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 goes inelastic.
MOMENT FRAME DAMAGE
Expected 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 similar work.
Expected 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
Expected 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 DAMAGE
Expected structural damage of Coupled Concrete Shear Walls
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 DAMAGE
Expected structural damage of an base isolated building
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 DAMAGE
Expected structural damage of Moment frames Systems in Concrete
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 DAMAGE
Expected 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 will split.
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 high.
- 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 construction.
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 DAMAGE
Expected 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 DAMAGE
Expected structural damage of Concrete tilt-up wall buildings
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.