EMPIRICAL DESIGN METHOD

The traditional design assumes that failure will occur in flexure, however, field and laboratory tests show that the ultimate strength of the concrete deck is controlled by its capacity to resist punching shear. This is the observation for empirical method.

When using the empirical deck design method, take note that it is only applicable to interior bays but not the overhang.

For the previous example:

Materials used are the following:

Slab:                   Normal weight concrete, f'c = 35 MPa

Wearing surface:   Asphalt concrete

Thickness of future wearing surface:         60mm

Thickness of top integral wearing surface: 12mm

Traffic barriers are composite to the deck, and the deck is composite to the steel beams.

Full-depth diaphragms are used at the ends of the span.

There is no skew.

EFFECTIVE LENGTH

Assuming that you are going to use W250x200x67.

Properties are as follows:   depth, h=257mm

                                       width,  w=204mm

                                        web thickness = 8.9mm

             

CHECK DESIGN CONDITIONS

AASHTO 9.7.2.4 specifies the design conditions for empirical deck design method, the designer only needs to check if the project is OK with the following requirements:

1. Cross-frames or diaphragms are used throughout the cross-section at lines of support.
2. For cross-section involving torsionally stiff units, such as individual separated box beams, either intermediate diaphragms between eh boxes are provided at a spacing not to exceed 8000nn, or the need for supplemental reinforcement over the webs to accommodate transverse bending between the box units is investigated and reinforcement is provided if necessary.
3. The supporting components are made of steel and/or concrete.
4. The deck is fully cast-in-place and water cured.
5. The deck is of uniform depth, except for haunches at girder flanges and other local thickening.
6. The ratio of effective length to design depth does not exceed 18.0 and is not less than 6.0.
7. Core depth of the slab is not less than 100mm.
8. The effective length does not exceed 4100mm.
9. The minimum depth of the slab is not less than 175mm, excluding a sacrificial wearing surface where applicable.
10. There is an overhang beyond the centerline of the outside girder of at least 5.0 times the depth of the slab; this condition is satisfied if the overhang is at least 3.0 times the depth of the slab and a structurally continuous concrete barrier is made composite with the overhang.
11. The specified 28-day strength of the deck concrete is not less than 28.0 MPa.
12. The deck is made composite with the supporting structural components.

*A minimum of two shear connectors at 600-mm centers shall be provided in the negative moment region of continuous steel superstructures.

SELECT STEEL REINFORCEMENT

For the empirical deck design method, four layers fo isotropic reinforcement shall be provided according to AASHTO 9.7.2.5.

The minimum amount of reinforcement shall be:

             of steel for each bottom layer

         of steel for each top layer



* Reinforcing steel shall be Grade 420 (Grade 60) or better.
* Spacing of steel shall not exceed 450mm.
* All reinforcement shall be straight bars, except the hooks.

Assuming D25mm reinforcing bars for both bottom longitudinal and transverse directions.
           

          

          

Use 450mm

Assuming D20mm reinforcing bars for both top longitudinal and transverse directions.
       
           

           

Use 450mm