Concept on positioning of Columns

Question 1:

Comment on the layout of the framing plan concerning earthquake resistance in the two horizontal directions X or Y (dots are columns, lines depict beams). Look into aspects of symmetry, redundancy and bi-directional stiffness and ductility.

Ans.

In this layout of the framing plan is Symmetric

A symmetrical well distribution plan layout of structural elements is appropriate to acheive uniformity. The use of many, evenly distributed structural elements increases the redundancy of the structure, that is the structure becomes more reliable to resisit the earthquake effects.

• Symmetry

It observed that the distribution of mass, stiffness is across the plan has good symmetrical aspects. This tends to reduce the horizontal displacement in both of two horizontal displacement in both of the two Orthogonal Horizontal Directions.

It also has a very good uniformly also.

• Redundancy

There are missing structural elements in the periphery of the plan. This causes the uneven distribution of earthquake action effects in non linear range across the plan.

The energy dissipation across the structure is very low which result in non-relaible structure.

• Bi-directional stiffness

The building plan may not be able to resist the horizontal sesmic actions in the shorter direction (Y-axis) and also the resistance and stiffness in the shorter direction (Y-axis) is less when compared to longer direction (X-axis).

• Ductility

The building may not be able to resist the elastic earthquqke forces in shorter direction (Y-axis). So it may result in elastic response.

The reduction in design force in the shorter direction (Y-axis) is to be compensated by Ductility.

In the plan there might be a chance of arising plastic hinges. to calaculate the limit ,oad for finding the plastic hinge limit load =4MP/L

where, MP= Plastic moment.

Question 2:

What are the possible sources of irregularity in the building shown here? Explain how, as an engineer, each of these irregularities should be dealt with.

Ans.

A building is said to be irregular when the maximum horizontal displacements of any floor in the direction of lateral forces at one end of the floor is more than 1.5 times its minimum horizontal displacement at the far end of the same floor in that direction.

In such a case the building configuration is must be revised.

Out of the many poosible irregularities in building structures, the final structures, the final design in absolute terms, avoid the two most dangerous irregular situations.

• Soft storey failure:

The first must have lesser strength and stifness as compared to upper stories which are stiffened by masonary in fills walls.Increase flexibility at the first storey results in extreme deflections. The damage may also happen due to the collapse and buckling of columns, especially where parking places are not coveredc appropriately.

Floating columns:

In the upper stories the perimeter of the ground story is discontinued and floatings columns rest at the tip of taper overhanging beams without considering the increased vulnerablilty of the lateral load resisting system due to vertical discontinuity. This type of construction does not create any problem under vertical loading conditions but during an earthquake, a clear load apth is not available for transferring the lateral forces to the foundation.

• Mass irregularity

The upper stories the perimeter of the ground story is dicontinued and flaoting columns rest at the tip of the taper overhanging beams withput considering the increase vulnerability of the lateral load resisting system due to vertical discontinuity. This type of construction does nt create any problem under vertical discontinuity. This type of construction does not create any problem under vertical loading conditions but during an earthquake, a clear load path is not available for transefering the lateral forces to the foundation even when the sesimic weight of any floor is more than 150 percent of the floor of that floor above.

Both case correspond to extreme irreggular stifness distrbution the former regarding the distribution in height/elevation with a very weak and later regarding the distrubtion in plan, with the concentration of the stiffness at the center or at corner.

Question 3:

A 4-storey hotel building has an open ground floor for the restaurant. Story 2 to 4 have one row of rooms along each long side in plan, separated by a corridor. The two short sides of the perimeter are fully in-filled in all storeys, except for certain openings at the ends of the corridor at storeys 2 to 4 and along the right-hand side of the ground floor. There is a staircase near the upper left-hand corner, with straight flights between landings at floor levels and in-between floors. Cross-section dimensions are written next to the member no. in meters.

Comment on the features of the structural design and of the layout of infills which are important for earthquake resistance and seismic performance. How do they relate to the almost full collapse of this building (the extreme left-hand bay with the staircase survived, as well as one long-side façade in the rear end, and the frame along the right-hand side in plan).

Ans.

The first must have lesser strength and st t tipifness as compared to upper stories which are stifened by masonary in fill walls.increase flexibility of the first story results in extreme deflections. The damage may also happen due to the collapse and buckiling of columns, especially where parking places are not covered appropriately. In the upper stories, the perimeter of the ground story is dicontinued and floating columns rest at the tip of taper overhanging beams without considering the increased vulernability of lateral load resisting system due to vertical discontinuity. This type of construction does not cratet any problem under vertical loading conditions but during esrthquake, a clear load path is not available for transerfering the lateral forces to the foundation. In the upper stories the perimeter of the ground story is discontinued and floating column rest at the tip of taper overhanging beams without considering the increasing vulnerability of latersal load resisting system due to vertical discontinuity. This type of construction does not create any problem under vertical loading conditions but during an earthquake a clear load path is not available for transfering the lateral forces to foundation.

The failure is basically due to the stifning effect of infills panels which is the caused of

a. Unequal distribution of lateral forces in different frames and overstressing of some of the building frames

b. Soft story

c. Short column

d. Torsional effect

e. Cracking of the infill walls

As the staircase is isolated from the primary structural system therefore it would have saved itself form damage due to earthquake

• There is no proper provsion of redundancy in the vertical direction.
• Improper distribution of symmetry (or non-uniformly).
• Provision of the weak storey on left hand bay.
• Improper placement of masonary infill wall.