Why is the concrete under a freeway over pass designed like an I beam?

Question

Is it for looks, or strength? what is the difference between it being in an I beam shape and just Pouring concrete like a big thick slab?

I assume it is for strength but I dont know why.

Answer 1

Yes it is for the max strength with the least material. Why? Under a pavement load, the top layer takes compression whereas the bottom layer takes tension. The middle layer remains relatively unstressed so it would be a waste to put lot of concrete there.

Concrete is very good at sustaining compression but is relatively poor at taking tension. Steel wires, which have the same thermal expansion coefficient as concrete, are placed in the bottom layer of the I beam to improve the tensile strength of this layer.

For long-spanning beams that are factory pre-casted, the steel wires at the bottom layer are pre-stretched to effectively displace the layer of no stress from the middle towards the bottom serif of the I. Such a steel pre-stretched beam with no load bends upward slightly producing two desired results : (1) putting the top layer concrete at smaller compression load preparing it for a higher payload, and (2) geometrically attenuating the downward displacement ratio per unit of pavement payload thus reducing the vibration amplitude.

Answer 2

If you study beams, you will find they are all quite similar. They have flanges and a web, an I beam. This is because that is the most efficient shape to resist bending and shear.

If you were to just pour a big slab, it would fail because it would collapse under it's own weight. You could make a box girder out of concrete, since it is hollow and lighter.

The thing to learn is that moment, or bending, which maxes out in the middle of the span, is resisted by the flanges. One flange is in compression, the other in tension. The further away the flanges are from each other, the more resistance you have for the same size flanges. Thus is you increase the depth of a beam, it's moment resistance increases geometrically.

Answer 3

The I beam does give the best resistance to bending for the amount of material used. Keep in mind that concrete does not predictably resist tension, but does great with compression. The lower part of the beam is reinforced (usually steel) to resist the tension. Also, it is the web that resists bending not just the flanges. The flanges resist warping, bending side to side, and add strength to the area of the most tensile/compressive forces. Take a wooden ruler, hold it flat and bend it. Now rotate it 90 deg and try to bend it. Notice how much harder it is to bend, yet it wants to buckle and bend from side to side? Put flanges on it and it won't do that as easily and it will be able to resist even greater forces downward.

Answer 4

An I beam shape allows the most strength with the least material.

Answer 5

It is to save money. They could use a big slab but by using the ibeam shape they can use less concrete. Ususally this is "high performance" concrete that has better tensile strength than regular concrete.