A load-bearing structure of a reinforced concrete building with an area of 100 sq.m. consists of 24.5 cubic meters of reinforced concrete. (Excluding the bases)
Each cubic meter of concrete weighs 2450 kg.
The 24.5 cubic meters of the carrier body weigh 60025 kg
Each cubic meter of concrete is reinforced with 140 kg of steel.
The 24.5 cubic meters of the bearing body carry 3430 kg of reinforcement.
A prestressing steel with cross section Φ / 40mm has lifting strengths of 120000 kg.
That is, a pre-tensioned tendon Φ / 20mm has the ability to lift in the air a load-bearing body with an area of 100 sq.m.
The question that arises is why do we arm the constructions with thousands of kilos of steel reinforcement, increasing the cost of the construction excessively and under strong seismic excitation we also have serious failures?
I will try to answer this question for the reasons that cause this waste of steel reinforcement which raises the cost of construction.
Each material is characterized by its strengths when forces act on it.
The forces, depending on the direction they have, are characterized as tensile forces, compressive, coronary, shear and bending forces.
Every building material such as concrete and steel react with different strengths to these forces.
Concrete has excellent resistance to compressive forces, while it has minimal resistance to tensile forces, shear force and all other forces.
For example, concrete is 12 times more resistant to compressive strength than it is to tensile strength.
Steel withstands all forces, but what makes it necessary in reinforced concrete structures is its excellent resistance to tensile forces.
So here we have two materials, concrete and steel, where the first withstands compressive forces and the second with tensile forces and together they are reinforced concrete.
The cooperation between concrete and steel is achieved with relevance.
With the term relevance. defines the combined action of the mechanisms that prevent the relative slippage between the reinforcement bars and the concrete that surrounds them.
The individual mechanisms of relevance are adhesion, friction and, in the case of ribbed steel bars, the resistance of the concrete trapped between the ribs.
During the rocking of the construction, the trunk of the lever arm of the column bends.
During bending, one side receives compressive forces, and the other tensile forces. Its compressive forces are received by the concrete and the tensile strength from the steel.
However, the steel does not receive the tension on its own, but also needs the cooperation of the concrete through the mechanism of relevance.
And here the problems begin.
The combined action of relevance mechanisms is considered equivalent, with the development of shear stresses, in the concrete and steel interface. When these stresses reach their limit value, relevance is destroyed, in the form of concrete failure, along the bars and detachment of the steel bars.
In short, the concrete has no shear strength and fails, with the result that the ultra-tensile strength of steel is canceled.
This is a serious problem of relevance and is created by the tensile strength of the steel, which turns the failure into a shear form, which is extremely brittle because the concrete is unable to pick it up.
Conclusion.
Using the relevance mechanism, the tensile specifications of the steel are canceled when the concrete breaks and destroys, caused by the development of shear stresses at the interface of the two materials.
Are there solutions to improve this problem?
Yes, there are.
1) The increase of the failure limit value is achieved by increasing the strength of the concrete.
2) The presence of transverse reinforcement acts favorably by limiting the opening of the developing cracks.
3) Partly the reduction of stresses is achieved by increasing the coating concrete and reducing the diameter of the reinforcement bars.
4) However, it is best to use the relevance mechanism for the secondary reinforcement of concrete and as the main reinforcement of the columns to use the precast steel co-operation mechanism of the prestressing in which the shear stresses do not exist, so they do not cancel the specifications of steel in tensile
My opinion.
Elasticity has limits then it is lost. In large earthquakes elasticity does not work. Plasticity serves for a greater response. But dynamics do the job in an earthquake. The dynamics increase only when we combine construction and ground + loads in the cross sections of the structural vertical elements of the bearing organism.
Plasticity is the tool of the science of seismic structures made of reinforced concrete, with which manages better the inability to dynamically control the inelastic deformations and failures of structures under strong seismic excitation. Three factors cause structural failure.
The bending, the overturning moment of the walls, and the problematic mechanism of concrete and steel cooperation, that of relevance.
We stop the bending of the vertical elements by applying compressive force to their cross section.
By the same method, we increase the response to the base shear.
The overturning moment of the wall is prevented when we join all its ends with the foundation ground.
And in this post issue, what we need to do, to eliminate the problems of the relevance mechanism.
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