What is a less reinforced beam and what are its failure characteristics

As soon as cracks appear in the concrete in the tensile area of the broken beam with less reinforcement, the reinforcement will be reached quicklyYield strength, through the flow amplitude section into the strengthening section. A very wide crack often appears in the destructionDeflectionIt's too big to work properly.

The failure characteristics of the less-reinforced beam are: once the concrete in the tension zone is cracked, the tensile reinforcement will yield and quickly enter the strengthening stage through the entire flow amplitude. This kind of damage is very sudden, it is brittle failure, and it is not allowed to be used in bridge engineering.

Once the beam head is cracked, the tensile steel bar immediately reaches the yield strength, and sometimes quickly enters the strengthening stage, and the crack is too wide, which marks the failure.

Despite the cracking of the rear beam.

It is still possible to retain a certain bearing capacity, but the beam has undergone severe cracking and sagging, and this part of the strength is practically unusable. The failure of the less reinforced beam is also a brittle failure. Less reinforced beams are uneconomical and unsafe, so in the building structure.

It is also not allowed to be adopted.

Other types. 1) The stress of the tensile steel bar first reaches the yield strength.

2) After the concrete in the compression zone reaches the limit, the compressive strain is crushed.

3) There are obvious main cracks and large deflection on the members before failure.

4) The tensile reinforcement does not reach the yield strength.

5) The compressed concrete first reaches the ultimate compressive strain and is crushed.

6) The number of cracks is large, the width is fine, and the deflection is relatively small.

When the longitudinal reinforcement ratio is too low, once the beam is cracked, the longitudinal reinforcement will yield, and even enter the strengthening stage, the bearing capacity of the beam is equivalent to the plain concrete beam of the same section, the beam is destroyed because the reinforcement ratio is too low, the failure process is short, the ductility is poor, it is called the less reinforcement failure, and the corresponding beam is called the less reinforced beam.

The cross-sectional size of the less reinforced beam is too large, so it is not economical, and it is a brittle failure, so the use of less reinforced beam and super-reinforced beam should be avoided in the actual project.

Failure process: under the load of the beam, once the concrete in the tension zone is cracked, the tensile force borne by the concrete of the crack section is almost all transferred to the tension reinforcement, because the longitudinal reinforcement is too little, the reinforcement immediately enters the yield stage, and even is pulled off, while the concrete in the compression area has not been crushed.

Failure characteristics: once the tension area cracks the beam, the damage is very sudden, it is a brittle failure, and the less reinforced beam is not allowed in the project.

When the reinforcement ratio of the component is lower than a certain value, the component not only has a low bearing capacity, but also as long as it cracks, the crack will be sharply developed and the tensile force at the section is all borne by the reinforcement, and the reinforcement yields due to the sudden increase in stress, and the component immediately undergoes failure, which is called the failure of less reinforcement. The destruction of less rib has brittle properties, there is no obvious sign before the failure, the consequences are serious, and the material is not fully utilized.

The failure process of a well-reinforced beam is divided into three stages: elastic stage, crack work stage, and failure stage. Resiliency Phase:

The upper part of the positive section is compressed and the lower part is tensile and the stress pattern is linearly distributed, and the tensile stress of the concrete at the lower edge has not reached the limit of concrete tensile strength, and there are no cracks. The deformation can be restored after the load is removed; With the increase of load, the neutral axis of the cross-section moves upward, the area of the compression zone decreases, and the plastic deformation begins, and the stress distribution no longer has a linear law, and slowly changes from a triangle to an approximate rectangle, and the cracks in the lower part of the beam are gradually developed until the crack width exceeds the allowable stage; Destruction Phase:

From the time the crack width exceeds the allowable width, the loading continues until the upper concrete is crushed, and at the same time, the lower reinforcement reaches the yield and loses the bearing capacity. There are two critical points in these three stages: cracks in the concrete in the tension zone between the first and second stages, and cracks in the second and third stages are when the cracks develop beyond the allowable width.

Therefore, the calculation of the limit of the flexural bearing capacity of the positive section is based on the end of the second stage. In the calculation of the normal serviceability limit state of the positive cut-off sensitive sock surface, the crack is not allowed to appear in the first stage, which is based on the first stage; The permissible occurrence of permissible cracks in the serviceability limit state calculation is based on the beginning of the second stage;

In the rare occurrence of **, there are signs before the failure of the suitable reinforcement beam, which is the ductile failure accepted by the seismic structure.

The failure of the reinforcement beam is that the tensile and compressive steel bar first reaches the yield strength, and then the concrete in the compression area is crushed at the same time, which belongs to plastic failure. Whereas, both low-rib and over-reinforced beams are brittle failures.

When the reinforcement beam is overloaded and failed, the concrete in the compression area is crushed at the same time, and the stress of the longitudinal tensile steel bar basically reaches yield, which is characterized by a precursor and belongs to ductile failure. When the over-reinforced beam is overloaded, the concrete in the compression zone is first instantly fragmented and unstable, which is a brittle failure.

Suitable reinforcement beams belong to plastic failure, while less reinforcement and super-reinforcement are brittle failures and should not be used.

Over-reinforcement is when the reinforcement exceeds the allowable reinforcement ratio, so that the concrete has been crushed before the reinforcement reaches the design strength.

Less reinforcement is that the cross-sectional height does not meet the requirements, and the stressed reinforcement is too few, and these two kinds of damage are sudden, and there is no precursor to failure. Suitable reinforcement is in the state between the excess reinforcement and the less reinforcement, when the reinforcement is in the state of failure, the concrete also begins to develop cracks, from the force exceeds the standard to destroy the complete structure is completely broken, there is a process of development and change.

Over-reinforcement refers to the fact that the reinforcement exceeds the allowable reinforcement ratio, so that the concrete has been crushed before the reinforcement reaches the design strength.

Less reinforcement is that the cross-sectional height does not meet the requirements, and the stressed reinforcement is too few, and these two kinds of damage are sudden, and there is no precursor to failure. Suitable reinforcement is in the state between the excess reinforcement and the less reinforcement, when the reinforcement is in the state of failure, the concrete also begins to develop cracks, from the force exceeds the standard to destroy the complete structure is completely broken, there is a process of development and change.

1. Failure of less reinforced beams: the longitudinal reinforcement rate is too low, once the beam is cracked, the longitudinal steel bar will yield, and even enter the strengthening stage, the bearing capacity of the beam is equivalent to the plain concrete of the same section, the failure process is short, the ductility is poor, and it is a sudden change.

How to prevent the blocking: the reinforcement amount of the component is required not to be lower than the minimum reinforcement ratio.

2. Failure of over-reinforced beams: the longitudinal reinforcement ratio is too high, the longitudinal reinforcement has not yielded, the concrete in the compression area is crushed, the beam is destroyed because the concrete is crushed, the failure process is short, the ductility is poor, and the failure has obvious brittleness, which is a sudden change.

How to prevent: The relative compression zone height of the component section is required not to exceed its relative limit compression zone height.

1) Less-tendon failure pattern.

The component will break down without warning, which is "brittle failure". Failure to make full use of the compressive strength of concrete.

2) Failure form of suitable reinforcement.

The tensile steel bar yields first, and the concrete in the compression area is crushed later, and there is an obvious warning before the failure - cracks and deformation develop sharply, which is "plastic failure".

3) Super-rib failure patterns.

The concrete in the compression area is crushed first, the steel bar does not yield, and there is no obvious warning before the failure, which is "brittle slag failure". The tensile strength of the rebar is not fully utilized.

The suitable reinforcement beam is calculated according to the load force of the suspect, when the force is damaged, the force is transmitted to the concrete and then to the steel bar, and the force is evenly stressed together, and then the concrete is broken, the steel bar is bent and deformed, and the whole beam is also deformed.

The over-reinforced beam is a steel bar greater than the force, and the steel bar can withstand the damage force when damaged, while the concrete is not good and cracks.

Less reinforcement beam is not calculated according to the load force, most of the self-built houses in the countryside have such a problem, the destructive force destroys the steel bar, concrete, concrete cracks, steel bending.

The failure characteristics of the well-reinforced beam are ().

a.The tensile reinforcement yields first, and then the concrete in the compression zone is crushed.

b.The concrete in the compression zone is first crushed, and then the tensile reinforcement yields.

c.When the tensile steel bar yields, the concrete in this area is crushed by the silver at the same time.

d.The tensile reinforcement yields first, and then the concrete is pulled off.

Correct answer: The tensile steel bar yields first, and then the compression zone is crushed.

Hello dear, glad to answer for you; Answer: the bearing capacity of the less-reinforced beam when it fails is smaller than the theoretical value because the suitable reinforcement beam and the super-reinforced beam are ultimately the concrete crushing in the base area resulting in the failure of the component, the difference between them is whether the reinforcement in the pull area yields during the failure, and the super-reinforced beam is difficult to achieve the yield strength due to the more reinforcement bars in the configuration, and it is very sudden when it is destroyed, and it is not as large as the reinforcement beam. Therefore, the final bearing capacity of the well-reinforced beam and the over-reinforced beam is only related to the strength of the concrete, and the preparation skin is theoretically the same under the condition of excluding other factors.

The failure of the less-reinforced beam is due to the yield of the erection reinforcement in the tensile area, and the crack expands upward, and the concrete of the nipple zone with his imitation branch difference does not fully play a role, but because of the crack expansion, the neutralization axis continues to move upward, resulting in the tension of the component, and its bearing capacity should be the smallest.

Characteristics of shear damage of non-abdominal rib beams:

In the failure form of lifting reeds, once the oblique crack appears, the critical oblique crack is quickly formed, and it is quickly extended to the point of concentrated load at the top of the beam, until the whole section is cracked, and the beam is broken by the oblique pull into two parts.

Shear failure, in which vertical cracks and several microscopic oblique cracks appear first.

Oblique compressive failure: in this failure form, a plurality of roughly parallel oblique cracks first appear in the beam belly part close to the bearing, and the beam belly is divided into several inclined compression columns, and with the increase of load, the excessive main compressive stress crushes the beam belly concrete and destroys.

There are characteristics of shear damage to the abdominal tendon beam with a smile:

Cable-stayed failure, when >3, and the number of stirrup configurations is too small, cable-stayed failure will occur. The failure characteristics are the same as those of the non-abdominal rib beam, and the stirrups are pulled off when they are broken.

Baroclinic failure, where the shear span is relatively small or the number of stirrups is too large, will occur, which is characterized by the fact that the concrete diagonal column is crushed, but the stirrups do not yield.

Shear compression failure occurs when the number of stirrups is appropriate and the shear-span ratio is between the shear-span ratio of the cable-stayed failure and the diagonal compression failure (1< <3). It is characterized by the tensile yield of stirrups, the crushing of concrete in the shear zone, and the bearing capacity of the oblique section increases with the increase of stirrup ratio and stirrup strength.

The oblique section of the beam without abdominal reinforcement is subject to shear failure, and there are three main failure modes:

1. Cable-stayed failure: In this failure form, once the oblique crack appears, it will quickly form a critical oblique crack, and quickly extend to the concentrated load application point at the top of the beam until the entire section is cracked, and the beam is broken by the oblique pull into two parts, as shown in Figure 5-2-2aIt is characterized by the rapid and sudden failure process of the whole failure process, and the failure load does not increase much compared with the load when the oblique crack appears.

Its failure is similar to that of a regular section less reinforced beam, and this failure is called cable-stayed failure.

2. Shear compression failure: in this failure form, vertical cracks and several fine oblique cracks appear first, when the load increases to a certain extent, one of them forms a critical oblique crack, although this critical oblique crack extends to the oblique upward, but still retains a certain concrete section of the shear pressure zone without cracking, until the concrete of the nipple zone at the top of the oblique crack is crushed and destroyed under the combined action of shear stress and compressive stress, as shown in Figure 5-2-2bIt is characterized by a relatively slow failure process, and the failure load is significantly higher than that of the oblique crack.

3. Baroclinic compression failure: in this failure form, a number of roughly parallel oblique cracks first appear in the beam belly part close to the bearing, and the beam belly is divided into several inclined compression columns, and with the increase of load, the excessive main compressive stress crushes the beam belly concrete and destroys, as shown in Figure 5-2-2c

The above three main failure modes, considering the shear bearing capacity, have the lowest oblique tensile failure, higher shear failure and highest oblique failure for the same component. However, in terms of its failure nature, because the mid-span deflection of the beam is not large when the three failure conditions reach the failure, and all of them are due to the failure caused by concrete, they all belong to brittle failure without warning.

The shear failure mode of the oblique section of the abdominal rib beam.

Similar to the beam without abdominal reinforcement, there are three main shear failure modes of the oblique section of the beam with abdominal reinforcement: oblique compressive failure, shear compression failure and cable-stayed failure.

1. Cable-stayed failure: When >3, and the number of stirrups is too small, cable-stayed failure will occur. The failure characteristics are the same as those of the non-abdominal rib beam, and the stirrups are pulled off when they are broken.

2. Baroclinic failure: If the shear span is relatively small or the number of stirrups is too large, baroclinic failure will occur. It is characterized by the fact that the concrete diagonal column is crushed, but the stirrups do not yield.

3. Shear failure: the number of stirrups is appropriate and the shear-span ratio is between the shear-span ratio of the oblique tensile failure and the diagonal compression failure (1