Behavior of RC structure

Training to LB technical staff on Nepal National Building Code

Binay Shrestha

Why Buildings fail in an Earthquake

Lack of

• Strength.

• Ductility.

Introduced by

• Design deficiency (failure to understand failure mechanism, deficient system, calculation).

• Construction deficiency (lack of quality assurance: material, construction details).

Measures

Design

• System design.

•Component design.

Detailing

•Better behavior.

•To improve ductility of building.

Poor Performance of RC Frame Buildings

I. Design concept ignorance 

Strong column weak beam

Soft story

Short column

II. Construction Defects:

Weak / Cold joint 

Lap & end anchorage

Ductility

Poor Performance of RC Frame Buildings

I. Other

Unacceptable shape

Defective load path

Local soil condition. 

Hammering/ Pounding

Design Concept Ignorance:

Strong column weak beam

(Pancake type damage)

Collapse of a multistory RC frame building due to weak column-strong beam design (Bhuj, India 2001)

Multiple-story collapse in a six-story building due to strong beam-weak column design in the 1999 Turkey earthquake

Strong-Column Weak-Beam Concept

• Failure of a column can affect the stability of the whole building, but the failure of a beam causes localized effect.

• Hence it is better to make beams to be the ductile weak links than columns.

• This method of designing RC building is called the strong-column weak-beam design method.

Columns should be stronger than beams and foundations should be stronger than columns.

Connections between beams & columns and columns & foundations should not fail so that beams can safely transfer forces to columns and columns to foundations.

Strong-Column Weak-Beam Concept

Sum of moment capacities of the columns for the design axial loads at a beam column joint should be greater than the sum of moment capacities of the beams along each principal plane.

Mcolumns > 1.2 Mbeams

The shear reinforcement should be adequate to ensure that the strength in shear exceeds the strength in flexure and thus prevent a non-ductile shear failure.

Soft Story

Soft Story building have two distinct characteristics, namely:

(a) It is relatively flexible in the ground storey, i.e., the relative horizontal displacement it undergoes in the ground storey is much larger than what each of the storeysabove it does.

(b) It is relatively weak in ground storey, i.e., the total horizontal earthquake force it can carry in the ground storey is significantly smaller than what each of the storeysabove it can carry.

Soft Story

Weak columns

Brick infill

Open floor Open floor

Ground shaking Ground shaking

Soft-story Collapse Mechanism

• Collapse of intermediate story in a6-storey RC frame commercial building at Bhuj.

Olive View Hospital, which nearly collapsed due to excessive deformation in the first two stories during the 1972 San Fernando earthquake

3rd floor collapse. (Kobe, Japan)

Design of Soft Storey Elements

The Code suggests that the forces in the columns, beams and shear walls (if any) under the action of seismic loads specified in the code, may be obtained by considering the bare frame building (without any infills).

However, beams and columns in the open ground storey are required to be designed for 2.5 times the forces obtained from this bare frame analysis.

Short Column Effect

Explicit Examples

Implicit Example

Short Column Effect due to staircase

Short Column Effect

• Tall column and a short column of same cross-section move horizontally by same amount ∆ .

• However, the short column is stiffer as compared to the tall column, and it attracts larger earthquake force.

• Stiffness of a column means resistance to deformation – the larger is the stiffness, larger is the force required to deform it. If a short column is not adequately designed for such a large force, it can suffer significant damage during an earthquake. This behaviour is called Short Column Effect.

• The damage in these short columns is often in the form of X-shaped cracking – this type of damage of columns is due to shear failure .

Lateral Stiffness = 12EI/(L3)

F= K∆

If short and tall columns exist within the same storey level, then the short columns attract several times larger earthquake force and suffer more damage as compared to taller ones.

short column damage

Construction Defect:

Joints

Beam Column Joint

Beam Column Joint

Beam Column Joint

Weak Joints

Beam-column joints may not be able to develop the strength of the connected members, and this can lead to sudden brittle failure of the joint

Cold Joint

Cold Joint

Shear key to avoid Cold Joint

Weak Joints

Lapping

Improper lapping and anchorage

The pictures show damage concentration in the region of bar lapping. Because of interaction between overlapped bars and concrete for load transfer the overlapping section suffers higher level of damage. This interaction is further coupled with lack of stirrups which has led to buckling of bars, loss of concrete

Improper lapping and anchorage

Failure due to improper Detailing(a) buckling of vertical column rebars due to inadequately spaced horizontal ties (b) severe damage of a ground-floor column due to improper confinement of

concrete and lapping of large number of longitudinal bars(c) typical infrequent horizontal ties with 90° hooks, which were unable to confine

the concrete core

Improper lapping and anchorage

Cover

Stirrup spacing

Improper lapping and anchorage

Improper anchorage of transverse reinforcement has resulted in failure of confinement in columns during the 1985 Mexico earthquake

Column tie spacing and tie hooks: failure of quality control

Deformability (ductility) of reinforced concrete members is a necessity. Note the obvious differences of capability of concrete columns to take load after earthquake damage. The reinforced column with more stirrups (ductile reinforcing) has an obvious capacity to carry much more load than the column with less stirrups

Shape

TorsionSeismic force at each level acts through Center of Mass of each floor and is resisted by the building through its center of rigidity.

Buildings have unequal vertical members. They cause the building to twist about a vertical axis.

One side open ground storey building twists during earthquake shaking.

TorsionCenter of Mass(CM )= Center of gravity of floor masses 

Center of Rigidity (CR)= A point through which a horizontal force is applied; resulting in translation of the floor without any rotation.

Effect of Torsion: different portions of

the same floor level moves horizontally by different amounts.

This induces more damage in the columns and walls on the side that moves more.

Remedies:• Symmetry in Plan through uniformly distributed mass & uniformly

placed vertical members .• Otherwise include additional shear forces in the design of columns as

per codal provisions.

Torsion

These are not Symmetrical

Unacceptable shape

Excessive long cantilever

Many staircase towers, called "mumty," which project about 2 mabove the surrounding construction in masonry houses, collapsed.

Unsymmetrical loadCollapse of one-half of the14-storey RC frame residential apartment building in Ahmedabad;the collapsed portion hada swimming pool on theroof, unlike the other halfthat is standing.

Disturbed load path• Connections between

beams and columns; Columns and foundations should not fail so that beams can safely transfer forces to columns and columns to foundation.

• Each joint should achieve proper ductility

Undefined Load Path

Eccentric beam column joint: Torsion in column

Indirect loading to column: conceptual mistakes

Hammering by Adjacent Building

Hammering by Adjacent Building

Pounding between a six-story and a two-story building (Golcuk, Turkey 1999 earthquake)

Inappropriate – Liquefiable area

Slender Building – not checked for overturning

THANK YOU !