Moment frames are an inherited high interstory drift system. Considerable nonstructural 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.

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.

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 boundry element.

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 schmatic representive of coupled shear walls.

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.

All information, opinions, comments, etc, expressed on this web page are this structural engineer's professional opinions, comments, etc. and are not intended to harm any other engineer's professional opinion, comments, buildings or projects.

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Seismic Resisting Systems damage