CLASSROOM
C V R M urty Indian Institute of Technology
Learn ing Earthquake Design and Construction
K a n p u r , K a n p u r 2 0 8 0 16 16 , In In d i a
22.. Why are Short Column 22 Column s more Dama ged
Em a i l : e q t i p s @ i i t k . a c . i n
During Earthquakes? Which Columns are Short?
During past earthquakes, reinforced concrete (RC) frame buildings that have columns of different heights within one storey, suffe suffered red m ore damage in th e shorter columns as compared to taller columns in th e same same storey. Two examples examples of build ings with short columns are shown in Figure 1 – buildings on a sloping ground and buildings with a mezzanine floor. Poor behaviour of short columns is due to the fact that in an earthquake, a tall column and a short column of same crosssection sec tion m ove horizon tally by same amoun amoun t. ( Figure 2). However, th e short column is stiffer as compared to th e tall column column , and and i t attracts larger earthquake force. Stiffness of a column means resistance to deformat deformat ion – th e larger larger is th e stiffness, stiffness, larger larger is th e force required to deform it. If a short column is not adequately designed for such a large force, force, it can suffer significant d amage during an earthquake. This behaviour is called Short Column Keywords
damage in t hese short short col column umn s is often often in the form of Effec Ef fect. Th e damage
Ea r t h q u a k e , RC RC b u i l d i n g , b e a m -
X-shaped cracking – this type of damage of columns is due to
c o l u m n j o i n ts ts .
shear failure (see I IIT IT K -B -BM M T P C E ar arthq thquake uake T ip 19).
Figure 1. Buildings with short columns – two explicit examples of common occurrences.
88
R ES O N A N C E
⎜ October 2005
CLASSROOM
Figure 2. Short columns are stiffer and attract larger forces during earthquakes – this must be accounted for in design.
T he Sh ort Column Behaviour
Many situations with short column effect arise in buildings. When a buildin g is rested on sloped ground ( Figure 1a), during earthquake shaking all columns move horizontally by the same amount along with the floor slab at a particular level (this is called rigid floor diaphragm action; see II T K -BM T PC E arthquake Tip 17). If short and tall columns exist within the same storey
level, then the short columns attract several times larger earthquake force and suffer more d amage as compared t o taller ones. The short column effect also occurs in columns that support
Figure 3. Short columns ef-
mezzanine floors or loft slabs that are added in between two
fect in RC buildings when
regular floors (Figures 1b). There is another special situation in
partial height walls adjoin
buildings when short-column effect occurs. Consider a wall (masonry or R C) of partial height built to fit a window over the
remaining h eight . Th e adjacent columns behave as short columns
columns – the effect is implicit here because infill walls are often treated as non-structural elements.
due to p resence of th ese walls. In many cases, other colum ns in th e same storey are of regular height, as there are no walls adjoining them. When the floor slab moves horizontally during an earthquake, the upper ends of these columns undergo the same displacement (Figure 3). However, th e stiff walls restrict horizontal movement of the lower portion of a short colum n, and it deforms by the full amount over the short height adjacent to the window
R ES O N A N C E
⎜ October 2005
89
CLASSROOM
Figure 4. Effective height of column over which it can bend is restricted by adjacent walls – this short-column effect is most severe when opening height is small.
opening. On th e other h and, regular columns d eform over the full height. Since the effective height over which a short column
can freely bend is small, it offers m ore resistance to h orizontal motion and thereby attracts a larger force as compared to the regular column. As a result, short colum n sustain s more damage. Figure 4 shows X-cracking in a colum n adjacent t o the walls of
partial height. T he Solution
In new buildings, short column effect should be avoided to the extent possible during architectural design stage itself. When it is not possible to avoid short columns, th is effect m ust be add ressed in structural design. The Indian Standard IS:13920-1993 for ductile detailing of RC structures requires special confining reinforcement to be provided over th e full height of columns th at are likely to sustain short column effect. The special confining
90
RESO N A N CE
⎜ October 2005
CLASSROOM
Figure 5. Details of reinforcement in a building with short column effect in some columns – additional special requirements are given in IS:13920 -1993 for the short columns.
Related – Earthquake Tip Tip 6: How Architectural Feat u r e s A f f e c t Bu i l d i n g s D u r i n g Earthquakes? Ti p 17 : H o w d o Ea r t h q u a k e s A f f e c t Re i n f o r c e d C o n c r e t e B u i l d ings?
reinforcement (i.e., closely spaced closed ties) must extend b eyond th e short column int o the column s vertically above and below by a certain distance as shown in Figure 5. See II T K -BM T PC E arthquake T ip 19 for details of the special confinement
reinforcement. In existing buildings with short columns, different retrofit solutions can be employed to avoid damage in futur e earthquakes.
Tip 19: How do Columns in RC B u i l d i n g s Re s i s t Ea r t h q u a k e s ? Resource Material IS 13920, (1993), I n d i a n St a n d a r d C o d e o f P r a c t ic e f o r D u c t i le D e t a i l i n g o f Re i n f o r c e d C o n c r e t e St r u c t u r e s Su b j e c t e d t o S e i s m i c Fo r c e s , Bureau of Indian Standards, N e w D e l h i.
Where walls of partial height are pr esent , the simplest solution is to close th e openings by build ing a wall of full h eight – th is will eliminate the short column effect. If that is not possible, short column s need to be strength ened using on e of the well established retrofit techniques. The retrofit solution should be designed by a qualified structural engineer with requisite background.
Suggestions/comments may b e s e n t t o : e q t i p s @ i it k .a c . i n .
R ES O N A N C E
⎜ October 2005
91
