REINFORCED CONCRETE FRAME BUILDINGS ~ ENGINEER'S LAB

Reinforced concrete (RC) frames consist of horizontal elements (beams) and vertical
elements (columns) connected by rigid joints. These structures are cast monolithically—
that is, beams and columns are cast in a single operation in order to act in unison. RC
frames provide resistance to both gravity and lateral loads through bending in beams
and columns (Figure 2). There are several subtypes of RC frame construction:

• Νonductile RC frames with/without infill walls
• Νonductile RC frames with reinforced infill walls
• Ductile RC frames with/without infill walls

Figure 2: A plan of a typical RC
frame building in Ahmedabad,
India; note the portion that
collapsed in the 2001 Bhuj
earthquake (WHE Report 19, India)

The current WHE database includes over twenty reports describing RC frame
construction. The most prevalent type is RC frame with masonry infill walls (Figure 3).
This construction is still practiced extensively in many parts of the world, especially in
developing countries. This construction comprises approximately 75% of the building
stock in Turkey, about 60% in Colombia, and over 30% in Greece. Details of this
construction type including regional variations are contained in the WHE reports from
Cyprus (WHE Report 13), India (WHE Report 19), Palestinian Territories (WHE Report
48), Turkey (WHE Report 64), and Romania (WHE Report 71). RC frames with concrete
infill walls, also known as dual systems, are very common in earthquake-prone areas.
The WHE reports from Chile (Report 6) and Syria (Report 59) describe details of this
construction type.

Code requirements related to design and detailing of RC frame buildings in seismic zones
were significantly changed in the early 1970s. Earlier codes focused on the strength
requirements—that is, on providing adequate strength in structural members to resist
the lateral seismic forces. However, based on research evidence and lessons learned
from earthquakes in the early 1970s, code requirements have become more focused
on the proportioning and detailing of beams, columns, and joints with the objective to
achieve a certain amount of ductility in addition to the required strength. Ductility is one

of the key features required for desirable seismic behavior of building structures. It can
be defined as the ability of a material to stretch (deform) significantly before failure.

Steel (and some other metals) exhibit ductile behavior. For example, a metal paper clip
can be bent back and forth without breaking. However, other materials are brittle (the
opposite of ductile). A piece of chalk will break as soon as we try to bend it. In reinforced
concrete, concrete behaves like chalk, whereas steel reinforcement behaves like a
paper clip. Therefore, steel reinforcement has a key role in ensuring ductile behavior
of reinforced concrete structures in earthquakes. Earthquake engineers spend a
considerable amount of time trying to ensure that the amount and distribution of steel
reinforcement are adequate for a specific design. That part of seismic design is called
seismic detailing, or sometimes the art of detailing. The principles and rules of seismic
detailing of reinforced concrete structures have been emerging over time and are
mainly reflected in seismic provisions of building codes.

Thus, pre-1970 nonductile concrete frames, although often designed to resist lateral
forces, did not incorporate modern ductile seismic detailing provisions. As a result, the
main seismic deficiencies of the pre-1970s concrete frame construction include (ATC-
401):

• Inadequate column detailing. The two main detailing problems include
inadequate column lap splices for main flexural reinforcement and a lack of
adequate transverse reinforcement (ties) within the column (Figure 4). As an
example, column lap splices were typically placed just above the floor level
in the zone of high stresses. In addition, the column lap splices were generally
too short, often in the order of 30-bar diameters, or less, and were typically not
confined with closely spaced column ties (as required by modern codes).

• Lack of strong column/weak beam design approach. A capacity design
approach was not followed in the design of the beam flexural reinforcement, as
the beams were generally designed for the code level forces. The effects of postyield
behavior were not considered, thus increasing the chances for undesirable
shear failure in either the beams or columns. Shear failure is rather brittle and
sudden, and should be avoided in reinforced concrete structures located in
seismic zones.

• Inadequate anchorage of beam reinforcement. The top reinforcing bars in
beams were often terminated 6 to 8 feet away from the column face, whereas
the bottom bars were typically discontinued at the face of the supporting column
or provided with only a short lap-splice centered on the column.

• Excessive tie-spacing. Spacing of ties in beams and columns was excessively
large by today’s standards. Column ties often consisted of a single hoop with
90 degree hooks spaced at 12 to 18 inches on center. Today’s ties generally
require 135 degree hooks to ensure adequate confinement. Beam ties, often
sized only for gravity shear loads, were spaced closely near the column face but
were widely spaced or even discontinued throughout the mid-span region of the
beam.

• Inadequate beam/column joint ties. The lack of ties in the beam/column joint
created a weak zone and likely failure mechanism within the joint.