equals or exceeds the ultimate shear.

Values in the lower left of the table in bold type represent conditions where shear reinforcement is required. The shear capacity provided by the steel (Vs), does not exceed 8c  bwd [ACI 318-89, Section 11.5.6.8].

Full scale load tests have been performed at Concrete Technology Corporation to verify the shear capacity of Double Tees without web reinforcement. Test results also confirm that, with a rough screeded top flange, full composite action is achieved without the use of mechanical ties to develop the horizontal shear requirements of ACI 318-89, Section 17.5.

Contact CTC's Marketing Department for further information concerning test results or special design conditions.

CONCRETE RELEASE STRENGTH and TOP FLANGE TENSION REINFORCEMENT requirements have been checked for conformance to ACI 318-89, Section 18.4.1, and are reflected in the Span-Load Tables.

CONCRETE STRENGTH varies depending on the total amount of prestress and the length of the Double Tees. All unhighlighted values in the Span-Load Tables use f'ci = 4000 psi at release and f'c - 6000 psi at 28 days. For composite Double Tees, the strength of the topping at 28 days is a minimum 3000 psi. Designs using higher values of f'c for both the Double Tees and topping slab are also possible. Where higher design values are desired, contact CTC's Marketing Department for capacities.

PRESTRESSING STEEL is ½" diameter, 270 ksi, 7 wire, Iow-relaxation strand. Initial jacking stress is 0.7fpu = 189 ksi. Long term losses are estimated at 35 ksi, for a final effective stress of 154 ksi.

HARPED STRAND DESIGNS are calculated for a midpoint harp and a single row of stacked strands (see page 5). For heavily stranded Double Tees, significant improvement in eccentricity may be achieved by using a double row harp design. However, little capacity improvement is achieved by two-point harping at 0.4L, so the single point harp is recommended for economy. Strand sleeving for control of concrete stresses at release is also available. Contact CTC's Marketing Department for economical alternate strand configurations.

HIGHLIGHTING of the values in the Span-Load Tables represent the following requirements:
12
Double Tees in this span range weigh in excess of 54 kips.
234
f'ci is between 4000 - 5000 psi,    f'c = 6000 psi
345
f'ci is greater than 5000 psi,    f'c = 7000 psi
456
Top flange tension reinforcement is required in addition to the flange mesh. For thick-flange Double Tees, this will exceed the minimum reinforcement required for temperature.

COMPOSITE DOUBLE TEE DESIGNS have already considered the weight of the cast-in-place topping. This should not be deducted from the allowable superimposed load. Double Tees which are designed with composite topping will have a rough screeded top to insure good bond to the topping.

STRUCTURAL LIGHTWEIGHT CONCRETE, with unit weights as low as 110 pcf and 28 day strengths of 5000 psi, can be used where reduced dead load can be of advantage. In considering its use, the designer should be aware of the reduced elastic modulus resulting in cambers approximately twice the value of those estimated for normal weight concrete. Large differential cambers often result from large absolute cambers. Variation in camber also results from less precise control of the concrete mix due to the inherent fluctuation of density, gradation, and moisture content in the lightweight aggregate. Contact CTC's Marketing Department for job-specific information.

SPECIAL WIDTHS are available, although it is desirable to maintain the 10' nominal width to minimize cost. If a layout suggests a non-standard width be used, it is most economical to use as many 10' units as possible, and fill the remainder with non-standard widths. The minimum width of a Double Tee is 6'-2".

The Span-Load Tables are calculated for the standard 10' nominal width. Narrower widths will allow heavier loading conditions than are reflected in the tables for a given span. In these cases, special consideration must be given to the prestressing profile, shear and end-bearing reinforcement, and the top flange thickness. Contact CTC's Marketing Department for span-load capacities of special-width Double Tees.

THE FIRE RESISTANCE RATING of Double Tees varies depending on many factors, including but not limited to cover over the prestressing, type and thickness of topping slab and/or roofing, normal or lightweight concrete, etc. In many cases, additional prestressed or mild reinforcement can be added to improve the fire resistance rating. Two excellent design guides are the PCI Design Handbook, and PCI MNL-124-77, "Design for Fire Resistance of Precast Prestressed Concrete".

CAMBER (net upward deflection) due to the eccentricity of the prestressing force should be recognized and accounted for in the design process. Many variables affect camber, including the quality of concrete and strength at release, member length, number and placement of strands, placement of supports for storage, differential temperature, age of member prior to erection and placement of superimposed loads, relative humidity, etc. Associated building elements which may be affected by camber should be placed with adequate tolerances. It is not practical to deflect the formwork to produce desired cambers.

Suggested methods for calculating cambers and deflections are described in the PCI Design Handbook. Contact CTC's Marketing Department with any questions about camber.