外文翻译
Loma Prieta Earthquake
1. Commercial Buildings
The Loma Prieta Earthquake and its subsequent aftershocks resulted in widespread damage to a variety of commercial structures. A large geographical area was affected, as is typical for an earthquake of this magnitude. The affected area encompasses eight counties, from Monterey and San Benito in the south to San Francisco, Alameda, and Contra Costa in the north. In total, building structures experienced damage over an area of approximately 3,000 square miles.
Although damage was widespread, it was also quite sporadic. As would be expected, areas closest to the epicenter including Hollister, Los Gatos, Santa Cruz, and Watsonville experienced the most concentrated damage. Farther away, heavy damage was generally limited to buildings of very poor construction founded on soft soils that failed or amplified the earthquake ground motions. This is similar to the effects noted in the 1985 Mexico City Earthquake.
Earthquake effects also tended to be highly directional. Most damage occurred within a narrow band that extends northwest to southeast, approximately paralleling the San Andreas Fault. Thus many communities along the margins of San Francisco Bay escaped serious damage.
2.Unreinforced Masonry Buildings
As has been observed in past California earthquakes, the most concentrated and severe damage to building structures occurred in unreinforced masonry (URM) bearing-wall buildings. URM buildings, constructed of wood-frame roof and floor systems supported by thick unreinforced brick walls, were commonly constructed throughout California until the 1930s, when the adoption of building codes with seismic-resistive provisions prevented their further development. As a result, URM buildings are typically found in the central business districts of older California cities Failures of URM buildings result from inadequate anchorage of the masonry walls to roof and floor diaphragms, as well as the limited strength and ductility of the basic building materials and poor construction workmanship. Deterioration of the sand-lime mortar and wood framing due to weather exposure frequently contributes to poor performance. California has recently enacted legislation (SB 547) requiring cities to
identify URM buildings and develop plans to reduce the risk they present.
Damage to URM buildings in the Loma Prieta Earthquake ranged from dramatic collapses near the epicenter to fallen parapets in Martinez, more than 70 miles away. Life-threatening collapses also occurred in Hollister, Los Gatos, Oakland, and the San Francisco financial district. The roofs and floors in many buildings with collapsed walls seemingly defied gravity by continuing to stand after losing their load-bearing support. Generally, buildings with through-wall anchorage to floor and roof framing performed better than buildings without this feature.
Most URM buildings in the region survived the earthquake without collapse or obvious substantial damage. However, field investigations show that many of these structures have experienced extensive cracking of the masonry and are therefore weakened. If not repaired, some of these buildings are likely to collapse in future earthquakes.
URM buildings with more than three or four stories were generally constructed with steel frames to carry the gravity loads. Masonry walls in these buildings were primarily provided for building closure and partitions and to add lateral shear-resistance to the structure. These steel-frame infill masonry buildings have generally performed better in past California earthquakes than the smaller bearing-wall buildings. Nonetheless, these buildings were included in California legislation addressing the unreinforced masonry hazard.
In the Loma Prieta Earthquake, many steel-frame buildings with infill masonry walls performed quite poorly, although no collapses occurred. Several major structures of this type in San Francisco and Oakland experienced extensive damage including partial loss of the exterior masonry walls, shattering of interior clay tile partitions, and cracking and spalling of terra-cotta veneers.
Many older reinforced concrete structures have very limited seismic resistance. These buildings tend to be quite heavy, resulting in large seismic forces. In addition, concrete itself is quite brittle and requires extensive amounts of reinforcing steel to perform properly in earthquakes. Most concrete structures designed prior to the mid-1970s do not have adequate reinforcing steel to ensure good performance. Termed non-ductile concrete structures by engineers, these structures have collapsed in past earthquakes. The Cypress Viaduct structure that dramatically collapsed was a non-ductile concrete structure.
Fortunately, there are relatively few non-ductile concrete buildings in the region and no buildings of this type collapsed. Many mid-rise concrete structures did experience extensive damage, however. Damage typically consisted of large diagonal cracking of shear walls, occasionally accompanied by spalling of large pieces of concrete from the building
A fifteen-story concrete shear-wall structure in downtown Oakland was extensively damaged. The lightweight concrete shear walls at the first story literally shattered, exposing the reinforcing steel to view. The presence of a redundant steel frame within the building may have prevented the collapse of this structure.
A six-story concrete shear-wall building in San Francisco also experienced substantial damage. This building had recently been seismically strengthened with the addition of steel braces on the building perimeter. Due to poor design and construction workmanship, the attachment of the braces to the building was inadequate and most of the bolts failed. Building deflections following failure of the bracing caused an interior column to punch through a floor slab. Extensive damage to shear walls above door openings also occurred.
Concrete tilt-up buildings are the most common form of modern low-rise industrial and commercial construction throughout California. They usually are constructed with plywood sheathed wood-frame roofs supported by perimeter concrete walls. They are called tilt-ups because the perimeter walls are constructed lying flat against the floor slab and then tilted-up into position around the building. Extensive damage to these buildings has been observed in past earthquakes, including the 1987 Whittier and 1971 San Fernando events.
Thousands of tilt-up buildings are present in the affected region. Although some significant damage was experienced near the epicenter, the greatest concentrations of these buildings are located in areas that experienced very weak ground motion. Most of these buildings therefore had little damage.
Several buildings in Hollister were investigated. Several tilt-up building partially collapsed as the result of inventories of stacked cans containing tomato product impacting the walls during the earthquake. Damage was extensive. Much of the inventory was lost or severely damaged. This underscores the importance of addressing seismic issues relating to equipment and contents, as well as structural designThe buildings appeared to be late 1960s or early 1970s vintage and as such were observed not to have roof-to-wall anchors. These anchors were required following collapses of tilt-ups during the San Fernando Earthquake in 1971. Much of the roof of a half of one building occupied by the adjacent tenant also collapsed, causing extensive interior damage
In Watsonville, several panels of a large tilt-up owned by a food packager bowed outward at mid-height and approached collapse. Concrete spalled off of several of the pilasters in the building wall, exposing the reinforcing steel, some of which buckled. In another, newly constructed Watsonville tilt-up, interpanel connections at a skewed corner were damaged and roof sheathing was severely torn.
5.Steel Buildings
Modern steel-frame buildings performed excellently in this earthquake, as they have in the past. Damage to these structures was typically limited to cracking of cladding and interior partitions and widespread disarray of contents. The nonstructural damage sustained by steel-frame buildings may largely be attributed to their flexibility, which results in very large displacements.
A steel-frame building with corrugated metal siding in Hollister serves as a warehouse for a food company. As with the concrete tilt-up buildings in Hollister, this tomato packing company experienced severe damage to its steel-frame building and significant loss of inventory. Canned tomato products stacked over 30 feet high on standard pallets were mobilized by the earthquake. Most stacks collapsed, impacting and ripping through the corrugated metal side walls.
Impact forces were so great that several tapered steel columns and beams were severely damaged. These steel members twisted, bent, and failed, taking down an entire section of the building. Much of the inventory was lost or severely damaged. As with concrete tilt-up warehouses, this incident highlights the effect that stored inventory can have on the integrity of a building during an earthquake.
Loma Prieta地震
1.商业建筑
Loma Prieta地震和它的余震导致了一系列商业结构的大面积顺坏损坏.正如这个量级的典型地震一样,有一大批地理范围受到了影响.被波及的地区包括8个郡,从南部的Monterey和San Benito,到北部的San Francisco, Alameda, 和 Contra Costa .总共有大约3000平方英里面积的建筑结构遭受到了破坏.
尽管地震带来的破坏面积很广,但是这些破坏很零散.据估计,最接近震中的地区,包括Hollister, Los Gatos, Santa Cruz, 和 Watsonville 经历了最集中的破坏.再远一点,仅仅是那些地基建在松软的土质上的质量非常差的建筑物遭到了严重的破坏,这些土质在地震的运动中中被破坏和坍塌.这和1985年的墨西哥地震产生的影响是相似的.
地震的影响是具有很强的方向性的.大多数破坏发生在从西北向东南方向延伸的狭窄地段,大约与San Andreas Fault平行.因此沿着 San Francisco 湾的许多地区逃脱了这场截难.
2.不稳固的石质建筑物
正如在过去的California 地震中所发现的那样,最集中和严重的建筑物结构破坏发生在不稳固的石质建筑物结构中.不稳固的石质建筑物结构包括木头框架的屋顶和地板,这些屋顶和地板是由很厚的不稳固的墙砖支撑的,这种结构在California一直流行到30年代,那时抗震材料在建筑上的应用阻止了它进一步的发展.因此,不稳固的石质建筑物结构通常在 California 的老城区中的集中商业地段被发现.
不稳固的石质建筑物结构的失败之处在于,屋顶和地板的隔离膜的砖墙抛锚点不够,还有就是基础建筑材料的承重和柔韧性的限制,以及低质的建筑技术.由于天气频繁的作用导致了灰泥和木质框的变质和低质.
California 近来颁布了法令(SB 547)要求鉴别城市中的不稳固的石质建筑物结构,然后制订计划减少它们所带来的风险.
在Loma Prieta地震中导致的不稳固的石质建筑物的破坏,从在震中大的坍塌,到70英里以外的.Martinez的栏杆的倒塌.在Hollister, Los Gatos, Oakland, 和San Francisco的商业区也发生了危及生命的倒塌.许多建筑物中的屋顶和地板以及倒塌的墙壁在失去支撑时仍然保持直立,似乎否定了万有引力的作用.一般说来,
在屋顶和地板之间有连通抛锚点的建筑物比没有这个结构的建筑物的性能要好得多.
在这个地区的大多数不稳固的石质建筑物在地震中没有倒塌或者有明显的损坏.但是,地质调查显示,大多数这种结构的建筑物被过度地拉扯和破裂,因此被削弱了质量.如果不修理,这些建筑物有可能在以后的地震中倒塌.有超过3,4层不稳固的石质建筑物一般是通过钢筋框架来承重的.在这些建筑物中的石墙是用来为建筑分层的,给结构增加侧面抵抗性.这些钢筋框架填充的建筑物在California 过去的地震中一般比小一些的
墙砖结构的建筑物的性能要好.但是,这些建筑物在California 的立法中被归为不稳固石质建筑.在Loma Prieta 地震中,许多石墙钢筋框架的建筑性能很差,尽管它们没有倒塌.在San Francisco 和 Oakland的一些这种类型的大型建筑经历了过度的破坏,包括失去部分外墙,内部黏土层脱落,以及陶瓦板的散裂.
3.混凝土建筑
许多加强型的老式混凝土建筑在抗震方面是非常有限的.这些建筑物非常沉重,导致巨大的地震力量.另外,混凝土本身十分易碎,它要求大量的加强的钢筋在地震中发挥作用.在70年代中期以前设计的大多数混凝土结构没有足够坚固的钢筋来保证其良好的性能.由工程师设计的没有延展性的混凝土结构,在过去的地震中倒塌了.倒塌了的Cypress Viaduct 结构就是一个没有延展性的混凝土结构.
幸运的是,在这个地区的没有延展性的混凝土结构建筑物相对较少,所以这种类型的建筑物没有倒塌的.然而,很多中期兴起的混凝土结构建筑物没有经历过过度的破坏.主要的破坏包括承重墙的斜方向的拉裂,有时伴有从建筑物上落下来的大块的混凝土碎片.一个在Oakland中心的15层的混凝土结构被严重的破坏了.在第一层的轻质混凝土墙逐步破裂,露出了加强钢筋.在建筑物中的额外的钢筋框架可以阻止这种结构的建筑物的倒塌.
在San Francisco的一个6层高的混凝土支撑墙建筑也受到了一定的破坏。这座建筑近来被加以防震措施，在它周围加上了钢筋带。由于设计和建筑工艺处理得不好，建筑物得边框不适当，大多数都垮了。建筑的偏差引起了边框的倒塌，使得内部的柱子打穿了地板的厚板。在门户上的承重墙也受到了普遍的破坏。
4.上倾斜建筑物
混凝土上倾斜建筑是贯穿整个 California的低层工业商业结构中最普遍的形式。他们通常是由盖着木头框架的屋顶的夹板构成，周围是由混凝土支撑的。它们被称为上倾斜，因为它的围墙是平的，而地板是倾斜的，在整个建筑物周围都是斜
的。在过去的地震中，像1987年的Whittier 和1971年的 San Fernando地震。
上千栋上倾斜建筑物位于受影响的地区。尽管一些严重的破坏是在震中发生的，大多数这种结构的建筑是位于受到微震的地区。因此大多数这种建筑物几乎没有遭到破坏。
我们对Hollister 的一些建筑进行了调查。在地震时，由于装着番茄的罐子对墙进行压迫，一些上倾斜建筑物倒塌了。这强调了和地震有关的设备和内容的话题的重要性，还有建筑设计的重要性。这些建筑是在60年代晚期或者70年代早期建造的，因此一般没有屋顶到墙的锚。这些锚导致了1971年San Fernando 地震中上倾斜建筑物的崩溃。有一个建筑物，它一半的屋顶都居住着人的，也倒塌了，引起了内部的广泛的破坏。
在Watsonville，有一个食品包装商的大型的上倾斜建筑，它的一些面板在中间向前方拱出，差一点就倒塌了。柱子上的混凝土碎片脱落了，露出了加固钢，一些被扣住了。另外，一些新近建成的上倾斜建筑物，在歪斜处的的内部连接也遭到破坏，屋顶也被严重地扯坏了。
5.钢结构建筑物
现代钢架建筑物在这次地震中表现得很出色，正如它们在过去的地震中的表现一样。这些建筑遭到的破坏非常有限，仅仅限于覆盖层和内部区间的破裂，以及建筑物中物品的混乱。钢架建筑物的非结构性损坏很大一部分归结于它们的弹性，可以允许大范围内的移位。
在Hollister的一个钢架建筑，周围是皱起的金属，它是一个食品公司的仓库。由于它和在Hollister的上倾斜建筑是一起的，这个番茄加工公司的钢架结构和遭受了严重的破坏，货物也受到了严重损失。放在30英尺高的标准货价上的番茄听装罐在地震中被震落下来。大多数罐子都落下来，打穿了金属墙。
冲击的力量是如此之大，以至于好几个锥形钢柱和梁被严重破坏。这些被扭曲，弯曲和打碎的钢制物件占据了建筑物的整个部分。这次事故强调了在地震中，存放的货物可能对整个建筑物造成的影响。