Failures and solutions in seismic design.
Author Ioannis Lymperis
In an earthquake, the displacement of the ground creates an opposite inertia force, the magnitude of which depends on the weight of the mass and the acceleration of the structure.
The construction is deformed and creates failures up to collapse. If we stop the deformation of the construction we will stop the damages since these two factors are directly related. To stop the deformation there is no other solution than to impose reaction forces, that is, forces in the opposite direction from these forces of inertia in order for a balance of forces to occur and the deformation to stop.
Today the science of civil engineering makes two big mistakes.
The first mistake is that it tries to create opposite balancing forces with the small cross sections of the beams and columns and walls. In small earthquakes it succeeds, but in large earthquakes with duration it has a problem. The problem they face is that as they increase the cross sections and reinforcement to increase the response of the structure to the earthquake, so do the mass of the structure which creates greater inertia intensities. The higher the height of the structure, the greater the inertia and strain of the cross-sections around the nodes. The result is to increase the cross sections of the reinforcement and at the same time the cost of the constructions without achieving a result of static adequacy in high acceleration and duration of earthquakes.
The fact that the structures do not collapse is due to the fact that these earthquakes do not happen often.
The second mistake they make in design is that they put together two materials that one is in favor of tensile strength and the other is in favor of compressive strength. I am referring to steel and concrete. Steel has excellent tensile strength and concrete has high compressive strength. They place them to work together using the mechanism of relevance. This mechanism has a problem because it forces the concrete to receive shear forces. Concrete does not withstand shear, and the correlation mechanism forces it to absorb large shear forces, which are created at the interface of the coating concrete and steel. The result is a breakdown failure in the overlay concrete, the loss of concrete-steel cooperation, and the cancellation of the use of steel to obtain tensile so that the structure collapses.
This problem grows, because the mechanism of relevance, among other things, turns the wall into a huge lever which lowers high torques at the base.
This means that it creates a critical failure area,
that is, an area in the trunk of the wall near the base where the tensions multiply and create a potential difference in traction.
The result is catastrophic, because the potential difference in adhesion below the critical failure area is smaller, so that the reinforcement can be easily extracted through the concrete.
And the construction collapses.
Consider that a construction of a finished floor of 100 sq.m. weighs 100 tons and has 3500 kg of reinforcement inside. A single reinforcing steel of the 40 mm diameter construction lifts 120 tons before failing. The problem that 3500 kg of reinforcement is not enough to keep the construction upright in a big earthquake is due to what I mentioned above.
When you identify the exact problem you can find the solution and I did this to save the structures from collapse in the great earthquake with duration.
I will tell you the solution to the above problems, which reduces the steel reinforcement and increases the dynamics of reinforced concrete structures.
1) Civil engineers design so that the cross sections of the bearing create the required equilibrium of equilibrium forces.
This is incorrect. The right thing to do is to drive the inertia forces out of the structure before they are directed to the cross-sections, and send them into the ground.
2) Response forces should not come from the cross sections of the load-bearing elements, but from external forces which are transferred to the structure to balance the inertia forces and prevent deformations and failures.
3) The response forces must not have mass (kg) so as not to increase the inertia intensities.
4) Steel should only be subjected to tensile strength (in which it has super strengths), and concrete only compressive forces (where it has ultra strengths.)
5) We must exclude any shear failure resulting from either bending, tensile, or due to the ultra-tensile strength of steel which turns the failure into a "shear shape" in the mechanism (of relevance.)
6) We must take soil samples in each construction to know what soil the construction is based on, what risks it hides and if we need to strengthen it.
7) We must create seismic energy damping mechanisms and systems that prevent the coordination mechanism. That is, to prevent the the same period of soil and construction which leads to the increase of displacement and deformation of the elements to infinity, within the time duration
How do we achieve all this by reducing the cost of construction?
Let's start with the critical failure area. When you bend a pillar, one side is stretched and the other is compressed.
Tensile is the intense state in which a body exerts opposite forces on a body that tend to lengthen it.
The force of compression is the intense state in which opposite forces are exerted on a body that tend to compress it.
These forces of compression and tension have something in common and that is that they have the opposite direction.
Opposite direction means that the forces either meet somewhere or somewhere separate their direction. The point where forces meet for compression and the point that separates their upward and downward direction for tension is called the "critical failure zone". In the critical failure zone the forces have their maximum value and that is the reason the column cross-section fails in this "critical failure area" A wall or wall is more difficult to bend than a square-cross-section column. The critical failure area in the elastic column is created by the bending of its trunk, while in the rigid wall it is created by shear. Shear exists where there is a tensile failure and always has a direction intersecting with the direction of the tensile axis.
How do I remove the critical failure area from bending or shear?
That is, I eliminate bending and shear.
In order for there to be a critical area of failure due to bending and shear, there must be tension. Without tension, none of this exists. How do we remove tensile? We remove the tension by applying opposite compressive forces to compress the cross section and create a balance of forces. In this way a balance of forces occurs and the tension is eliminated and with it the bending, the shear failure and the critical area.
This method is called prestressing and is not my invention.
Force that intersects its cross section near the base
The acceleration of the earthquake and the reaction of the mass of the structure in the opposite direction create the inertia equal to the mass in kilograms on the acceleration. The inertia and the force that intersects the cross section near the base are the same. This force is the one that cuts in two a column of books that we are going to move abruptly. The same force cuts the wall at a sharp acceleration of the earthquake.
If we put a force up and down with our hands the column of books will not be cut in two. The same goes for the wall. If we apply compression to its cross section we will not fail This is called prestressing and it is not my invention.
Shear failure of the coating concrete due to the ultra-tensile strength of the steel.
The cooperation between concrete and steel is achieved through the mechanism of relevance. The term relevance defines the combined action of the mechanisms that prevent the relative slippage between the bars of the reinforcement and the concrete that surrounds them. The individual mechanisms of relevance are the adhesion, the friction and, in the case of ribbed steel bars, the resistance of the concrete which is trapped between the ribs. The combined action of these mechanisms is considered equivalent to the development of shear stresses in the concrete and steel interface. When these stresses reach their limit value, the relevance is destroyed, and the coating concrete along the bars is destroyed and the steel bars are detached.
Basically this failure problem arises because there must normally be an equivalent balance of forces between the two materials which does not exist. When the steel is stretched the concrete tries to hold it. However, the ultra-tensile strength of steel and the low shear strength of overlapping concrete are incompatible. For this reason, in an earthquake, the overlapping concrete breaks and the cooperation of the two materials ceases with catastrophic results. For this reason you will never see in the rubble of earthquakes even an iron cut.
Is there a solution to this problem?
Yes there is. We use the mechanism of relevance as a secondary reinforcement, and as a primary reinforcement we use prestressing. That is, we impose compression on the cross section of the wall, with prestressed tendons without relevance, which the concrete can withstand just fine and we eliminate the failure from shear. This is the reason that while we use 3500 kg of reinforcement in a construction of 100 sq.m. (because concrete does not withstand shear)
So far I have explained to you that with the pre-tensioning method we quantitatively eliminate the failure from the cutting base, the shear failure, how we eliminate the bending and the critical failure area where the greatest intensities are concentrated.
How do we eliminate the potential difference around the critical failure area?
Relevance shows a failure mechanism always close to the trunk of the wall near the base. The forces of relevance from the critical area and below are smaller than those that have the opposite direction and act from the critical area upwards when attracted.
A potential difference is created and the steel is extracted even more easily from the bottom of the critical area. In the pre-tension there is no potential difference since there is no critical separation area in the direction of the intensities.
How do I remove the coordination.
When the construction ground period coincides to be the same, coordination occurs.
During the coordination of ground construction, each displacement of the nodes of the highest level of the structure grows more and more towards infinity, with the result that within the seismic duration the structure is destroyed. So far this problem is unsolvable.
I solved it by applying prestressing to the cross sections to control the deformation due to bending, and anchoring the prestressing tendon to the foundation ground using deep ground anchorages, to stop the construction ground coordination. That is, to control the displacement of the nodes of the highest level with an external force coming from the ground. By controlling the deformation of the wall trunk through the pre-tensioning mechanism and the rigid walls, and controlling the overturning torque of the wall by anchoring it to the ground on all its sides, I ensured the control of the construction ground coordination, preventing torque transfer to the joints. in this way I ensured the control of the deformations of the bearing organism. And we know that without inelastic deformation there would be no failures since by controlling the deformation you also control the failures.
With the method of designing, anchoring the nodes of the highest level with the ground, I hope to deflect the lateral inertial intensities of the earthquake in the ground, thus removing large tensions and failures over the load-bearing body of the building while ensuring a stronger capacity. of the foundation soil. With the appropriate sizing design of the walls and their placement in appropriate places, I also prevent torsional buckling that occurs in asymmetric and metal high-rise structures.
How to remove the wall lever that lowers high torques at the base?
Without torque in the trunk of the wall and without tension, there is no lever arm since the forces of torque are directed into the ground. Without tensile there is no bending deformation or shear failure. And where do these tensions lead? They are driven into the ground.
The design method I use consists of strong ground anchors which, after first being firmly anchored to the ground, transfer with unrelated tendons the anchoring force on all sides of the walls in order to create forces in response to the forces of inertia and deflect them in. to the ground excluding their transport over the cross sections.
Basically, the anchoring of the sides of the walls to the ground prevents the wall from overturning and the transfer of torques to the cross-sections around the nodes, by shifting these forces into the ground and if we apply pre-tension with the same tendons at the cross-section of the walls we prevent bending. of their trunk the shear failure of the overlapping concrete and the cross-sections, and we quantify the response to the cutting base. The drilling of holes for the placement of our anchors shows the composition of the soil as well as the dangers it hides. The expandable anchoring mechanism together with the concrete filling grout of the drilling, provide strong support in the construction even on soft ground.
https://www.researchgate.net/publica...seismic_design