The description of damage of each seismic resisting system is a generic type of
damage that pertains to some level of earthquake input energy that allows the
seismic resisting system to go beyond yielding and into the inelastic range.
Expected structural damage of Moment frames Systems in Steel.Moment frames
are an inherited high interstory drift system. Considerable non-structural
damage will occur. Depending the stiffness of the moment frame and the inelastic
properties, the interstory drift will be in the 1″ or 2″ to 3″ range for a
15-foot story. This is high and the exterior facade as well as the interior
partitions will be severely damaged. There will be structural damage also. It
wasn’t a surprise that many moment frames welded joints were damaged in the
Northridge earthquake. But now that we are trying to fix the welded joints, has
anyone ever seen what a beam looks like at the plastic hinge after a few cycles.
There is severe distortion of the flanges with possible tearing of the web to
flange connection. I still do not know how to fix the beam except to remove the
whole beam and this will cost a bundle. It may be easier to provide a very large
number of welded joints using small beam sections with a similar pre-Northridge
joint detail. I am assuming that some of the welds will fail but not all of them
and that the remaining welds are adequate to prevent the building from
collapsing. It may be cheaper to fix a welded joints than replacing the entire
beam. This is one concept but I still think this is not the right direction.
Owners should be told up front about the expected damage by using a moment frame
system. The building will be a total lost in the long run. An appropriate
solution to minimize the inelastic damage of moment frames is to have a backup
energy dissipation system that absorbs the energy before the moment frame beams
MOMENT FRAME DAMAGEExpected Structural damage of Eccentric Brace Frames
The structural damage will be concentrated at the link beam. The floor slab will
be severely cracked up due to the movement of the link beam. In addition, any
non-structural elements at or near the link beam, such as, partitions, etc.,
will be severely damaged. It is recommended that any exit facilities, stairs or
exit access not be located at the link beams. Exit doorways will be frozen shut
and the occupants will not be able to escape after an earthquake. If EBF is
located at the exterior facade, the exterior should not be attached to the link
beam at all. Some additional framing should be added to bypass the link beam.
The reason for not attaching the exterior facade to the link beam is that there
will be such large beam rotations the exterior connections will be severely
damaged causing the facade to fail and fall. The link beam will have the same
type of damage as the moment frame, i.e., severe beam flange rotation and
possible tearing of the flange to web connection at the point of the plastic
hinge. The result will be that the link beams will have to be replaced. This
operation will require the removal of the brace, floor, and link beam or some
EBF DAMAGEExpected structural damage of Concentric Braced frames
Buckling of the compression braces in the CBF will occur in large or great
earthquakes. The result is the tension braces will absorb the additional seismic
forces beyond the buckling load. There are three points in the compression brace
where the buckling will occur and is dependent on the connection to the gusset
plate. If the brace to connection plate with a hinging mechanism, then one
bucking point may exist and that is at the mid-length of the brace. The columns
of a CBF shall always be larger enough not to buckle at all and the anchorage at
the base shall be able to resist the maximum possible tension load. Therefore,
the repair of a CBF after a large or great earthquake should only be replacing
the buckled braces. To minimize repair cost, the original designer should
provide easy access to the braces for easy removal and replacement. Even though
CBF is a stiff system, the interstory drift will be significant enough when
considering all the inelastic displacements of each component of the CBF system
that the exterior facade should be articulated. The interstory drift of a braced
frame system should be in the 1″ to 1 1/2″ range. This kind of drift will cause
damage to the facade and the interior partitions
CBF DAMAGEExpected structural damage of Concrete shear walls
No matter what, concrete shear walls will crack up. The larger the earthquake
the more cracks that will occur and the size of the cracks will be larger. Under
cyclic axial loads from overturning moments, the concrete in the boundary
element will degrade. Confinement ties will keep the concrete from blowing out,
but removal of the concrete may be required. The tension steel will yield
leaving the permanent deformation or cracks in the boundary element
CONCRETE SHEAR WALL DAMAGEExpected structural damage of Coupled Concrete
Like concrete shear walls, Coupled concrete shear walls will crack up. The
exterior boundary elements will degrade from the high vertical axial loads. The
coupling beams will crack and in some cases, chunks of concrete will fall. It is
recommended to enclose the coupling beam with chicken wire to keep the concrete
from falling. Somewhat vertical cracks will occur at the coupling beam ends due
to the yielding of the tensile diagonal bars. refer to my home page for
schematic reprehensive of coupled shear walls.
COUPLED SHEAR WALL DAMAGEExpected structural damage of an base isolated
There is very minimal structural damage in a base isolated building because the
primarily structural system will not yield. In addition, the lateral
displacements above the isolators will be very low which will minimize the
non-structural damage in the facade and partitions. Also the accelerations will
be low which will reduce the demand on the anchorage of other non-structural and
structural elements. There is some potential structural damage only if the
isolators fail and the building lands hard on the secondary support system. In
general, the overall additional cost of the base isolated system will negate the
post earthquake repair cost.
BASE ISOLATED DAMAGEExpected structural damage of Moment frames Systems in
Concrete moment frames damage will be different from steel moment frames.
Concrete moment frames will crack up and spell especially at the beam plastic
hinges. The spalling of the columns exposing the reinforcing will occur. For
large to great earthquakes, there will be severe cracking at the beam-column
joints and the damage to columns will consist of derogation of the column core
with possible pouring out of degraded concrete between the confinement ties.
There is one positive good for concrete moment frames verses steel moment
frames. The repair of the concrete moment frame beams may be earlier than steel
repair if the damage is confined to the beams. It is easy to chip out the
damaged concrete and replace either with high strength grout or concrete.
Repairing severely damage columns will be extremely difficult. This level of
damage will result in a possible red tag of the building. The building official
would require the owners to demolish the building or removed the building and
send the bill to the owner. Therefore, the design of the building should provide
for oversize columns and small beams. In addition, there should be a large
number of columns, i.e., considerable amount of frames (redundancy) The solution
to minimize the inelastic damage of moment frames beams is to have a backup
energy dissipation system that absorbs the energy before the moment frame beams
and columns goes inelastic.
CONCRETE MOMENT FRAME DAMAGEExpected damage of bearing wall building
Given each material used in a bearing wall/ shear wall building, there will be
different type of damage.
Expected Structural damage of Wood frame buildings
The damage to wood shear walls tend to be the result of poor construction and
design for earthquake forces. An example is that the compression post from
overturning forces tend to squash the sill plates. At the upper floors,
overturning forces may taken by blocking between the joists or the joist
themselves. Hold-downs that do not properly work due to the oversize holes in
the wood or holes enlarged due to shrinkage. Under severe seismic loads, nails
in wood shear walls will pop, plywood will buckle , finishes will crack, wood
The observations from pass earthquakes show most wood frame buildings tend to
survive an earthquake provided they have the following:
- They have significant number of shear walls and their stresses are not too
- They are anchored to the foundation with an significant number of bolts.
- They do not have cripple walls – discontinuous shear walls.
- They do not rely on torsion due to discontinuous walls, etc.
- They are adequately tied together with appropriate collectors or drag members
and have adequate diaphragm chords.
- There is continuity from foundation to roof of all floor ant roof elements.
Most residential housing units can be dated by level of damage and type of
New homes, built in the last fifteen to twenty years, will probably have plywood
and gyp board walls that will perform reasonably well.
Homes built in the fifties and sixties may not have plywood sheathing but
exterior finishes that consist of straight sheathing or stucco and gyp board in
the interior. These homes will perform moderately well but they will have much
higher damage level than the newer homes using plywood. If the damage is quite
severe, these homes may be yellow tag by the building official. Basiclly, the
straight sheathing can not act as a good shear wall and the gyp board is a poor
shear wall material.
Stucco or plaster walls are very brittle and will crack easy.
Homes built before the fifties used more plaster type material. Many of these
homes will be severely damage and there is an hazard in these homes, falling
plaster. Homes built before the thirties tend not to have anchor bolts and will
slide off the foundation.
Many multi-family housing units or apartment buildings will not perform well
because they usually have discontinuous walls on one or two sides at the ground
level for parking or garages. In the 1989 earthquake many of the Marina district
apartment buildings were severely damage or collapse because of this. Yet, there
are many more apartment buildings in San Francisco that were not damage in the
earthquake and have not been retrofitted. The reason for these buildings not
being damaged, is because they sit on rock or hard material that saw very low
ground accelerations from a long distance earthquake. The next earthquake closer
to San Francisco will have higher ground accelerations and these buildings on
rock will be damage or they will collapse.
WOOD SHEAR WALL DAMAGEExpected structural damage of concrete block buildings
The damage to reinforced masonry building is dependent on the size of the
building and the amount of masonry walls. Large buildings using reinforced
masonry walls(cmu block) as shear walls will be severely damage. The type of
damage will consist of shear cracking of the wall and face shell failure at high
compression zones. Diagonal cracks at the corners of doors and windows will
occur or increase in size due to the earthquake. But the most problematic damage
is the face shell damage in high compression zones. It is extremely difficult to
build a boundary element confinement zone at high compression zones. A pilaster
with confinement reinforcing at these locations is a good solution but the face
shell will degrade. If a 16 inch by 16 inch pilaster was used as the boundary
element at the end of the reinforced masonry shear wall. The confinement zone
would be the thickness of the block plus approximate 3/4 inch or about 13 by 13
inches. What happens to the outside part of the pilaster or face shell, it will
explode as the shear wall is pushed into the in-elastic range. Concrete block is
a very brittle material and has limited ductility. It is the grout and the
reinforcing steel that gives the concrete block wall it strength. Concrete
masonry walls are good for short and small buildings where there is a need for
large overstrenght. Concrete block shall be fully grouted, no partially grouted
block should be used in high seismic areas and in high wind areas.
CMU BLOCK WALL DAMAGEExpected structural damage of Concrete tilt-up wall
Earthquake damage to a tilt-up building could be extensive. All wall panels will
resist the seismic load based on their individual stiffness and fixity at the
base. When the stiffer wall panels start to yield and crack, the less stiff wall
panels will resist a higher seismic force. Many engineers analyze a tilt-up
building assuming the individual wall panels are uncrack sectional properties
and fixed at the base. In reality, the stiff wall elements will crack and the
weak wall panels will be resisting a much higher seismic load then originally
detailed. Thus, there will be some potential failure of these weak elements. But
the main issue of tilt-up construction is the anchorage of the wall panels at
the base and the thickness of the wall panel at the boundary element.
Out-of-plane buckling of the wall panel at the boundary element is possible.
Tension failure of the hold down at the other end is possible. When one or both
occurs, the wall panel will rock substantially and the resulting displacements
will cause damage. Furthermore, these large displacements will affect the
out-of- plane loads in the diaphragm anchorage. The other major damage is the
diaphragms themselves for more see diaphragm web page.