同济大学：《建筑结构抗震》课程教学资源（课件讲稿）Chapter 2 Site, Subsoil and Foundation.pdf_大学文库

2015/9/22 2 (2) Unfavorable to earthquake resistance: • Soft soil; • liquefiable soil; • stripe-shaped protruding ridge; • high isolated hill; • non-rocky steep slope; • river banks and edges of slopes; • soil strata having obviously heterogeneous distribution in plane (such as abandoned and filled river beds, fracture zone of fault, and hidden swamp(沼泽地), creek（溪）, gully（沟） and pit（矿井）, as well as subsoil with partial excavation and filling. 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 7 (3) Hazardous to earthquake resistance: Places where landslide, avalanche, subsidence, formation of cracks and debris flow (mud-rock flow) are liable to occur during an earthquake,orthe location of active fault. Beichuan, Sichuan Province Landside 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 8 (3) Hazardous to earthquake resistance: Places where landslide and the location of active fault. 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 9 Yiliang Earthquake: Landslide: The quakes caused landslides that blocked roads in the area of the hardest hit region, the town of Luozehe (洛泽河) in Yunnan Province. The affected areas are densely populated and mountainous. Power and Communication stopped: cutting off highways and communications, as well as power supply Lost and Losses: 81 people killed, 800 people injured, and more than 6,600 houses were flattened, while many thousands more were damaged. In Guizhou Province, collapsed houses were reported in Weining County, and houses were damaged in Hezhang County. Direct economic losses were estimated at 3.5 billion yuan . 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 10 • Structures that lie atop the surface trace of an active fault can be damaged or destroyed by “surface fault rupture” during an earthquake.( Fig 2.1) Fig. 2.1 Crop rows offset by a lateral strike- slip fault shifting in the 1976 Earthquake that shook El Progresso, Guatemala. • A structure that lies partially or entirely w ithin an Earthquake Fault Zone may be subject to requirements for site-specific geologic studies before any new or additional construction may take place. ( Fig.2.2 ) Fig. 2.2 Railroad tracks shifted by the 1976 Guatemala Earthquake 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 11 2.2.2 Construction site categories 1. Seismic influence of construction site Local site condition Play an important role Responses of structures Q: How the performance of a Highrise buildings founded on deep soft soils compare with the one on rock? Why? A: Earthquake records on soft to medium clay sites indicate that the soil amplification factors for long-period spectral accelerations can be significantly large. WHY ? HOW ？ 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 12

2015/9/22 3 (1) The site soil should be classified according to the shear wave velocity of the soil layer. 2. Classification of site soil Type of the soil Shear wave velocity of soil layer (m/s) Rock Vs＞800 Stiff soil 800≥Vs＞500 Medium-stiff site soil 500≥Vs＞250 Medium-soft site soil 250≥Vs＞150 Soft site soil Vs≤150 Table 2.1 Classification of site soil (GB50011-2010 表4.1.3) 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 13 (2) If no measured data of shear wave velocity for Building Type C （丙类建筑）to Type D（丁类建筑） buildings is available, the soil may be classified according to Table 2.2 Type of soil Geotechnical description Rock Stiff and integral rock Stiff soil Stable rock, dense gravel Medium-stiff soil Dense, medium dense, or slightly dense gravel; coarse or medium sand; cohesive soil and silt with fak ＞150 Medium-soft soil Slightly dense gravel, coarse or medium sand; fine and silty sand other than that which is loose; cohesive soil and silt with fak≤150; fill landwith fak＞130 Soft soil Muck and mucky soil; loose sand; new alluvial sediment of cohesive soil and silt; fill land with fak≤130 Table 2.2 Classification of soil (GB50011-2010 表4.1.3)  2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 14 (3) When the site contains several types of soil layers, the equivalent shear wave velocity should be used to determine the type of soil. It is determined by the following formula: v d t se  0     n i i si t d v 1 where: —— the equivalent shear wave velocity of soil layer —— calculation depth in meters, the thickness of the overlaying layers （覆盖层）, but no larger than 20 m —— thickness of the i-th layer in meters se v 0 d di —— travel time of shear wave from the ground level to calculation depth —— shear wave velocity in the i-th layer/second —— the number of soil layer t si v n the layer which shear wave is less than 500m/s. 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 15 3. Construction site categories  Construction sites shall be classified into four categories according to the equivalent shear wave velocity and the overlaying thickness at the site.  Type I0, I1, II, III, IV Equivalent shear-wave velocity (m/s) Construction site categories I0 I1 II III IV Vse＞800 0 800≥Vse＞500 0 500≥Vse＞250 ＜5 ≥5 250≥Vse＞150 ＜3 3~50 ＞50 Vse≤150 ＜3 3~15 15~80 ＞80 Table 2.3 the thickness of overlaying layer at various site 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 16 Given:The soil profile at a site is shown in Table 2.4. Question:To determine the category of the construction site Example 2.1 Bottom depth of soil layer (m) Thickness of soil layer (m) Soil profile name Shear-wave velocity (m/s) 9.5 9.5 Sand 170 37.8 28.3 Very soft clay 138 48.6 10.8 Sand 240 60.1 11.5 Very soft silty clay 200 68.0 7.9 Fine sand 330 86.5 18.5 Gravelly sand 550 Table 2.4 Data of soil exploration at site 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 17 Step 1: Determine the overlaying thickness of the site The shear-wave velocity of the soil layer beneath 68 meter is more than 500 m/s, so the overlaying thickness of the site is equals to 68 meters (>20m). Step 2: Determine the type of soil within the range of 20 m under the ground level The equivalent shear-wave: The equivalent shear wave velocity: 250 m/s> >150 m/s, so the type of the soil is medium-soft soil. Solution: t d v  s n i i si 9.5 170 (20 9.5) 138 0.132 1        v d t m s se  0  20 0.132 151.6 s v 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 18

2015/9/22 4 Step 3: Determine the construction site category The equivalent shear wave velocity is between 250 m/s and 150 m/s and, the overlaying thickness of the site is larger than 50 meters, so the soil is categorized as Type III accordingto Table 2.3. Table 2.3 the thickness of overlaying layer at various site Equivalent shear-wave velocity (m/s) Construction site categories I0 I1 II III IV Vse＞800 0 800≥Vse＞500 0 500≥Vse＞250 ＜5 ≥5 250≥Vse＞150 ＜3 3~50 ＞50 Vse≤150 ＜3 3~15 15~80 ＞80 L 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 19 The mean pressure on the base of the foundation and the maximum pressure at the edge of the foundation shall comply with the following equations: aE p  f aE p 1.2 f max  where: —— mean design value of pressure of combined seismic action on the foundation base; —— maximum design value of pressure of combined seismic action at the edge of foundation base. p pmax 1. Checking Earthquake Resistance of foundation on natural soil 2. 3 Seismic checking for subsoil and foundation 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 20 aE a a f   f where: —— adjusted characteristic value of seismic bearing capacity of subsoil; —— characteristic value of force bearing capacity of subsoil with the modification of width and depth; a f —— adjusting coefficient for seismic bearing capacity of subsoil, and shall be taken from Table 2.5.  a seismic Seismic adjusting coefficient Q: it is greater than 1.0 or less than 1.0? 2. Checking of the earthquake resistance on natural soil Seismic bearingcapacityof subsoil: aE f 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 21 Geotechnical description Rock; dense gravel; dense gravel, dense coarse or medium sand; cohesive soil or silt with fak≥300 1.5 Medium dense or slightly dense gravel; medium or slightly dense gravel; coarse or medium sand; dense and medium dense fine or silty sand; cohesive soil or silt with 150≤fak＜300; Stiff loess 1.3 Slightly dense fine sand and silty sand; cohesive soil or silt with 100≤fak＜150; plastic loess 1.1 Silt, silty soil, loose sand, fill, newly alluvial sediment deposit of loess or flowing mucky loess 1.0 Table 2.5 Adjusting coefficient Hard Soft A: The factor, , is equal or greater than 1.0 .  a  a 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 22 But， In some satiation that bearing capacity of natural subsoil and foundation need not be checked for earthquake resistance. Reasons: 1）capacity of natural subsoil have more redundancy under static vertical loads, because most of them were designed dominantly with deformation limitation of soil but capacity. 2) under long time loaded, the soil was solidified (固结) and the strength was risen. 3) the strength of soil is higher under dynamic loads than under static loads. (except of liquefaction) 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 23 (1) Structures, of which seismic checking NEED NOT be conducted for their superstructures,specified in the code. (2) Some structures, of which NO soft clay layer exists in their bearing subsoil, listed as follows: • ordinary single-story factory buildings, and single-story spacious buildings, • masonry buildings, • multi-story framed civil buildings, • multi-story framed and frame-shear wall workshops. 3. Requirements for Needn't Checking: Except the building listed above, the bearing capacity of natural subsoil need be checked for earthquake resistance. 2015/9/22 熊海贝，同济大学土木工程学院 xionghaibei@tongji.edu.cn 24