rail overpass - stress bed beams

Pre-Stressed Concrete

NT Alice Springs Stuart Highway Rail Overpass

THURSDAY 4TH DECEMBER – 1:00 PM

By Jad Daet

2Agenda

What Is Pre-Stressed Concrete?

How Does A Pre-Stressed Concrete Beam Work?

Example Model Construction of Pre-Stressed Concrete

Stressing & De-Stressing Procedure

Q & A

Stress Bed Constructed At The Overpass

3What is Pre-Stressed Concrete?

Method for Overcoming Concrete’s Natural Weakness

Accomplished In Three Possible Ways:

1. Pre-Tensioned Concrete

2. Bonded Post-Tensioned Concrete

3. Unbonded Post-Tensioned Concrete

4How Does A Pre-Stressed Concrete Beam Work?

5How Does A Pre-Stressed Concrete Beam Work?

6

Picture of Stop End

Strands contain massive amounts of force, Overpass strands contain either 147kN or 187kN of force

Strands are stretched, concrete poured around strands

Once specified compressive strength is reached, strands are then gradually released one by one. Thereby transferring force from strands into the concrete. Thus Compression into the beam.

Notice the strands at the bottom of the beam, reason being is, not only do we wish to apply compression, but we want to apply a bending moment, because the overall position of the strands are eccentric, ie removed from the centre of the beam, that creates the bending moment which helps to counteract the loads on the beam itself.

Once construction is complete of the pre-stressed concrete beam, you will notice that the beam itself will result in the beam cambering up several milimetres.

Beneficial for two reasons, deflection checks for structural steel/concrete calculations. The other is for the aesthetics, as we do not want to see visual sagging of the beam itself once the load is applied.

How Does A Pre-Stressed Concrete Beam Work?

7How Does A Pre-Stressed Concrete Beam Work?

8Model Construction of Pre-Stressed Concrete

Step 1 Step 2

9Model Construction of Pre-Stressed Concrete

Step 3 Step 4

10Model Construction of Pre-Stressed Concrete

Step 5 Step 6

11Model Construction of Pre-Stressed Concrete

Step 7 Step 8

12Model Construction of Pre-Stressed Concrete

Step 9 Step 10

13Stressing & De-Stressing Procedure

De-Stress Order:De-Stress Is Performed In Reverse To Stress Order, A0, A1, B0, B1 etc.

Stressing Order:D4, D7, C4, C7, B4, B7, D3, D8, C3, C8, B3, B8, D2, D9, C2, C9, B2, B9, D1, D0, B1, B0, A1, A0

14

Beams are intended to carry flexural and shear forces, however the flexurewhich is carried by the beam results in tension in at least one face of thebeam and as we know, concrete is not strong in tension. Thus pre-stressconcrete methodologies are required.

Therefore, in pre-stressed concrete design, we apply compression toovercome the tensile stress on the beam due to flexure.

Summary

15Q&A