(6) Main major modes of Failure in Beams

What is a failure in beams?

Beams are the structural flexural members which are generally subjected to compression and tension simultaneously. As earthquake forces act on a building, sometimes the reversal of stresses may take place which may lead to failure of beams. This can be better understood by analyzing and designing members for seismic forces. 

Role of beams in RCC buildings

Beams are a composite section, consisting of concrete and steel reinforcement, placed horizontally on masonry load-bearing walls or RCC columns for the effective transferring of a load of slab/floor to the vertical members. Under the action of loads, they act together, with columns, as a frame transferring the forces from one to another and then to the foundations below. 

All the beams in the structure are designed for the worst possible combinations of dead load, live load (superimposed), and earthquake loads. The unexpected seismic loads should be accurately estimated and analyzed as per codal provisions for designing the beams of a given span. Beams in RCC buildings generally have two types of steel reinforcement that define the following given below.

(a) Main longitudinal steel placed along the effective span for resisting the tensile stresses. 

(b) Closed loops in the form of shear stirrups which are used 

• To tie up the longitudinal steel at proper spacing while pouring concrete.
• To give the desired shape to the section.
• The most important function is to resist the shear forces effectively, therefore also called shear reinforcement.
• This prevents brittle shear failure which occurs due to lack of shear reinforcement.

Main steel in the simply supported beam is always· provided at the bottom (tension portion) and in case of the cantilever beam, it is provided at the top portion which is under tension.

All RCC members (beams & columns) and structure (frame) as a whole should be well proportional so that it develops enough resistance at all sections for bending stresses, shear stresses, or a combination of both and other types of failure in concrete beams.

The main identification of damages (failure) in RCC Beams 

1. Bending (Flexural) failure in a beam
2. Shear failure in a beam
3. Diagonal compression failure in a beam
4. Inadequate development length in a beam
5. Inadequate splicing in a beam
6. Change in Nature and magnitude of loads in a beam

1. Bending (Flexural) failure in a beam

Due to the load coming over the simply Supported beam, it sags (deflect). Due to this deflection, top portion compresses and the bottom portion of the beam stretches in tension. As a result, compressive stresses are developed at top and tensile stresses are developed at the bottom. 

Concrete itself is strong under compression to take the compressive stresses but is weak in tension, therefore, steel reinforcement bars are provided at the bottom to take up the tensile stresses. 

The section can be over reinforced (steel more than required) or it can be under reinforced (less steel). Among these two, under reinforced sections are preferred for earthquake resistant point-of-view because in under reinforced sections, steel reaches its peak value of stress e compared to concrete.

Note - Under reinforced sections are always preferred as compare to over reinforced and Steel, reinforced sections.

Steel, being a ductile material, yields giving ample of time before failure. But in case of over reinforced sections. Steel is more therefore concrete reaches its peak value of the stress earlier and concrete being a brittle material, it does not give enough time to escape out during the earthquake.

2. Shear failure in a beam

This type of failure in the beam occurs due to the shearing action of forces acting upon them. Cracks inclined at 45 degrees with the horizontal area generally develops due to the shearing action. Generally, the cracks start from the bottom or middle depth and progress towards the top and bottom faces of the beam. This type of failure takes place when a shear force is large in relation to bending moment in beams.

Shearing action can be prevented by providing suitably designed shear reinforced which can be in the form of mainly three types of shear failure in beams.

• Vertical shear stirrups (also called minimum shear Reinforcement)
• Bent up bars (also called negative Reinforcement)
• Combination of both (a) and (b)

From an earthquake point of view, this type of failure/damage is of great importance as these cracks (damages) occur near the supports leading to the failure joints (of column & beam) This type of shear failure is brittle and must be avoided.

This type of failure can be prevented by placing shear stirrups closely spaced near the supports or providing bent up bars at a distance l/7 to l/4 where l is effective span.

3. Diagonal compression failure in a beam

This type of failure generally takes place in beams when the concrete in the compression portion (top) is crushed due to the presence of point load. As a result, the diagonal crack is formed. These types of cracks start from the top and proceed to the bottom.

Shear reinforcement is essentially provided to prevent such type of damages in beams· Codal provisions should be strictly followed which clearly indicates the maximum allowable shear stress permitted in a beam.

4. Inadequate development length in a beam

Development length in a section can be defined as the minimum length of the bar which should be embedded in concrete beyond any section to develop by sufficient bond, between steel and concrete, a force equal to the tensile force developed in the bar at that section. It is generally expressed in terms of the diameter of the bar. Bond around the steel bar by concrete is necessary so that it does not slip while taking stresses allotted to it, but if the bond length is not embedded properly in the concrete, then chances of slip from the concrete may cause the failure of the member.

5. Inadequate splicing in a beam

Splicing means the overlapping of steel reinforcement bars for joining them to increase their lengths. Steel reinforcement bars are usually available in lengths of 12 m in the market, but casting members of length greater than 12 m will require joining of steel bars effectively by splicing. At the lapping location, the bars transfer huge magnitude of forces from one to another, therefore welding of two bars is done and tied with binding wire which helps in an effective load transfer mechanism. These locations must riot occur near the supports in tension-steel as the bars can yield resulting in collapse. At the location of splicing, the vertical shear stirrups are closely placed so that chances of failure due to large stresses can be avoided. This type of failure will be brittle because steel bars do not reach their maximum stress value due to inadequate lap and slips, therefore concrete fails. 

6. Change in Nature and magnitude of loads in a beam

Beams should be checked for the worst combination of dead load. live load and earthquake loads.

Beams are generally safe against the designed dead load and live loads but due to unexpected earthquake forces acting on them, the combination of loads increases to manifolds to the values for which it was designed for. It becomes mandatory for builders and designers to take care of earthquake loads properly as per guidelines laid down by Indian Standard Seismic Codes.