Hy-Span | Design

Hy-Span's® are designed as three member rigid frames with hinges supports at the bases of the legs. Haunches are incorporated at the ends of the slab so that all members have varying moments of inertia along their lengths.

Hy-Spans are designed for:

Dead Load

This includes the weight of the (top) slab, earth cover, pavement, sidewalks parapets and fixture wearing surface as applicable. If earth cover is not specified, the dead load considered is:
weight of slab - 3" of asphalt concrete - future wearing surface @ 30 PSF

Live Load

Typically, either AASHTO HS 20 or Alternate Military Loading is used. however, any purchaser specified live load can be used.

Impact

The magnitude of the impact factor decreases with increasing depth of earth cover, with a maximum of 0.3 of the live load.

Lateral Earth Pressure

Usually, active earth pressure is applied to the legs with 2 feet of soil surcharge to account for live load on the surface of the fill.

Loads Imposed During Handling and Transportation

Once the locations of the pickup points are determined, the stresses due to handling can be calculated. Similarly, once the (position) orientation of the Hy-Span® and blocking during transportation are known, the stresses during transportation can be calculated.

Special Loading Conditions

In a usual case, the design loads are: Dead Load, Live Load plus Impact, Earth Pressure

Checking Stresses

After the design is completed, the handling and transportation stresses can be checked. When checking handling stresses, the strength of the concrete (based on its age at the time of handling) must be assessed to determine permissible values.

Horizontal Reaction

The horizontal reaction is provided by friction in the footing key between the Hy-Span® leg and the top of the footing, and by friction between the bottom of the footing and the soil. If this friction is not sufficient to provide the calculated horizontal reaction, the earth pressure will switch from active to passive. If the force associated with the passive earth pressure distribution is large enough, the footing will not slide and the hinged end condition still exists. If the force associated with the passive earth pressure distribution does not exceed the calculated horizontal reaction, the footing will slide. Hence, it will be necessary to account for this and modify the calculated internal forces. (This is equivalent to one support becoming a restrained roller.)

Structural Analysis

Structural analysis of a Hy-Span® can be completed using a classical method (such as Slope-Deflection) modified to account for varying moments of inertia, or using a finite element program (such as PLANHy-Span'E® or STRUDEL). The analysis is performed on a one foot wide strip of Hy-Span® with the live wheel load plus impact applied to this strip determined by dividing the actual live wheel load plus impact by the ASHTO distribution width for a wheel load (for less than 2 feet of earth cover). Note that the live load must be positioned at several locations across the slab to determine the maximum design forces. For bending moment, this usually occurs with the live load at mid-span.

The design phase is in accordance with AASHTO and follows the strength design procedure with the applicable load factors. Items considered are:

flexural reinforcing
shear
shrinkage and temperature reinforcing
distribution reinforcing
crack control
fatigue
immediate live load plus impact deflection

Compressive axial forces are neglected in the design. reinforcing steel is calculated for bending moment only. The effect of the axial force can then be checked to verify that the design is compressive force for the balanced condition. Hence, a tensile failure is to be expected and neglecting the compressive force leads to a conservative design. Applying AASHTO 8.1.4.3, the axial force can be neglected if Pu<.1 F'c Ag




Sanitary Sewer Pipe Storm Sewer Pipe Culverts Elliptical Pipe Jacking Pipe Fittings and Specials Box Culverts Hy-Span Bridges Advantages Suggested Uses Design Jurisdictions Temporary Median Barrier Hy-Span \| Design Hy-Span's® are designed as three member rigid frames with hinges supports at the bases of the legs. Haunches are incorporated at the ends of the slab so that all members have varying moments of inertia along their lengths. Hy-Spans are designed for: Dead Load This includes the weight of the (top) slab, earth cover, pavement, sidewalks parapets and fixture wearing surface as applicable. If earth cover is not specified, the dead load considered is: weight of slab - 3" of asphalt concrete - future wearing surface @ 30 PSF Live Load Typically, either AASHTO HS 20 or Alternate Military Loading is used. however, any purchaser specified live load can be used. Impact The magnitude of the impact factor decreases with increasing depth of earth cover, with a maximum of 0.3 of the live load. Lateral Earth Pressure Usually, active earth pressure is applied to the legs with 2 feet of soil surcharge to account for live load on the surface of the fill. Loads Imposed During Handling and Transportation Once the locations of the pickup points are determined, the stresses due to handling can be calculated. Similarly, once the (position) orientation of the Hy-Span® and blocking during transportation are known, the stresses during transportation can be calculated. Special Loading Conditions In a usual case, the design loads are: Dead Load, Live Load plus Impact, Earth Pressure Checking Stresses After the design is completed, the handling and transportation stresses can be checked. When checking handling stresses, the strength of the concrete (based on its age at the time of handling) must be assessed to determine permissible values. Horizontal Reaction The horizontal reaction is provided by friction in the footing key between the Hy-Span® leg and the top of the footing, and by friction between the bottom of the footing and the soil. If this friction is not sufficient to provide the calculated horizontal reaction, the earth pressure will switch from active to passive. If the force associated with the passive earth pressure distribution is large enough, the footing will not slide and the hinged end condition still exists. If the force associated with the passive earth pressure distribution does not exceed the calculated horizontal reaction, the footing will slide. Hence, it will be necessary to account for this and modify the calculated internal forces. (This is equivalent to one support becoming a restrained roller.) Structural Analysis Structural analysis of a Hy-Span® can be completed using a classical method (such as Slope-Deflection) modified to account for varying moments of inertia, or using a finite element program (such as PLANHy-Span'E® or STRUDEL). The analysis is performed on a one foot wide strip of Hy-Span® with the live wheel load plus impact applied to this strip determined by dividing the actual live wheel load plus impact by the ASHTO distribution width for a wheel load (for less than 2 feet of earth cover). Note that the live load must be positioned at several locations across the slab to determine the maximum design forces. For bending moment, this usually occurs with the live load at mid-span. The design phase is in accordance with AASHTO and follows the strength design procedure with the applicable load factors. Items considered are: flexural reinforcing shear shrinkage and temperature reinforcing distribution reinforcing crack control fatigue immediate live load plus impact deflection Compressive axial forces are neglected in the design. reinforcing steel is calculated for bending moment only. The effect of the axial force can then be checked to verify that the design is compressive force for the balanced condition. Hence, a tensile failure is to be expected and neglecting the compressive force leads to a conservative design. Applying AASHTO 8.1.4.3, the axial force can be neglected if Pu<.1 F'c Ag