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  • 1
    ISSN: 1435-5663
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract In the conceptual phase of structural design a designer develops and investigates many potential alternatives for safe and economic transfer of loads that are to be carried by the structure. A methodology for automating conceptual structural design is presented in this paper. Some of the salient aspects of the methodology are: (1) an explicit representation of the structural form, function, and behavior; (2) modeling the structural engineering domain as well as the strategy employed by expert designers; (3) usingcost/value ratio as an intrinsic measure of the merit of a design alternative; and (4) reduced reliance on heuristics with more emphasis on first principles and fundamental knowledge. The categories of knowledge that need to be represented in a computer system to support the reasoning for conceptual structural design are identified. The use of such knowledge is illustrated through examples based on several different types of structures. A constraint classification system (to organize the constraints that arise from structural and exogenous considerations) is also proposed.
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  • 2
    ISSN: 1435-5663
    Source: Springer Online Journal Archives 1860-2000
    Topics: Computer Science , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Technology
    Notes: Abstract The function of the structural system in a building is to transfer gravity as well as lateral loads from their points of origin to the ground. Floor framing generation involves providing a path to transfer the gravity loads to the ground through various structural elements in an architectural plan while meeting the requirements imposed by other entities, such as the architect, the mechanical engineer, and the contractor, involved in the design/construct process. In this paper a formal approach for generating floor framing plans for steel office buildings is presented. We describe the knowledge and the reasoning behind a computer system, FFG (floor framing generator), which generates floor framing schemes for steel office buildings that are rectangular in plan and have a single service core. Constraints arising from structural as well as exogenous considerations are enumerated and their effect on framing schemes is identified. We also elaborate on the evaluation mechanism for ranking alternative schemes, in addition to providing details of the computer implementation.
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  • 3
    Publication Date: 1991-10-01
    Description: We have used 24 broadband teleseismic and 48 components of local strong-motion velocity records of the 1989 Loma Prieta earthquake in a formal inversion to determine the temporal and spatial distribution of slip. Separate inversions of the teleseismic data (periods of 3 to 30 sec) or strong-motion data (periods of 1 to 5 sec) result in similar models. The data require bilateral rupture with relatively little slip in the region directly updip from the hypocenter. Slip is concentrated in two patches: one centered 6 km northwest of the hypocenter at a depth of 12 km and with a maximum slip of 350 cm, and the other centered about 5 km southeast of the hypocenter at a depth of 16 km and with a maximum slip of 460 cm. The bilateral nature of the rupture results in large amplitude ground motions at sites located along the fault strike, both to the northwest and the southeast. However, the northwestern patch has a larger moment and overall stress drop and is, consequently, the source of the largest ground motion velocities, consistent with the observed recordings. This bilateral rupture also produces relatively modest ground motion amplitudes directly updip from the hypocenter, which is in agreement with the velocity ground motions observed at Corralitos. There is clear evidence of a foreshock (magnitude between 3.5 and 5.0) or a slow rupture nucleation about 2 sec before the main part of the rupture; the origin time implied by strong-motion trigger times is systematically 2 sec later than the time predicted from the high-gain regional network data. The seismic moment obtained from either of the separate data sets or both sets combined is about 3.0X10 (super 26) dyne-cm and the potency is 0.95 km (super 3) .
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 4
    Publication Date: 1991-10-01
    Description: Two weeks after the 18 October 1989 Loma Prieta, California, earthquake, 18 three-component digital seismometers were deployed in the epicentral area to form three six-station subarrays. The subarray configuration allowed us to investigate the presence of direction- and frequency-dependent site resonances. We measured the shear-wave polarization from the recordings of 10 aftershocks from the Loma Prieta earthquake. Our observations show that the site response has a strong azimuthal dependence and that both the shear-wave polarization and the spectral amplitude of the ground motions are affected by site characteristics. In the frequency range from 1 to 18 Hz, the majority of stations examined showed preferred azimuths of ground motion for the scattered waves that did not depend either on the earthquake location or on the polarization of the shear waves expected from the known focal mechanism. The measurements were made from 5-sec windows that included direct and scattered shear waves, which contain the largest amplitude motions in the near-source region and are therefore of most interest to earthquake engineers. However, in the 0- to 2-Hz frequency range, the first pulse of shear waves shows a polarization that is well predicted by the mechanism and location of the earthquake. The rapid spatial variation of the preferred directions and their corresponding frequencies indicate that geologic structures within a distance of the order of 50 m probably control these site effects. We suggest that the site amplified the motion of scattered waves in one preferred direction, altering the resulting polarization and modulating the spectral amplitude.
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  • 5
    Publication Date: 1991-10-01
    Description: Following the 18 October 1989 Loma Prieta earthquake, triggered seismographs were deployed in and around the Marina District to investigate site amplification. In the 3 weeks after the mainshock, 16 aftershocks were recorded by two or more of these stations. Two additional stations, deployed in March 1990, recorded three earthquakes that occurred in Contra Costa County. By recasting the method of spectral ratios into a generalized inverse problem, we combine the shear-wave spectra from these 19 aftershocks to estimate the relative site amplification as a function of frequency. All five stations located in the Marina District are amplified by factors of 6 to 10 at 1 Hz and 2 to 4 at 3 Hz, relative to a station located on Franciscan sandstone at Fort Mason, irrespective of whether they are sited on artificial fill or beach sand. Two stations located on dune sand just outside the Marina District are amplified by factors of 2 to 4 for frequencies above 1 Hz. Four stations located on Franciscan sandstone in Pacific Heights, Nob Hill, Rincon Hill, and Diamond Heights show little relative amplification. Conditional estimates of the mainshock spectra can be determined at stations that only recorded aftershocks, if they can be linked to stations that recorded the mainshock. These estimates are predicated on the assumption that the ground behaved linearly in the mainshock, an assumption that is clearly violated for those sites where there was ground failure. Using this method of extrapolation for a site in the Marina District that showed no evidence of ground failure yields mainshock spectra that are slightly greater than the mainshock spectra from the Outer Harbor Wharf in Oakiand and near the Pacific Park Plaza in Emeryville and that significantly exceed the mainshock spectra from all the free-field accelerograms located in San Francisco.
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  • 6
    Publication Date: 1991-10-01
    Description: Following the Loma Prieta earthquake, a study was initiated to evaluate the liquefaction potential of fill soils in San Francisco. Field investigations were conducted at several sites along the San Francisco waterfront where pre-earthquake data were available and/or the field performance during the 1989 event was well documented. Based upon the interpretation of cone penetration test data, several areas underlain by zones of dune sand present in the fills appear to have densified when compared to pre-earthquake data. These fill sands were in a loose-to-medium dense state prior to the 1989 earthquake. Although several steps of the interpretation require assumptions at this stage of the research project, the liquefaction assessments made for Loma Prieta type conditions correlate well with the observed performance of the different sites. It is shown that a number of locations would suffer severe damage during a postulated magnitude 7.5 event occurring close to San Francisco, and many sites would be affected to a lesser degree. Even engineered fills may suffer some level of distress because of zones of looser material at shallow depths.
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  • 7
    Publication Date: 1991-10-01
    Description: We considered the effects of source, propagation path, and local site conditions on strong ground motion from the Loma Prieta earthquake in the following steps. First, the local site effect was removed from the strong motion records applying the weak-motion amplification factors estimated from nearby stations of USGS regional network using the coda method. Then, we confirmed the approximate validity of the I/R law of geometrical spreading factor for the crustal model applicable to the region, and we determined the attenuation parameter 0(f) and source spectrum S(f) empirically from the site-effect-corrected spectral amplitude. Both Q(f) and S(f) are consistent with the results obtained earlier for major California earthquakes. We then synthesized the time history of ground acceleration by applying the stochastic simulation technique. The agreement between the observed and predicted records was good for duration and spectral content. We found, however, a strong systematic discrepancy with regard to peak acceleration; namely, for distances less than 50 km, the predicted peak acceleration consistently overestimated the observed for sediment sites and underestimated it for the Franciscan formation sites. Since the above systematic discrepancy applies to all the available stations within about 50 km from the hypocenter, it is unlikely that the effect is due to the radiation pattern, near-source structure, or topography. It appears that the strong difference in the amplification factor between the Franciscan and the sediment sites observed for weak motion disappears at acceleration levels higher than about 0.1 to 0.3 g. if this is due to the nonlinear response of the sediment sites, it is in the range expected from geotechnical engineering studies.
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  • 8
    Publication Date: 1991-10-01
    Description: A total of 137 ground-level accelerograms recorded during the 18 October 1989 M (sub s) = 7.1 Loma Prieta, California, earthquake were used to study the dependence of peak horizontal acceleration on distance, azimuth, and site geology. According to this analysis: (1) peak accelerations recorded on alluvium at distances of 50 to 79 km from the inferred seismogenic rupture zone exhibited little or no attenuation with distance; (2) peak accelerations recorded on alluvium at seismogenic distances exceeding 50 km were significantly higher than those predicted from attenuation relationships available from the literature; (3) both the amplitude and rate of attenuation of peak accelerations recorded on alluvium exhibited a strong dependence on azimuth, with accelerations recorded at azimuths corresponding to San Francisco and Oakland having 68% higher amplitudes and a lower rate of attenuation than those recorded at more northerly azimuths; and (4) peak accelerations recorded at all azimuths exhibited a strong dependence on site geology, with Bay Mud sites having 88% higher accelerations, soft-rock sites having 15% lower accelerations, and hard-rock sites having 32 per cent lower accelerations than alluvial sites. The dependence of peak acceleration on azimuth is consistent with the geographic distribution of Modified Mercalli intensity. Although it is possible to explain both the low rate of attenuation and the constant level of acceleration at distances of 50 to 79 km by the arrival of critical reflections from the base of the crust (Moho), it is also possible that source directivity and radiation pattern, path-dependent attenuation, and systematic differences in local site conditions could have been partly responsible for these observations.
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  • 9
    Publication Date: 1991-10-01
    Description: I have analyzed records from 20 strong-motion instruments that recorded the 1989 Loma Prieta earthquake (M (sub L) = 6.9) to determine the spatial and temporal distribution of slip using a tomographic back-projection technique. I find that the earthquake ruptured bilaterally at approximately 80% of the local shear-wave velocity over a distance of slightly more than 30 km from 13 km northwest to 20 km southeast of the hypocenter. Slip within the rupture zone was highly variable both in magnitude and direction. Slip at the hypocenter and immediately up dip was relatively low, about 1 m. There were two main areas of concentrated slip, one centered 7 km northwest of the hypocenter at a depth of 14 km and the other centered 6 km southeast of the hypocenter at a depth of 12 km. Peak slip amplitude on these high-slip patches exceeded 4.5 m. A surprising aspect of the rupture model is that rake varies from being predominately strike-slip to the southeast of the hypocenter to being predominately reverse slip to the northwest. Despite this variation in rake, the average rake determined from the strong-motion data is in agreement with that determined teleseismically. There is a correlation between areas of high slip and areas of low aftershock activity. A simple model to explain this observation is that the areas of high slip are also areas of high strength, which slip only during infrequent large mainshocks. Other areas of the fault are weaker and may slip both seismically and aseismically. Most of the slip in the Loma Prieta earthquake occurred between 9 and 16 km depth on a structure that dips to the southwest and runs underneath the surface trace of the San Andreas fault at a depth of 8 to 10 km. This observation complicates the assessment of seismic hazard on the southern Santa Cruz Mountains segment of the San Andreas fault.
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  • 10
    Publication Date: 1991-10-01
    Description: A northwest-trending valley in the bedrock surface is buried by firm Pleistocene bay clay, a dense Pleistocene sand layer, soft Holocene bay sediments, loose to dense Holocene beach and dune sands, and artificial fill that have an aggregate maximum thickness of about 90 m (300 ft). Artificial filling of a cove at the site of the Marina District proceeded gradually from the late 1860s to 1912, when major hydraulic filling was done for the Panama-Pacific International Exposition. The remains of thousands of piles driven for the Exposition very probably still exist and have had unknown effects on long-term ground settlement and earthquake-related ground displacements. Intensity maps of the 1906 earthquake, and seismic recordings and severe building damage in 1989, reported by others, indicate that ground motion was amplified on both natural and artificial ground. This suggests that the configuration of the bedrock surface and the location and thickness of various clay and sand deposits underlying the fill had an important effect on the shaking. However, most of the settlement and liquefaction and the damage to pipelines, building foundations, streets, sidewalks, and curbs occurred in areas of artificial fill consisting mainly of loose sand.
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