Abstract
I. Purpose
Low-rise RC terrace buildings, continuously constructed along street sides,
are common in Taiwan. According to the 921 earthquake reconnaissance, most
severely damaged or collapsed buildings are street buildings, which were
generally failed of soft story mechanism at ground floor. It is because fewer
walls were placed ground floors than upper floors in parallel of street direction.
To date, most low-rise RC street buildings still use similar RC frame system
with masonry or RC infill walls. In practice, architects and engineers would like
to neglect these infill walls in structural design. Notably, column dimensions for
the low-rise buildings are relatively small, and therefore the infill walls may
attract a large portion of lateral forces. Captive beams and columns may
dominate structural response beyond anticipation.
Due to our residential culture and needs, open fronts and backs at ground
floor are very common for low-rise RC street buildings. It is almost impossible
to add walls or bracings at fronts and backs. An alternative option is to arrange
one or two RC walls aside the stairwell in the interior frame of each unit street
building. This arrangement could make up the deficiency of soft story and
enhance the earthquake-resistant capacity of street buildings.
II. Method and Steps
This study includes pushover analysis and experiments. We investigated
available literature and typical configurations for the RC street buildings in
Taiwan. Using modern pushover analysis methods, we conduct seismic
梯間牆對低層RC 造沿街連棟建築物耐震性能之影響
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evaluations of typical street building models with different arrangement of
stairwell walls. The results may change from flexure-critical to shear-critical
because of captive beams adjacent to the stairwell walls. The capacity curves
may be quite different because of different failures.
Based on the seismic evaluation of typical street buildings, two model
frames, two-story by two bay, partially infilled with RC walls, were constructed
to simulate an interior frame aside the stairwell of a street building. Cyclic
loading tests were carried out to study their failure mechanism and seismic
performance.
III. Main Findings
Based on literature survey, peer discussions, pushover analysis of street
buildings, and experimental investigations, following findings were drawn:
1. Many non-engineered RC street buildings collapsed in the 921 earthquake.
The primary reasons were inadequate system without walls parallel to the
street direction. This could be made up by adding walls in the interior
frame aside the stairwell in each unit street building. Good arrangement of
stairwell walls could prevent building collapse in a major earthquake and
ensure life safety of people.
2. Due to architect reasons, door openings in RC infills can not be avoided.
Captive beams and horizontal wall segments adjacent to the door openings
may be shear-critical and dominated the lateral-force-resisting response.
Special detailing should be considered in the design to preclude
non-preferred shear failure of captive beams.
3. The rigidity of wall foundations affects the results of seismic evaluation by
pushover analysis. Tie foundation beams are usually used in street
buildings. Whenever an RC infill wall is framed and foundation-supported,
摘要
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such three-sided infill wall should be included into the model as an
equivalent column for pushover analysis.
4. Experimental results showed that the lateral strength of the model frame
was dominated by the flexure of columns and walls with ductility, when
the clear length of the captive beams was four times beam depth. The
lateral-force-resisting behavior can be captured in the pushover analysis
using beam elements.
5. Experimental results showed that the lateral strength of the model frame
was dominated by shear failure of the captive beams and horizontal wall
segments with limited ductility, due to the door openings above and below
the captive beams. The failure mechanism turned to D-region shear failure
and it could not be analyzed by beam theory. It is recommended to use
strut-and-tie model for the D-region element.
6. In the conditions of typical partition wall thickness and reinforcing details,
the extreme vertical reinforcement buckled following the crushing of
concrete at the wall edge. Such wall section could attain the flexure
strength with limited ductility in cyclic loading reversals. Boundary
elements with special detailing are needed for more ductility.
IV. Main Suggestions
This project comes to the immediate and long-term strategies.
For immediate strategies:
1. Incorporating stairwell walls into seismic evaluation of street buildings
2. Experimental investigations on the seismic performance of wall buildings
For long-term strategies:
Seminar for promoting new technologies of seismic design and evaluation