Design of Concrete Structure II Sessional

(CE-3103)

A Presentation on

Reinforced Slab Bridge Design(ASD)Prepared by-Shekh Muhsen Uddin Ahmed

Department of Civil Engineering

Submitted to-Md. Rezaul Karim, Ph.D.Associate Professor&Sukanta Kumar ShillAssistant Professor

Dhaka University of Engineering & Technology

Contents

Introduction to Bridge StructuresTypes of Reinforced Concrete BridgeLoads that Concerned with Slab BridgesComponents of Slab BridgeDesign Steps for Slab and Edge Beam

A Short Introduction to Bridge Structures

The first Bridges were made by nature as simple log fallen across a stream. The first bridge made by humans were probably spans of wooden log on planks and eventually stones, using simple support and cross beam arrangement.

Most of these early bridges could not carry heavy weights on withstand strong current. It was these inadequacies which led to the development of better bridges.

Types of Reinforced Concrete Bridge

Reinforced Slab Bridges

Beam and Slab Bridges

For short spans, a solid reinforced concrete slab, generally cast in-situ rather than precast, is the simplest designUp to about 25m span, such voided slabs are more economical than prestressed slabs.

Beam and slab bridges are the most common form of concrete bridge in today. They have the virtue of simplicity, economy, wide availability of the standard sections, and speed of erection.The precast beams are placed on the supporting piers or abutments, usually on rubber bearings which are maintenance free.

Types of Reinforced Concrete Bridge

Arch Bridges

Cable-Stayed Bridges

Arch bridges derive their strength from the fact that vertical loads on the arch generate compressive forces in the arch ring, which is constructed of materials well able to withstand these forces.

For really large spans, one solution is the cable-stayed bridge. As typified by the Dee Crossing where all elements are concrete, the design consists of supporting towers carrying cables which support the bridge from both sides of the tower.

Box Girder Bridges

Types of Reinforced Concrete Bridge

For spans greater than around 45 metres, prestressed concrete box girders are the most common method of concrete bridge construction.

Incrementally launched

The incrementally launched technique creates the bridge section by section, pushing the structure outwards from the abutment towards the pier. The practical limit on span for the technique is around 75m.

Span-by-span

The span-by-span method is used for multi-span viaducts, where the individual span can be up to 60m.These bridges are usually constructed in-situ

Balanced cantileverIn the early 1950's, the German engineer Ulrich Finsterwalder developed a way of erecting prestressed concrete cantilevers segment by segment with each additional unit being prestressed to those already in position.

Integral Bridges

Types of Reinforced Concrete Bridge

One of the difficulties in designing any structure is deciding where to put the joints. These are necessary to allow movement as the structure expands under the heat of the summer sun and contracts during the cold of winter.

Expansion joints in bridges are notoriously prone to leakage. Water laden with road salts can then reach the tops of the piers and the abutments, and this can result in corrosion of all reinforcement. The expansive effects of rust can split concrete apart.

They are constructed with their decks connected directly to the supporting piers and abutments and with no provision in the form of bearings or expansion joints for thermal movement.

Suspension Bridges

Types of Reinforced Concrete Bridge

Concrete plays an important part in the construction of a suspension bridge. There will be massive foundations, usually embedded in the ground, that support the weight and cable anchorages. There will also be the abutments, again probably in mass concrete, providing the vital strength and ability to resist the enormous forces, and in addition, the slender superstructures carrying the upper ends of the supporting cables are also generally made from reinforced concrete.

Live Loads

Loads that Concerned with Slab Bridges

Truck Loading

Other roadway Loading

AASHTO specify two types of truck loadings(HS and H). Highways which may carry heavy truck traffic the minimum live load

shall be HS15-44

Bridge may be required to carry electric railways, railroad freight cars, military vehicles, or other extra ordinary vehicles.

Sidewalk Loading

Sidewalk floors, stringers, and their immediate Supports are usually designed for a live of at least 85 psf of sidewalk area.

Standard H and HS Loading

Impact Load

Loads that Concerned with Slab Bridges

Live load stresses due to truck loading are increased by vibration and sudden application of the load.

Impact Load= Live Load * Impact FractionWhere Impact Fraction, I= ≤ 0.30

Here l = loaded length

Components of Slab Bridge

Sub-Structure Footing Distributes super structures load on soil.

AbutmentActs as a load bearing wall which transfer supper structure load

on footing.

Super Structure Slab Support all kinds of live load and dead load and transfer them

on Abutment

Edge Beam Prevent cracking of slab edge and support stringer load

Slab Bridge

Components of Slab Bridge

Slab

Footing

Abu

tmen

t

Slab

Edge Beam

Fig- Reinforced Cement Concrete Slab Bridge

Stringer

Design Steps for Slab and Edge Beam

a) Slab Design

Span LengthSpan Length , S= Centre to Centre Distance of the Supports ≤ (Clear Distance between support+ Slab Thickness)

Bending Moment

i). Dead Load Moments(DDM)=

ii). Live Load Moments(LLM)

(For main reinforcement parallel to the traffic) When HS20 Loadings: Spans up to and including 50ft ,LLM= 900S ft-lb Spans 50ft to 100ft, LLM= 1000(1.30S-20.0)ft-lb When HS15 Loadings:

LLM =

iii). Impact Load Moments(ILM)

ILM= (Live load Moments * Impact Fraction) ft-lb

Design Steps for Slab and Edge Beama) Slab Design

Design Steps for Slab and Edge Beama) Slab Design

Effective Depth, d

Minimum Permissible Effective depth of slab,

Area of Main Reinforcement, Area of Main Reinforcement,

Design Step for Slab and Edge Beama) Slab Design

Area of Distributed

Reinforcement, Area of Distributed Reinforcement,

Where

Design Steps for Slab and Edge Beamb) Edge Beam Design

Bending Moment

i). Dead Load Moments(DDM)=

ii). Specified Live Load Moments(LLM) =0.10*

(Here we will be only consider Self weight and Stringer Weight)

Design Step for Slab and Edge Beamb) Edge Beam Design

Area of Tensile Reinforcement, Area of Tensile Reinforcement,

Typical RCC Slab Bridge Example

Reference

1. Auther H. Nilson,2. http://www.cbdg.org.uk/intro3.asp

Thanks To

Everyone