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.:- 1

1. Symmetrical aspect.:

Above layout has been evenly distributed along both the X and Y axis which allows short and direct transmission of the inertial forces produced in the distributed masses of the building.

The layout is symmetrically well distributed so that distribution of stiffness and mass becomes uniform, thus reducing eccentricity which causes torsional response and tends to increase horizontal displacement in some of the building which is unfavourable.

The symmetrical distribution of structural elements decouples the vibration modes of the building in two independent horizontal directions and thus its response to the seismic excitation is much simpler and less prone to torsional effects.

2. Redundancy.:

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

These elements will hold higher loads than other vertical loads due to their longer span. These columns will bear higher lateral forces.

3. Bi-directional strength.:

The arrangement of structural members in X direction is more when compared to the Y direction. Therefore the stiffness of building in X direction is higher than that in the y direction.

Structural elements should be arranged in an in-plan orthogonal stuctural pattern so that the building is able to direct the seismic action in any direction.

4. Irregularity in terms of ductility.:

We know that ductilty of a buidling is inversely proportional to the stiffness of the building.

So in this figure we can see that the building is less ductile in X direction means high stiffness and more ductile in Y direction means less stiffness.

Therefore the lateral forces in X direction will be larger than the lateral forces in Y direction.

The building may not be albe to resist the elastic earthquake forces in shorter direction (Y axis) which may result in inelastic response. The reduction in design force in the shorter direction (Y axis) is to be compensated by ductility.

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.:- 2

1. Irregularity due to in-plane flexibilty.:

As we can see from the figure the building is constructed in rectangular plan.

So it seems the length of the is more than 3 times the width of the building.

With aloor aspect ratio>3 it is assumed to have significant in plane- fexibilty which will make the floor flexible during the seismic action.

This irregularity can corrected by maintaining the floor plan aspect ratio within 3.

2. Irregularity in redundancy.:

As we can see from the figure the load carried by the vertical and horizontal structural elements differ drastically.

So during the seismic action the the building's response for the lateral force will varying for each columns with their load demand.

We can correct this irregularity by maitaining a uniform load distribution for all the vertical and horizontal members which will minimize the effect due to redundancy irregularity.

3. Soft storey effect due to absence of in-fill walls.:

As we can see in the figure the fifth floor is not having any walls for architectural purposes.

This will lead to to differences in storey stiffnes between successive levels.

This effect can be minimized by providing masonary walls in that level wherever possible.

4. Soft storey effect due to mass irregularity.:

The ratio of floor mass between successive floors may higher than 1.5.

So, this effect can be minimized by providing light weight concrete/low weight masonary in lower floors so that the total mass of the lower floor become nearly equal to upper floor mass.

5. Irregularity in vertical geometry.:

If we imagine seeing form the side then we will notice significant reduction in the floor extension upwards.

According to indian code the building is said to be irregular if the overall building width is 25 percent more than the top floor width.

This irregularity can be minimized by keeping all foor widths same as much as possible.

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.C

omment 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.:- 3

To investigate the given building structural plan and causes of its collapse after an earthquake.Looking at the plan of the building it consists of one way slab and beams.The RC are vertically one way and RC beams are oriented in X direction which transfers the floor load to columns. We can also see a cantilever RC slabs provided for the balcony purpose. We can also see that there are no RC beams in the y direction.

Elements responsible for the failure of the building:

1. In-fill walls.:

It does not matter whether we provide structural or non-structural in-fill walls, it possess mass and therefore supply stiffness to the region.

So in this building it is supported by full height masonary walls on either short sides which provides higher stiffness to the bulding during seismic period.

Thus the interior panels became more flexible leading to collapse of the majo portions and the stiffer members remained intact.

2. Staircase.:

As we discussed earlier the staircases provided sufficient lateral stiffness to its region as it behaves a bracing member connecting floor to floor.

Staircase provided more stiffness to its region as a bracing so that is why staircase remained intact after the earthquake.

3. Mass Irregularity.:

As we can see in the plan the top and bottom bays are provided full of rooms and middle bay is left unocuppied for corridor purpose which creates mass irregularity within the building.

This mass concentration will provide stiffness along direction but it will be vulnerable in the Y direction. So if the earthquake forces act from the Y direction then the top and bottom will exert more lateral forces and middle bay will be more flexible which in turn leads to internal collapse of the building during siesmic action.

4. Framing arrangement.:

As we can see in the plan that there are no RC beams in the vertical direction.

The absence of vertical RC beams creates RC slab of diaphragmatic behaviour.

So the in-plane flexibilty of the RC slab will influence the collapse of the building.

5. Column Orientation.:

As per the architectural and structural plan there does not seem to be any difference in center of mass and centre of rigidity.

However, the building must have experienced torsional irregularity due to positioning of columns and unavilability of any redundancy.

As we can see in the plan beccauuse of the dimensions of the column it will be more stiffer in the y direction.

So eventually it will experience more lateral forces in Y direction but the RC beams are not along the y direction so the lateral force will not dissipate from floor to frame properly which leads to the collapse of the building during seismic activity.

6. Comment on the features of the structural building plan.:

The given building structural plan consisted of One-way Slabs and Beams.

The RC slabs are vertically one way and the RC beams are oriented in X-direction which transfers all Slab load to columns.

There are no RC beams provided in Y-direction.

Also from elevation, we can see small cantilever RC slabs are provided for the balcony purpose of rooms.