13.4 FOUNDATIONS FOR SINGLE-FAMILY HOUSES

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1 13.32 CHAPTER THIRTEEN FIGURE The excavation for the grade beams is complete, and the tops of the prestressed piles are trimmed so that they are relatively flush. fissuring and sand boils, then this layer may provide passive resistance for the piles, caps, and grade beams. 4. Liquefaction of sloping ground: For liquefaction of sloping ground, there will often be lateral spreading of the ground, which could shear off the piles. One mitigation measure consists of the installation of compaction piles (see Sec ), in order to create a zone of nonliquefiable soil around and beneath the foundation FOUNDATIONS FOR SINGLE-FAMILY HOUSES In southern California, the type of foundation for single-family houses often consists of either a raised wood floor foundation or a concrete slab-on-grade.

2 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE Close-up view of one of the prestressed piles showing a trimmed top surface with the strands extending out the top of the pile. FIGURE Close-up view of the top of a prestressed pile with the steel reinforcement from the grade beam positioned on top of the pile. The strands from the pile are attached to the steel reinforcement in the grade beam.

3 13.34 CHAPTER THIRTEEN FIGURE Overview of the steel reinforcement positioned within the grade beam excavation Raised Wood Floor Foundation The typical raised wood floor foundation consists of continuous concrete perimeter footings and interior (isolated) concrete pads. The floor beams span between the continuous perimeter footings and the isolated interior pads. The continuous concrete perimeter footings are typically constructed so that they protrude about 0.3 to 0.6 m (1 to 2 ft) above the adjacent pad grade. The interior concrete pad footings are not as high as the perimeter footings, and short wood posts are used to support the floor beams. The perimeter footings and interior posts elevate the wood floor and provide for a crawl space below the floor. In southern California, the raised wood floor foundation having isolated interior pads is common for houses 30 years or older. Most newer houses are not constructed with this foundation type. In general, damages caused by southern California earthquakes have been more severe to houses having this type of raised wood floor foundation. There may be several different reasons for this behavior: 1. Lack of shear resistance of wood posts: As previously mentioned, in the interior, the raised wood floor beams are supported by short wood posts bearing on interior concrete pads. During the earthquake, these short posts are vulnerable to collapse or tilting.

4 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE The top of the steel pile separated from the concrete pile cap during the Kobe earthquake on January 17, (Photograph from the Kobe Geotechnical Collection, EERC, University of California, Berkeley.) 2. No bolts or inadequate bolted condition: Because in many cases the house is not adequately bolted to the foundation, it can slide or even fall off the foundation during the earthquake. In other cases the bolts are spaced too far apart, and the wood sill plate splits, allowing the house to slide off the foundation. 3. Age of residence: The houses having this type of raised wood floor foundation are older. The wood is more brittle and in some cases weakened due to rot or termite damage. In some cases, the concrete perimeter footings are nonreinforced or have been weakened due to prior soil movement, making them more susceptible to cracking during the earthquake. 4. Crawl-space vents: To provide ventilation to the crawl space, long vents are often constructed just above the concrete foundation, such as shown in Fig These vents provide areas of weakness just above the foundation. All these factors can contribute to the detachment of the house from the foundation. For example, Fig shows the sliding of the house off the foundation caused by the San Fernando earthquake.

5 13.36 CHAPTER THIRTEEN FIGURE The top of the concrete pile separated from the concrete pile cap during the Kobe earthquake on January 17, (Photograph from the Kobe Geotechnical Collection, EERC, University of California, Berkeley.) Besides determining the type of foundation to resist earthquake-related effects, the geotechnical engineer could also be involved with the retrofitting of existing structures. As previously mentioned, the raised wood floor with isolated posts is rarely used for new construction. But there are numerous older houses that have this foundation type, and in many cases, the wood sill plate is inadequately bolted to the foundation. Bolts or tie-down anchors could be installed to securely attach the wood framing to the concrete foundation. Wood bracing or plywood could be added to the open areas between posts to give the foundation greater shear resistance and prevent the house from sliding off the foundations, such as shown in Fig Slab-on-Grade In southern California, the concrete slab-on-grade is the most common type of foundation for houses constructed within the past 20 years. It consists of perimeter and interior continuous footings, interconnected by a slab-on-grade. Construction of the slab-on-grade begins with the excavation of the interior and perimeter continuous footings. Steel reinforcing bars are commonly centered in the footing excavations, and wire mesh or steel bars are used as reinforcement for the slab. The concrete for both the footings and the slab is usually placed at the same time, to create a monolithic foundation. Unlike the raised wood floor foundation, the slab-on-grade does not have a crawl space. In general, for those houses with a slab-on-grade, the wood sill plate is securely bolted to the concrete foundation. In many cases, an earthquake can cause the development of an exterior crack in the stucco at the location where the sill plate meets the concrete founda-

6 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS FIGURE Sliding of house off the foundation caused by the San Fernando earthquake in California on February 9, The house is located in the city of San Fernando, near Knox and Grove Streets. (Photograph from the Steinbrugge Collection, EERC, University of California, Berkeley.) tion. In some cases, the crack can be found on all four sides of the house. The crack develops when the house framing bends back and forth during the seismic shaking. For raised wood floor foundations and the slab-on-grade foundations subjected to similar earthquake intensity and duration, those houses having a slab-on-grade generally have the best performance. This is because the slab-on-grade is typically stronger due to steel reinforcement and monolithic construction, the houses are newer (less wood rot and concrete deterioration), there is greater frame resistance because of the construction of shear walls, and the wood sill plate is in continuous contact with the concrete foundation. Note that although the slab-on-grade generally has the best performance, these houses can be severely damaged. In many cases, these houses do not have adequate shear walls, there are numerous wall openings, or there is poor construction. The construction of a slabon-grade by itself is not enough to protect a structure from collapse if the structural frame above the slab does not have adequate shear resistance.

7 13.38 CHAPTER THIRTEEN California Northridge Earthquake The Northridge earthquake, which occurred in California on January 17, 1994, struck an urban area that primarily contained single-family dwellings. The type of foundation for the single-family houses was a major factor in the damage caused by the Northridge earthquake. Particulars concerning the Northridge earthquake are as follows (Day 1999, USGS 1994): The Northridge earthquake had a magnitude of 6.7 and occurred beneath the San Fernando Valley on a deeply buried blind thrust fault that may be an eastern extension of the Oak Ridge fault system. The fault plane ruptured from a depth of about 11 mi (17.5 km) upward to about 3 mi (5 km) beneath the surface. For 8 s following the initial break, the rupture propagated upward and northwestward along the fault plane at a rate of about 2 mi/s (3 km/s). Fortuitously, the strongest seismic energy was directed along the fault plane toward sparsely populated areas north of the San Fernando Valley. The earthquake deformed the earth s crust over an area of 1500 mi 2 (4000 km 2 ), forcing the land surface upward in the shape of an asymmetric dome. The dome manifests features and consequences of blind thrust faulting that might lead scientists to the discovery of similar faults elsewhere. The lack of clear surface rupture in 1994 may be explained by fault movement terminating at depth against another fault that moved in the 1971 San Fernando event. Studies of more than 250 ground-motion records showed that peak accelerations during the earthquake generally exceeded those predicted. At several locations, horizontal peaks were close to or exceeded 1g, and at one station, vertical acceleration exceeded 1g. Ground motions both near and far from the fault contained consistent, high-energy pulses of relatively long duration. Midrise to high-rise steel structures designed for lesser motions were particularly vulnerable to these pulses. In general, the ratio of horizontal to vertical shaking was similar to that of past earthquakes, and the motions, although strong, were not unusual. There was collapse of specially designed structures such as multistory buildings, parking garages, and freeways. In some areas, the most severe damage would indicate a modified Mercalli intensity of IX, although VII to VIII was more widespread. Because the Northridge earthquake occurred in a suburban community, damage to single-family houses was common. Numerous structural failures throughout the region were evidence of significant deficiencies in design or construction methods. Steel frames of buildings intended for seismic resistance were cracked, and reinforced concrete columns were crushed. Most highway structures performed well, but freeways collapsed at seven sites, and 170 bridges sustained varying degrees of damage. Damage estimates varied considerably. For both public and private facilities, the total cost of the Northridge earthquake was on the order of $20 to $25 billion. This makes the Northridge earthquake California s most expensive natural disaster. Given the significant damage caused by this earthquake, the number of deaths was relatively low. This was partly because most people were asleep at home at the time of the earthquake (4:31 a.m.). The observed foundation damage caused by the California Northridge earthquake indicated the importance of tying together the various foundation elements. To resist damage during the earthquake, the foundation should be monolithic with no gaps in the footings or planes of weakness due to free-floating slabs. For new construction in southern California, many single-family houses are being constructed with post-tensioned slab-on-grade (see Fig ). This type of foundation has an induced compressive stress due to the tensioning of the steel tendons embedded in the foundation concrete. Because of the compression

8 FOUNDATION ALTERNATIVES TO MITIGATE EARTHQUAKE EFFECTS stress and lack of free-floating slab elements, this type of foundation will probably perform even better during an earthquake than the conventional slab-on-grade PROBLEMS 13.1 Use the data from Prob and Fig and assume a level-ground site. A proposed building will have a deep foundation system consisting of piles that are driven into the Flysh claystone. Assuming that the piles are widely spaced and do not increase the liquefaction resistance of the soil, calculate the differential movement between the building and adjacent ground. Answer: Using Fig. 7.1, differential movement 20 cm. Using Fig. 7.2, differential movement 14 cm Use the data from Prob and an effective friction angle φ between the pile surface and the surface soil layer and sand layer of 28. Assume that k and that the last location for the earthquake-induced pore water pressures to dissipate will be just above the clayey fine sand layer. Further assume that the clayey fine sand layer and the silty fine sand layer are not anticipated to settle during the earthquake. If the piles are 0.3 m in diameter, calculate the down-drag load on each pile due to liquefaction at the site. Answer: Down-drag load 61 kn Use the data from Prob and Fig To prevent liquefaction-induced settlement of the building, what is the minimum length of piles that should be installed at the site? Answer: 20-m-long piles. FIGURE Construction of a post-tensioned foundation for a single-family residence.