Identification of Cracks

1) What is the mode of failure or damage of the beams in the figure above? Would you characterize the case as damage or as a failure? Are the damages repairable? Elaborate.

QUESTION-1

A)

•              This crack will be formed due to the flexural failure of a beam. The flexural strength, known as modulus of rupture, bend strength, or fracture strength, is a material property, defined as the stress in a material just before it yields in a flexure test. The flexural strength represents the highest stress experienced within the material at its moment of rupture. It is measured in terms of stress. To overcome this type of failure, main steel is provided at the bottom/top of the beam. The damages are repairable, To repair damaged beams, locate a local welder or metal shop and request angle iron, cut and drilled to the length of the beam being repaired.Remove any pipes, wires, etc. currently going through the beam. Be sure to turn off the water, power, etc. before doing so.Pry the joist hanger away from the beam.Mount the angle iron on the jack and the temporary post. Get the angle iron in place and slowly raise the jack until the angle iron is wedged up against the damaged beam.Screw the angle iron into the beam and remove the temporary post.Slide any pipes or wires previously removed back through the beam and turn the water and power back on.
• At the time of earthquake, the stress on the beam is reversal, so this is a kind of flexural failure.
• During the earthquake, beam gets in tension zone and the structure feels in compression due to the stress reversal support at top.
• Bottom reinforcement is not sufficient to resist the tension in beam and the top compression resist by the slab due to concrete
• The bottom reinforcement of the beam lost their elastic limit, so that it is impossible to repair the bottom crack.

b)   

           This cracks are formed due to the mistakes done in the construction and honeycombing in concrete. To avoid cracks in concrete, avoid using high slump concrete. Concrete of moderate slump (less than 5 inches) results in less cracking.  High slump concrete can only be used if it is controlled by concrete admixtures or designed such that segregation and bleeding of concrete is controlled. Concrete admixtures such as water reducers or super plasticizers can be used to increase slump while reducing addition of water content. Air entertained concrete can also be produced to increase slump especially when concrete is to be subjected to freeze and thaw cycles. The honeycombing occurs when the fine mortar fails to thoroughly fall and fill the gaps created by the coarse aggregate. As mentioned previously, poor concrete consolidation is usually the top cause of honeycombing, along with poor vibration, improper ratio of fine to coarse aggregate, dry concrete and construction conditions that are not up to standard.

• The reinforcement of the beam is not sufficient to anchor the vertical rebar of the column, that is insufficient development length of the rebar
• Due to inadequate reinforcement, beam not bonded to the column that’s why the horizontal member gets detached from the vertical member during the earthquake
• The lateral movement cannot be satisfied due to insufficient anchorage length of beam rebar so it keeps on separated from the column
• It is very tough to provide the development length in casted concrete between two members, so it is not repairable

C)      

           This cracks are formed due to the beam-column joint failure. In the strong column-weak beam design, beams are expected to form plastic hinges at their ends and develop flexural overstrength beyond the design strength. The high internal forces developed at plastic hinges cause critical bond conditions in the longitudinal reinforcing bars passing through the joint and also impose high shear demand in the joint core. The bond performance of the bars anchored in a joint affects the shear resisting mechanism to a significant extent.

• This is a plastic hinge failure, the deformation in beam means no capability to resist the moment and its permit to free rotation
• The plastic hinge failure is happened at the joint only, so the damage occurs in the junction
• The main reason of damage are cyclic lateral loading and stress reversal at the junction
• As per the image, the slab has tried for handle the tension, but still it got cracked, so it clear that the tension is more on the slab and slab is unable to control damage
• To recover from the damage, we can strengthen the column-beam junction

QUESTION-2

A)

•             This crack will be formed due to the flexural failure of a column. Columns have the most important role in all structural members of a building. According to the base civil engineering concept, columns failure wants to be occurring after beams and the other components damages. Performance of existing buildings in earthquakes indicates that the beams are less vulnerable to damage during earthquakes and their damage appears to be less critical to performance as compared with that of columns and column-beam joints.

• From these observations its clear, this is a soft storey failure, technically plastic hinge formation at column
• Due to lateral movement in the building and absence of walls, columns got affected because of plastic hinge formed at the upper side of the ground floor
• Based on spall out the concrete from column and the severity of column is to be decide either repairable or non-repairable

b)

           This crack will be formed due to the shear failure of a column. Lateral loads in a structure are carrying by vertical elements such as columns and shear walls. When there are no shear walls, columns carry these lateral loads. Lateral loads are generated by winds, earthquake loads, from retaining structures, etc. Shear links are provided to columns based on the shear forces excreted on them. Further, the size of the columns is increased in the direction of shear to increase the shear capacity.

• By observing the picture, we can conclude this is a pure shear failure because the crack is in diagonal direction and the complete concrete had spalled and crushed
• The shear reinforcements were not sufficient so it had detached and spalled throughout the column. It had compelled concrete to breakdown into pieces.

C)

            This cracks are formed due to the short column failure. In Short Columns, failure takes place due to yielding and we define this phenomenon as CRUSHING. Short Columns are classified based upon Slenderness Ratio which is the ratio of the effective length (Length between point of zero moments) and least lateral dimension of the column. Failure of column usually occurs due to anyone of the following stresses developed in the column. Short column will fail only because of direct compressive stress developed in the column or we can also say that short column will usually fail due to direct compression or crushing.

• From the observation we can conclude that the failure is short column failure.
• The column is attached with intermediate slab and the height of column is not similar then the other columns, so there is the heavy shear load on the column.
• The intermediate slab load is unintentional and not accounted in the design so this leads to the failure
• This failure is not repairable.