PT Slab sample details (Download a zipped set - 13 sheets; Microstation ".dgn" format)

Description

The Kansas Department of Transportation has developed standards for the design and construction of post-tensioned concrete haunched slab bridges. KDOT currently has standards for reinforced concrete haunched slabs from 12 m (39 ft) to 22 m (72 ft) center span range. Post-tensioned concrete haunched slabs standards will extend the span range capabilities of the shallow-depth slabs from 20 m (66 ft) to 28 m (92 ft), in addition to providing a high performance type bridge superstructure.

General Design

The slab bottom (soffit) has a parabolic profile with minimum depth at mid-span and maximum depth at pier. Longitudinal post-tensioning tendons are draped in a parabolic profile, which provide primary resistance to the loads. A nominal prestressing is provided in the transverse direction using straight tendons for distribution of loads and confinement. Nominal mild steel reinforcing is provided in both directions, at the top and bottom of slab, for temperature and shrinkage as well as to assist in the placement of the post-tension tendons.

Spans and Roadways

Four (4) span arrangements will be available in a continuous three (3) span configuration.

Design Loads

Dead Loads:

Concrete Dead Load = 22.8 kN / m3 ( 145 pcf)

Traffic Barrier Load = 4.0 kN / m ( 275 plf ) per rail

Future Wearing Surface = 1.2 kPa ( 25 psf )

Live Loads:

Live load design is accompanied by a corresponding allowable concrete tensile stress limitation.

3 * f'c^0.5 maximum tension for HL-93 Loads (LRFD).

Load Rating:

The goal of design load rating for the given span ranges, governed by the standard HS-20 Truck among all seven (7) Kansas load rating trucks, is 1.10.

The design of "larger" span groups C and D spans results in a minimum 1.10 rating for the HS-20 truck.

The design of "smaller" span groups A and B spans results in a minimum 1.10 rating for the HS-20 Truck for maximum positive and negative moments, and a minimum 1.05 rating for negative moment near mid-span of the exterior spans.

Load rating table shown on plans complies with HS-20 for Operating, Inventory and H.E.T. for operating.

Skews

An example of a 30 degrees skewed structure is available for download as reference material. Below is a partial list of considerations for skewed post-tensioned haunched slab bridges.

• Longitudinal post-tensioning details are expected to be substantially unchanged.
• Transverse post-tensioning details will require modifications to accommodate skew and potential conflict with rail post reinforcement.
• Anchor end recess will also require special details to accommodate the skew.

Multi-span Applications

The current standards for post-tensioned concrete haunched slabs are limited to three (3) continuous span arrangement. Unlike the standards for the conventional reinforced concrete haunched slabs, post-tensioned concrete slab design cannot be extended to multi-span application in a straightforward manner. The additional length of tendons results in increased loss of prestress, which may require additional prestress. KDOT may, in the future, develop standards for multi-span applications. In any event, the maximum total length of post-tensioned concrete slab bridges, without introducing a strand splice or interior span joints, is expected to be about 180 m ( 600' ).

Availability

The standards are available in three components:

1. The user's Manual – PDF file format
2. Plan standards – Microstation ".dgn" drawing file format
3. Project Special Provisions ( Construction specifications for post-tensioned concrete haunched slab bridges ) – Microsoft Word file format

Contact: 
Kansas Department of Transportation
Bureau of Design - State Bridge Office
Dwight D. Eisenhower State Office Building
700 SW Harrison Street
Topeka, KS 66603-3754
Phone: 785-296-3761
Attention: Mr. Loren Risch, PE – Bureau Chief
or Mr. Terry Fleck , PE – Assistant Bureau Chief

PB Booker Associates Inc. ( Consultant )
225 N. Market, Suite 350
Wichita, Kansas 67202
Phone: 316-263-6121
Attention: Mr. Raja Govindaswamy, P.E. – Engineering Manager