1835 Shear

Most building structures use shear walls or moment-resisting frames to resist lateral forces during earthquakes. Damage to these systems varies from minor cracks to complete collapse. Figure 18.48 is a photo of the Mt. McKinley Apartments after the 1964 Great Alaska Earthquake. It was a 14-story RC building composed of narrow exterior shear walls and spandrel beams (Figure 18.48), as well as interior and exterior columns and a central tower. During the 1964 earthquake, this structure suffered major structural damage to most of the load bearing members.

The most serious damage was to a shear wall on the north side of the building (Figure 18.49). A very wide shear crack split the wall in two directly under a horizontal beam. This crack was because there was not enough transverse reinforcement to hold the wall together as it moved transversely and also due to a cold joint in the concrete at that location. The spandrel beams between the walls had large "X" cracks

Kobe Earthquake
FIGURE 18.41 Nine-story SRC building immediately after the 1995 Kobe Earthquake. (Photo courtesy of NIST; NIST, The January 17, 1995 Hyogoken-Nanbu (Kobe) Earthquake, U.S. NIST, Gaithersburg, MD, 1996.)
FIGURE 18.42 Eventual collapse of nine-story SRC building after the Kobe Earthquake. (Photo courtesy of NIST; NIST, The January 17, 1995 Hyogoken-Nanbu (Kobe) Earthquake, U.S. NIST, Gaithersburg, MD, 1996.)
FIGURE 18.44 The Ji Lu cable-stayed bridge after the 1999 Ji Ji, Taiwan, earthquake.
FIGURE 18.45 Damage at end-supports to Ji Lu cable-stayed bridge. Copyright 2005 by CRC Press
FIGURE 18.46 Column damage to the Minatogawa interchange during the Kobe Earthquake.

associated with shear damage as the building moved back and forth. These cracks decreased in size on the upper floors.

There was also shear damage to many of the columns. Figure 18.50 is one of the exterior columns on the south side of the building with a diagonal shear crack. Again, the problem was insufficient transverse reinforcement to resist the large shear forces that occurred during the earthquake. The central tower was also damaged. However, the Mt. McKinley Apartment can be viewed as a success since there was no collapse and no lives lost during this extremely large earthquake.

Bridges are equally susceptible to shear damage. For instance, there was considerable shear damage to piers on elevated Route 3 during the Kobe Earthquake. The superstructure is mostly steel girders and the substructure is RC single-column bents. Between Piers 148 and 150, the superstructure is a three-span, continuous, double-steel box with span lengths of 45 m, 75 m (between Piers 149 and 150), and 45 m. Pier 149 is a 10-m-tall by 3.5-m square RC single-column bent supported on a 12-m square pile cap.

FIGURE 18.48 West elevation of the Mt. McKinley Apartment building after the 1964 Great Alaska Earthquake. (Photograph by Karl Steinbrugge, Earthquake Engineering Research Center, University of California, Berkeley.)
FIGURE 18.49 Damage to north side of Mt. McKinley Apartments. (Photograph by Karl Steinbrugge, Earthquake Engineering Research Center, University of California, Berkeley.)

Pier 150 is a 9.1-m-tall by 3.5-m square RC single-column bent supported on a 14.5-m by 12-m rectangular pile cap.

Figure 18.51 shows the shear failure at Pier 150. This damage was the result of insufficient transverse reinforcement and poor details. During the large initial jolt (amplified by near-field directivity effects) the transverse reinforcement came apart and resulted in the column failing in shear. Pier 149 was also severely damaged, but because it was taller (and more flexible), most of the force went to stiff Pier 150. The three-span continuous superstructure survived the collapse of Pier 150 with minor damage.

FIGURE 18.50 Damage to the south side of Mt. McKinley Apartments. (Photograph by Karl Steinbrugge, Earthquake Engineering Research Center, University of California, Berkeley.)
FIGURE 18.51 Shear failure of Pier 150 on Kobe Route 3.

18.3.6 Flexural Failure

Flexural members are often designed to form plastic hinges during large earthquakes. A plastic hinge allows a member to yield and deform while continuing to support its load. However, when there is insufficient confinement for RC members (and insufficient b/t ratios for SRC members), a flexural failure will occur instead. Often, flexural damage is accompanied by compression or shear damage as the capacity of the damaged area has been lowered.

The Dakkai subway station in Japan is a two-story underground RC structure. It was constructed by removing the ground, building the structure, and then covering it (the-cut-and-cover method). During the 1995 Kobe Earthquake, the center columns on both levels suffered a combination of flexural and compression damage that caused both roofs to collapse along with a roadway that ran above the station (Figure 18.52). Figure 18.53 shows the rather slender center columns at the lower level after the earthquake. The columns had insufficient transverse reinforcement at the location of maximum moment. The transverse reinforcement broke as the columns were displaced, allowing the longitudinal reinforcement to buckle and the concrete to fall out of the column.

FIGURE 18.52 Plan and section drawings of the Dakkai subway station after the Kobe Earthquake. (Drawing courtesy of JSCE, JSCE, Preliminary report on the Great Hanshin Earthquake, January 17, 1995, JSCE, 1995.)
FIGURE 18.53 Flexural damage to columns at lower level of Dakkai subway during the 1995 Kobe Earthquake. (Photo courtesy of JSCE, JSCE, Preliminary report on the Great Hanshin earthquake, January 17, 1995, JSCE, 1995.)

Steel columns experience similar damage when the flexural demand exceeds the capacity. In downtown Kobe, in the Nagata District, Route 3 splits into two parallel structures with the superstructure composed of 50-m-long simple spans with steel girders and a three-span, continuous, steel box girder section between Piers 585 and 588. The substructure consists of 2.2-m-diameter, 14-m-tall steel hammerhead single columns. The steel columns are bolted onto 4-m-diameter hollow, concrete 20-m-long shafts. The column bottoms are filled with concrete to protect against vehicular impact. During the Kobe Earthquake, these steel columns had damage varying from local buckles to a complete section buckle, and at a few locations, the steel shell had torn, splitting the column. Most of the buckling occurred in a thinner section of the column. In some cases (Figure 18.54), the column underwent an excursion in only one direction and consequently had a buckle on one side of the column.

In some cases the buckled face tore in the tension cycle (Figure 18.55). The tears occurred in low-ductility welds. After the earthquake, the columns were tilting dangerously to the side. Buckling occurred before a plastic hinge was formed. A few columns remained undamaged as a result of failed bearings. Although local buckling cannot be completely eliminated, its spread can be prevented by maintaining smaller b/t ratios. This is accomplished with thicker sections, more frequent stiffeners, and diaphragms, or by filling the steel shells with lightweight concrete.

FIGURE 18.54 Pier 585 on Kobe Route 3 during the 1995 Kobe Earthquake. (Photo courtesy of Hanshin Expressway Public Corporation.)
FIGURE 18.55 Torn buckle of steel column on Route 3 after the 1995 Kobe Earthquake. (Photo courtesy of Hanshin Expressway Public Corporation.)

18.3.7 Connection Problems

The most catastrophic type of structural damage is the failure of connections. When a bridge superstructure moves off its expansion joint or when the connection between building columns and beams fail, the result is too often the collapse of the structure.

Between Piers 39 and 43 on Kobe Route 3, the superstructure was a series of 52-m-long simple-span steel boxes supporting a concrete deck. Steel web plate restrainers with oversized holes at one end connected the girders together over each pier support. Fixed pin bearings and movable roller bearings supported each span. The substructure was hammerhead piers with 3.5-m-diameter circular or 3.5-m square concrete columns over 10 m in height. Each column was supported on a rectangular footing supported by 18- to 1.0-m-diameter by 16.5-m-long piles.

During the earthquake, the piers moved back and forth longitudinally, sustaining cracks or shear damage at the column bases. At the same time, the girder spans were moving west (toward Pier 43) as a result of the impact with the five-span, continuous, steel girder bridge east of Pier 38. The relative displacement between the piers and the simple spans exceeded the 0.8-m seat width at Piers 40 and 41 and the expansion end of Spans 40 and 41 fell off the piers (Figure 18.56). Most of the web plate restrainers failed in tension (Figure 18.57) and almost all of the fixed and expansion bearings were damaged, mostly due to the top shoe pulling out of the bearing (Figure 18.58).

RC structures must be carefully designed to allow the shear transfer at joints. A common kind of residence in Turkey was four- to eight-story buildings composed of concrete columns supporting concrete slabs and infill walls of unreinforced masonry blocks. During the 1999 Kocaeli Earthquake, these buildings collapsed at very low levels of acceleration, killing thousands of people. The large inertia force from these heavy structures had to be carried by the slender columns and by the inadequately reinforced connections (Figure 18.59). Sufficient concrete and reinforcement must be provided to resist the large tension and compression forces in moment-resisting joints during earthquakes. Joints of moment-resisting RC frames should be stronger than the elements that join them.

The connections of steel moment-resisting frames, with detailing recommended by design codes prior to 1994, suffered considerable damage during the Northridge (and Kobe) Earthquakes. During the Northridge Earthquake, over 100 steel moment-resisting frame buildings had some damage to the

Kobe Bridge Collapse
FIGURE 18.56 Superstructure collapse at Spans 40 and 41 on Kobe Route 3.
FIGURE 18.57 Broken restrainer at Pier 39. (Photo courtesy of Hanshin Expressway Public Corporation.)

column to beam connections. As previously mentioned, flexural elements are supposed to form plastic hinges during large earthquakes. However, during the Northridge Earthquake the connection fractured instead. In this connection the beam flanges are welded to the column with a column flange stiffener placed along the column web to provide continuity to the joint. The beam web is bolted to the column with a shear connection plate. As shown in Figure 18.60, a large crack fractured the flange and web of some columns. Although this damage was unexpected and looked serious, no building during the Northridge Earthquake was reported to have collapsed as a result of the failure of this connection. Still, a few more cycles of motion (from a much larger earthquake than the magnitude 6.7 event) could easily

FIGURE 18.59 Collapsed reinforced concrete building during the 1999 Kocaeli Earthquake.

tear apart this fractured column. Consequently, new connection details have been developed as well as retrofit details for the many buildings with this connection.

18.3.8 Problem Structures

Some types of structures have performed particularly poorly during previous earthquakes. Usually, this is the result of vulnerabilities such as weak connections, improper detailing, and eccentric loads that predispose these structures to severe damage and collapse during earthquakes.

Unreinforced masonry should never be used to resist lateral forces since it is very weak in tension and very heavy, resulting in walls that immediately fall over, seriously injuring anyone nearby (Figure 18.61). However, reinforced masonry walls have performed very well when thoughtfully designed.

Northridge Connections Failure
FIGURE 18.60 Steel moment-resisting frame connection after the 1994 Northridge Earthquake. (Photo courtesy of EQE International; EQE, EQE Int. Rev., Fall, 1-6, 1994.)
FIGURE 18.61 Rear view of damaged unreinforced masonry (URM) building facing Pacific Garden Mall, Santa Cruz Area, California. (Photo by James R. Blacklock, Loma Prieta Collection, Earthquake Engineering Research Center, University of California, Berkeley.)

Tilt-up buildings have also performed poorly during past earthquakes because the walls pull away from the roof diaphragm, because of the discontinuity at the vertical joints between panels, because of poor connections to the roof joists, etc. Similarly, precast prestressed bridges tend to fall apart due to inadequate connections between members.

Tanks, bins, silos, grain elevators, concrete mix plants, etc. are the most commonly damaged structures during earthquakes. They are tall, heavy, and too often designed with weak supports and inadequate anchors (Figure 18.62).

Construction sites are particularly dangerous places to be on during an earthquake. Too often, little thought is given to providing lateral strength to partially constructed structures. This can result in millions of dollars in damage as well as fatalities. For instance a new expressway was being built during the 1999 Ji Ji, Taiwan, earthquake. This included two simple-span, precast, and prestressed "I" girder bridges on two-column bents near the epicenter. The girders were 84 in. tall by 24 in. wide and sat on

FIGURE 18.62 California Water Service 150,000-gal tank on six legs; the typical failure of tanks is upside down, with riser on top (Bakersfield, California, photo by Karl V. Steinbrugge, Steinbrugge Collection, slide/image No. S64, photo date July 25, 1952, Kern County, California, earthquake, July 21, 1952, magnitude 7.69; courtesy of Steinbrugge Collection, Earthquake Engineering Research Center, University of California, Berkeley).

FIGURE 18.62 California Water Service 150,000-gal tank on six legs; the typical failure of tanks is upside down, with riser on top (Bakersfield, California, photo by Karl V. Steinbrugge, Steinbrugge Collection, slide/image No. S64, photo date July 25, 1952, Kern County, California, earthquake, July 21, 1952, magnitude 7.69; courtesy of Steinbrugge Collection, Earthquake Engineering Research Center, University of California, Berkeley).

FIGURE 18.63 Collapsed precast, prestressed T girders during construction of new expressway in Taiwan due to the 1999 Ji Ji, Taiwan, earthquake.

24 in. by 16 in. by 5 in.-tall elastomeric pads. The bents were at least 30 ft tall from the ground to the top of the bent caps. The bridge had 13 approximately 150-ft-long spans. At the time of the earthquake, the girders had been placed on the bents and the seat-type abutments, and the intermediate and end-diaphragms were just beginning to be cast between the girders. Wherever the intermediate diaphragms had not been placed, the girders had been shaken off of their supports and had fallen to the ground, breaking into pieces (Figure 18.63). This resulted in the loss of about 100 girders. The use of temporary supports before the casting of diaphragms could have saved about a million dollars.

All of these structures can be designed to perform adequately when they are provided with sufficient strength and ductility and a good understanding of their structural behavior during earthquakes.

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