It was still one earthquake. In some locations though, the soil filtered the seismic waves in a such way as to make it feel like two.

Had the slabs, during the shaking, been rigid enough to distribute the dynamic loads uniformly onto the different vertical elements, I think this building would not have survived. There was much destruction shown on the transverse walls. A detailed analysis could show if this could have been the case (or not). I am not aware of an ACI criterion for modeling a slab under seismic loads.

To Donald R. Logan:

Thank you for your nice words.

The behavior during the 1985 earthquake and the behavior during this earthquake were notably similar. Buildings with similar wall widths, 25 cm and larger, and non-well-detailed ends behaved the same. However, when wall widths were reduced, but without increasing the wall end details; then, the performance shown was dramatically different. These cases were the cases that showed the most damage. Longitudinal walls are still used, but not at the exterior grid lines (to make room for parking, and unobstructed views, sometimes, from floors second and up).

I do not think there is a specific requirement by the code in terms of shear walls and their distribution. The practice changed as explained in the previous paragraph, which can be summarized as sizing by ACI, detailing by Chilean practice to achieve an “efficient design.” In addition, it is my belief (to be confirmed) that in the analysis a rigid diaphragm approximation was used, which would under estimate the axial and in-plane bending loads going onto the vertical elements, and over estimate the shear. Furthermore, I think the design check might not have been done considering the interaction of all forces (axial, bending, shear) going into the wall.

The traditional Chilean building practice of multi-story buildings is based on a shear wall structure. Thus, all of the buildings in Concepción, those performing well as well as those that were damaged, are structured in base of shear walls. The difference was the “efficient design” of the buildings built starting in the late 1980s combined with poor detailing of the steel, plus non-conservative approximations, plus orientation of the weaker walls along the direction of highest acceleration, plus foundation rotation induced by soil-structure interaction.

In the 1985 Vina del Mar earthquake, the use of orthoganally oriented (or equivalent) thick shearwalls with no special detailing at their ends was prevalant, and those mid-rise reinforced concrete shearwall structures performed very well, most with virtually no damage. It appears from the above report that this prevailing practice has changed to using only transverse shearwalls, with serious consequences to these more recent structures.

Did the Codes change from requiring orthoganal layout of shearwalls to permit the current practice? Or was this always a matter of prevailing practice without specific requirements by the Codes?

In Concepcion. were there many mid-rise reinforced concrete shearwall structures that followed the earlier orthogonal shearwall concept, with simple detailing, and, if so, how did they perform?

Thank you,

Donald R. Logan, PE
Logan Structural Research Foundation

Thanks.

This quote (presented today by Jose Restrepo, just returned from earthquake reconnaissance in Chile; at the NCSEA Winter Institute in San Diego) is a variation of the long familiar: “The building experiences the earthquake, not the approved plans.” Peer review for buildings above three stories sounds good, but what is built may not be the same as what was shown on the plans.

Some brief history, as personally reported to me over the years. After the M 9.5 earthquake in 1960 . . .

http://www.drgeorgepc.com/Tsunami1960.html

Cloud, W.K., 1963, An engineering report on the Chilean earthquakes of May 1960 Period measurements of structures in Chile, in BSSA, v. 53, p. 359-380.
(There is a special BSSA volume on the 1960 Chile EQ, but outof-print)

http://en.wikipedia.org/wiki/List_of_earthquakes_in_Chile

It was reported to me that, after that earthquake, Chile sent 6 graduate students abroad and said: “Tell us what happened, and tell us what to do about it.” 2 went to U.C. Berkeley, 2 to M.I.T., and 2 went to UK. When they came back, they all said the same thing: (1) Build a frame to carry the gravity loads; (2) Detail the connections for ductility; and (3) Use shear walls to limit the drift.

After the death (under questionable circumstances) of president Salvador Allende during 1973 coup, and after the military rule established under General Pinochet; Chilean people were prohibited from meeting together in groups. Therefore, associations of structural engineers could not occur either. When the M 8.0 1985 Chile EQ occurred, it was realized that the Chilean building code had not been attended to, nor updated, for a long time. An effort was, therefore, undertaken to examine the performance of buildings in that earthquake, in order to determine the qualities of good performance, and to update the Chilean building code accordingly. A report was written, reviewing the earthquake performance (and the reasons for that observed performance).

One particular reason for the good performance was the very large number of walls in structures.

(See also: The 1985 CHILE EARTHQUAKE: OBSERVATIONS ON EARTHQUAKE RESISTANT CONSTRUCTION IN VINA DEL MAR. http://www.bing.com/search?q=1985+Chile+earthquake+building+code+review&src=IE-SearchBox )

During one tour for U.S. structural engineers, the late Rodriquez Flores was asked: “How do you get so many shear walls in your buildings?” Flores reportedly responded: “In Chile, we have domesticated our architects.”

Frame buildings did much worse than wall structures, but their survival was credited to a minimum base shear of 0.06W required in the building code (I’m remembering). After the Chilean building code was updated, a loophole was found to have been created: in that you could achieve the new design strength requirements — with far fewer walls than had been determined to have been the cause of the good performance.

Ground motions were quite variable over recorded distances and not amenable to representation by simple attenuation equations. Shear walls reported in the 1985 earthquake observations were generally described as of poor construction quality and lacking boundary elements (basically un-designed): but there were so many of them (much more than U.S. buildings, as described by Sharon Wood) that good performance of wall supported buildings was reported.

It would, therefore, be interesting to track the % of building area in walls in 1985 with buildings observed in 2010 to have been severely damaged. Some of the pictures (elsewhere, than right above here; i.e. on clearing house home page) are showing failures in boundary elements, with sometimes dramatic fracturing of very large rebars — due to lack of concrete confinement during the long-duration shaking (reportedly resulting from the use of 90 deg hooks being used in detailing, and therefore opening up during eq shaking).

I’m remembering Professor Bertero’s comment to one of his students from Mexico City during that 1985 EQ: “[Name], you must confine the concrete!” Bertero also expressed concerns for vulnerabilities of high-rise and long period structures at very long distances from the earthquake sources; which seems to me is being observed (at least in some cases) during this recent Mw 8.8 earthquake.

“Nature, to be commanded, must be obeyed.” – Sir Francis Bacon

It appears . . . we have not yet domesticated our earthquakes.

I suspect that Chilean earthquake engineers are as confused as U.S. engineers (particularly code writers) about what a response spectra really represents; and perhaps, in many of the present cases being observed in Chile, assumed incorrectly that that spectrum realistically characterized both their earthquake hazard and their associated earthquake design responsibilities.

( George Housner: EERI Oral History )
http://www.eeri.org/site/images/projects/oralhistory/housner.pdf

call me at +91 9906541770 or type M.G.Hassan Mukhtar in google search.