Earthquake Reaearch in China  2017, Vol. 31 Issue (3): 334-348
Study on Seismic Damage to Buildings from the 2016 Hutubi MS6.2 Earthquake
Chang Xiangde, Sun Jing, Tan Ming, Yao Yuan, Li Shuai, Luo Ju, Chen Jianbo     
Earthquake Administration of Xinjiang Uygur Autonomous Region, Urumqi 830011, China
Abstract: Based on the field investigation of the building types and damage caused to them by the Hutubi MS6.2 earthquake on December 8, 2016, we analyzed the damage characteristics and causes for different types of buildings. In conclusion we put forward some suggestions for the restoration and reconstruction in the earthquake affected area in future.
Key words: Hutubi earthquake     Building structure     Earthquake damage     Damage causes    

INTRODUCTION

At 13:15 p.m. on December 8, 2016 (Beijing time), an earthquake of MS6.2 occurred in Hutubi County, Changji Hui Autonomous Prefecture, Xinjiang Uygur Autonomous Region (43.83°N, 86.35°E). The epicenter of the earthquake was 54km from Manas County town, 57km from Shihezi City, and 60km away from Hutubi County. The earthquake was strongly felt in the areas of Changji Hui Autonomous Prefecture, Shihezi City, Urumqi City and Karamay City.

The earthquake was located in the northern piedmont of Tianshan Mountains, the southern part of the earthquake-stricken area is a high, mountainous, depopulated area, and the earthquake intensity survey sites were mainly distributed in the hills and alluvial plains of the northern part. A joint investigation team consisting of a total of 18 institutions from the China Earthquake Administration and Earthquake Administration of Xinjiang Uygur Autonomous Region conducted surveys of over 35 townships (towns, farms and sub-districts) of seven counties (cities) at 182 survey sites and 75 sampling points, successfully completing the earthquake disaster investigation, earthquake intensity mapping and disaster report release. The earthquake's macroscopic epicenter was located in Hutubi County. The elevation of the epicenter area is 1500m. The major axis of the isoseismal map trends mainly NWW, and the intensities are divided into three zones, of which the major axis of intensity in the Ⅷ area is 17km, the minor axis is 10km; the major axis of Ⅶ degree area is 58km, the minor axis is 38km, and the major axis of the Ⅵ degree area is 134km, while the minor axis is 116km (Earthquake Administration of Xinjiang Uygur Autonomous Region, 2016a, b).

The earthquake-stricken area mainly covers Changji City, Hutubi County and Manas County of Changji Hui Autonomous Prefecture, Shawan County of Tacheng Prefecture, Shihezi City of the Eighth Division of the Xinjiang Production and Construction Corps, and the Hutubi county seat, Manas County seat, Shihezi urban area and part of Wujiaqu City of the Sixth Division of the Xinjiang Production and Construction Corps. The earthquake caused three minor injuries and damage to residential housing, facilities and industrial properties. The direct economic loss is about 2.74 billion Yuan (RMB).

1 BUILDING TYPES AND SEISMIC HAZARD IN THE EARTHQUAKE-STRICKEN AREA 1.1 Building Types in the Earthquake-stricken Area

An earthquake mainly causes damage to civil, brick-timber, masonry and frame structure buildings. The houses built under the earthquake-resistant comfortable housing projects have a good seismic performance, and there was no obvious damage caused to these houses by this earthquake. The basic characteristics of building structure types of the earthquake stricken area are as follows:

1.1.1 The Civil Structure House

This type of building is mostly distributed in rural areas, townships(towns), regiment farms, villages in county seats and in urban-rural fringes. Most of the walls of this type of building are built with adobe bricks, and a few of them are built with compacted cohesive soil, and clay brick walls are built at side of some of the adobe walls. There is no effective and reliable bonding between the adobe and brick walls, being a typical brick-face wall. The adobe walls of some houses contain wooden pillars. Such houses mostly have purlin roofs, mostly wood.

1.1.2 Brick-timber Structure House

This type of housing is distributed in rural areas, townships(towns), the headquarters of regiment farms and county seats; they are houses mostly self-built by the residents. The housing walls are built with clay bricks, and no ring beams, structural columns or other seismic measures. Most houses have purlin roofs, and there are no measures of reliable bonding between the walls.

1.1.3 Brick-concrete Structure Building

The buildings of this type are mainly concentrated in the county seats, townships(towns) government sites and the headquarters locations of Corps Regiment Farms, and some of the houses are used for production and accommodation purposes for factories and mines. The load bearing walls are made of clay brick masonry, and the roof is a precast concrete slab or cast-in-place concrete slab. General seismic fortification measures, such as ring beams, structural columns and other seismic measures were taken in the public buildings and the multi-storey brick masonry houses in the county seats, townships and regiment farm headquarters (Lei Sijun, 2012), where the construction quality of these houses is better. Although the seismic design specifications adopted are based on different versions, the seismic performance of these buildings is relatively good (Tan Ming et al., 2012). The single-storey brick-concrete structure houses are mostly self-built, with mud masonry construction, and low material strength and masonry strength, and some houses were built a long time ago, with poor construction quality and a lack of anti-seismic measures, so their seismic performance is poor.

1.1.4 Frame Structure Building

Frame structure buildings are mainly concentrated in the county seats and urban areas, and some are distributed in townships, the headquarter locations of regiment farms and in some factories and mines. The load-bearing system of this type of building consists of reinforced concrete beams and other structural members, with a flexible layout, which can be divided freely into rooms, and thus easily meet the requirements of production and use. The framework itself is light in weight and can effectively reduce the earthquake load. Generally, reasonably designed frame structure buildings have good seismic performance, with good ductility (Xu Kaiming, 2012).

1.1.5 Houses Built under the Quake-proof and Comfortable Housing Project

The quake-proof comfortable houses are mainly distributed in the rural areas, rebuilt mostly on the original homesteads of the residents, and some are concentrated in build-up areas. The quake-proof comfortable housing project was initiated in 2004 in Xinjiang (Zhang Yong, 2006). Since 2011, in accordance with the requirements of "high starting point of planning, high level of construction, and high efficiency of supporting facility", the construction area of each household is designed to be not less than 80m2; the house layout would meet the demands of being practical, durable and well-appointed, while reflecting the geographical characteristics and cultural connotation. In order to ensure that housing under this project can meet the requirements of earthquake resistance, governments at all levels and relevant departments took a series of administrative and technical measures to provide guidance and services for house owners. Single-storey housing is a mainly brick-timber structure or brick-concrete structure, and multi-storey buildings are mainly brick-concrete structures. With the completion of construction year by year, the proportion of brick-timber structure and brick-concrete structure houses has increased rapidly and these structures will become the main structure types of housing in the rural areas of Xinjiang. The seismic measures of these types of housing meet fortification requirements, with good seismic performance, therefore, there was no significant damage from this earthquake.

1.2 General Situation of Earthquake Damage to Houses

The seismic intensity of this earthquake is delineated into three regions. The damage degree and damage form of different types of housing are different in different intensity regions. The factors that affect the damage degree and form are building materials, construction quality, construction age, and seismic measures taken. Serious damage in the quake-hit areas was mainly dealt to civil structure houses, old brick-timber and brick-masonry structure houses. Frame structure houses took slight damage in the earthquake, mainly to the filling walls and ancillary decoration works. Houses built under the quake-proof comfortable housing project performed well in the earthquake, and there was no damage caused by the earthquake. Table 1 shows the damage ratio of different types of buildings in the earthquake-stricken area. It should be noted that, because the brick-concrete structure houses in the intensity Ⅷ and Ⅶ areas are less distributed, and the majority are mud masonry, with very low overall strength and poor seismic capacity, these buildings are classified as brick-timber structures in the damage ratio calculation. In the high seismic intensity region, the frame structure buildings are mostly houses of factories and mines. These houses are small in number and scattered in distribution, so for such houses of factories and mines, we carried out individual on-the-spot investigation and did not calculate the damage ratio.

Table 1 Damage ratio of different types of building in the earthquake-stricken area (%)

It can be seen from Table 1 that the damage to civil structure houses is the most serious in intensity Ⅷ and Ⅶ regions, and brick-timber structure buildings rank second. In the Ⅵ degree area, 80% of civil structure houses were damaged, and 6.8% of brick-timber structure houses were severely damaged or ruined. Most of the brick-masonry or frame structure houses are basically intact and only a few were slightly damaged. There was no damage to the houses built under the quake-proof comfortable housing project, indicating that technical guidance and construction management have greatly improved the seismic performance of the buildings.

2 EARTHQUAKE DAMAGE OF BUILDINGS OF DIFFERENT TYPES AND ANALYSIS OF ITS CAUSES 2.1 Civil Structure Houses

The damage to civil structure houses is varied. In intensity Ⅷ and Ⅶ regions, 88% of civil structure houses were damaged, of which about 25% were seriously damaged and uninhabitable, and 63% were damaged to a certain extent and could only be used after repair. In the intensity Ⅵ region, 80% of civil structure houses were damaged, of which about 9% were seriously damaged and uninhabitable, and 71% were damaged to a certain extent and could only be used after repair. Earthquake damage to civil structure houses has the following features:

2.1.1 Partial Collapse of Walls and Roofs of Houses

In intensity Ⅷ region, partial collapse of walls occurred in a few civil structure houses. The falling wall was about 2m long (Fig. 1(a)). In intensity Ⅶ region, the main form of damage is falling roof reed mat and room mud (Fig. 1(b)), small-scale collapse of corners of external walls or the junction of the external longitudinal wall and the transverse wall (Fig. 1(c)), and the large-area collapse of the top of the external brick wall of the longitudinal wall together with the eaves in front of a brick-faced house (Fig. 1(d)).

Fig. 1 Partial collapse of wall and roof of civil structure houses (a)Houses in the third area of Que'ergou village in Que'ergou town.(b)House in the first area of Que'ergou village in Que'ergou town.(c)House in Que'ergou town.(d)House in the second area of Xigou village in Que'ergou town

Cause analysis: This kind of house structure is a typical brittle structure. The load-bearing walls are mostly rammed earth walls or adobe walls. The roof is made of wood beams and reed mats, straw and other simple waterproof materials, the strength of building materials is low, no seismic measures were taken, and there is lack of effective bonding between roof and walls, so the integrity of houses is very poor. Under the action of horizontal seismic force, the whole roof produced thrust that damaged the external load-bearing walls (Ge Ming et al., 2012; Wang Yongliang et al., 2014). Most of the civil structure houses were built years ago and are been in a state of bad repair; and the day before the earthquake, the region suffered heavy rain and snowy weather, this is also one of the reasons that the earthquake caused serious damage to civil structure houses.

2.1.2 Shear Cracks at the Corners of Door/Window Openings and on the Walls between the Openings

The damage to the corners of door/window openings of this type of structure is mainly in the form of diagonal cracks (Fig. 2(a)) and vertical cracks (Fig. 2(b)), and some of the diagonal cracks extend to the joints of longitudinal and transverse walls, causing cracking there (Fig. 2(b)). Through-wall X-shear cracks and diagonal cracks (Fig. 2(c)) appear on the walls of door openings in individual houses.

Fig. 2 Cracks around doors, windows and load bearing walls of mud-wood houses (a)House in the Akexi area of Keziletasi village of Que'ergou town.(b)House in Que'ergou town.(c)House in the third area of Que'ergou village of Que'ergou town.(d)House in Huangtaizi village of Taxihe township.(e)Accommodation room of 106 coal mine workers.(f)House in Huositielieke village of Que'ergou town.(g)The house in North City community of Manas county

Cause analysis: There are no lintels set on the top of door/window openings, or only narrow wood slats placed on them without sufficiently long lap joints, thus failing to act as lintels. The adobe brick wall itself has low strength and the strength of the slurry used in construction is also low, so when an earthquake hits, stress concentrates locally, generating cracks on the walls above the top corners of the openings (Tan Ming et al., 2012; Chang Xiangde et al., 2012). In the intensity Ⅷ region, the walls between openings are too narrow in individual houses, and penetrating cracks or X-shaped shear cracks appeared at the opening corners or narrow transverse walls.

2.1.3 Damage to Load-bearing Walls

Damage to load-bearing walls occurred mainly in two forms: First, cracks occurred at the junction of longitudinal and transverse walls, where the longitudinal wall tilted outwards. This type of damage is mainly distributed in the intensity Ⅶ degree and Ⅵ degree areas (Fig. 2(d)). In some of the houses, diagonal cracks appeared due to the outward tilting of the longitudinal wall which pulled the transverse wall, generating diagonal cracks between the wall and door opening; and the rear longitudinal wall of some houses tilted outwards, resulting in penetrating vertical cracks appearing between the longitudinal wall and the transverse wall (Fig. 2(e)).

The second type of damage to the walls is vertical cracks, which are distributed mainly in the intensityⅥ region and rare in the Ⅶ region, most of which are the widening and expansion of existing cracks. The vertical cracks appeared mainly on the transverse walls of rooms, and some cracks penetrated from the roof to the ground. In individual houses, the vertical cracks extended to the hot wall openings and were distributed around the openings (Fig. 2(f), (g)).

Cause analysis: There are no structural columns built in the four corners of the house and in the longitudinal and transverse wall joints, or in building the cross joints of the walls, bricks were not laid to overlap every other layer, and there is no effective bonding between the longitudinal and transverse walls. Roof purlins or girders were simply put on top of the rammed-earth walls (purlin roof), without beam pads under the beams, nor hidden columns or pillars under the beam (Tan Ming et al., 2012), cracks occurred on the wall under local compression. Under the action of ground motion, stress concentrated in the cracked walls, widening the vertical cracks.

2.1.4 Damage to Non-load-bearing Structural Members such as Parapets and Eaves

The main damage form in the intensity Ⅶ region is partial collapse (Fig. 3(a)) or outward tilt (Fig. 3(b)) of parapet walls, small-scale collapse of eave corners (Fig. 3(c)) and chimney collapse (Fig. 3(d)); In the Ⅵ degree area, the damage is mainly in the form of peeling of external walls of houses (Fig. 3(e)).

Fig. 3 Damage of parapet, eaves and non-load bearing of mud-wood houses (a)House in the second area of Xigou village in Que'ergou town. (b), (c), (d)House in Huositielieke village of Que'ergou town.(e)House in Shangzhuangzi village of Letuyi town

Cause analysis: Because the parapet wall is too high or lacks effective bonding, it will cause an obvious whipping effect, resulting in the collapse or tilt outward of the parapet (Tang Lihua et al., 2007); the water proofing of roof and eaves was poor and the perennial rain erosion decreased the strength of wood, so subsequently, the wood beams under the eave broke in the earthquake, causing local collapse (Fig. 3(c)).

2.2 Brick-timber Structure Houses

In intensity Ⅶ region, 85% of brick-timber houses were damaged, of which 21% were seriously damaged, 64% were damaged to a certain extent, only usable after repair. In the Ⅵ region, 77% of brick-timber houses were damaged, of which 7% were seriously damaged, becoming uninhabitable and dangerous, 70% were damaged to a certain extent and were only usable after repair. Earthquake damage to the brick-timber houses mainly had the following features.

2.2.1 Cracks in the Corners of Door/Window Openings

In the disaster area, the damage to brick-timber structure housing is mainly in the form of diagonal cracks at the corners of doors and windows openings (Fig. 4(a), (b)). Some of the diagonal cracks at the openings extended to the longitudinal and transverse wall joints, resulting in cracking in the wall joints (Fig. 4(a)). In the intensity Ⅵ degree area, horizontal cracks extending 2m-3m long appeared in a few houses (Fig. 4(c)).

Fig. 4 Earthquake damage of brick-wood houses (a), (b)House in Huositielieke village of Que'ergou town.(c)House in Lianfeng village of Dafeng town.(d)House in Hu'agen village of Ashili town.(e)House in Que'ergou town.(f)House in Hongshan village of Dafeng town.(g)House in Que'ergou town.(h), (i)House in Shang'ergong village of Manas town

Analysis of the causes: No lintels were set over the door and window openings, or the lintels were too short to play the corresponding role; in some of the housing, the roof beams were laid directly above the openings, or in the range of 45° above the opening (Fig. 4(b)). Individual houses were built with clay as the binder, which has low strength, so when the earthquake struck, local stress concentration would produce cracks in the upper part of the opening wall. Influenced by the height of openings, transverse cracks occurred along the door opening to the corner of the hot-wall opening in individual houses in the intensity Ⅵ region (Fig. 4(c)). The hot wall is also part of the wall under load, forming the opening.

2.2.2 Cracks of the Walls

This type of damage is mainly in the form of vertical cracks appearing intransverse walls of houses (Fig. 4(d)), a few of them are the transverse cracks (Fig. 4(e)) and there are one or two diagonal cracks extending towards door openings (Fig. 4(f)).

Analysis of the causes: The roof purlins or crossbeams are laid simply on the top of brick wall (roof purlin), and there is no beam pad under the beam. Under the ground motion, the wall under beams will be subject to a concentrated load locally, causing vertical cracks. For some rooms, the walls between windows were too narrow, and transverse cracks were generated under the earthquake. The pre-existing vertical cracks in the walls of some houses were widened due to foundation subsidence under the earthquake force.

2.2.3 Cracks at Longitudinal and Transverse Wall Joints

This type of damage is mainly the vertical cracks appearing in the corners of the longitudinal and transverse wall joints (Fig. 4(g), (h) and (i)), and in some houses, diagonal cracks occurred along the wall corners (Fig. 4(i)).

Analysis of the causes: There were no structural columns built in the four corners of houses and in the longitudinal and transverse wall joints, or in building the cross joints of the walls, bricks were not laid to overlap every other layer, and there was no effective bonding between the longitudinal and transverse walls.

2.3 Brick Masonry Houses

In the earthquake-stricken area, the damage to brick masonry houses (single-storey or multi-storey buildings) was mainly distributed in the intensity Ⅵ region. Most of these houses in the high seismic intensity regions are production or residential buildings of factories and mines. The earthquake damage to the brick masonry buildings has the following features:

2.3.1 The Through-wall Cracks

This kind of damage occurred mainly in the intensity Ⅷ and Ⅶ regions. According to the spatial distribution, the damage is distributed mainly along the major axis of the Ⅶ isoseismal line. In the epicenter area of intensity Ⅷ region, diagonal and transverse shear cracks occurred on the walls of single-storey brick masonry houses (Fig. 5(a), (b) and (c)), and X-shape shear cracks are directed to the corners of door/window openings. Along the major axis of the Ⅶ isoseismal line, there is no damage appearing in the concrete frame structure under the brick masonry gate chamber at the Taxihe Shimenzi reservoir which is near to the intensity Ⅷ region, but in the upper part of the brick masonry structure are penetrating X-shaped cracks on the southeastern gable wall (Fig. 5(d), (e)), penetrating diagonal and horizontal cracks on the northwestern wall (Fig. 5(f)), and two 2m-3m long horizontal cracks on the longitudinal wall. Other brick masonry houses around this site are all in good condition.

Fig. 5 wall cracks of brick-concrete buildings (a), (b), (c)Xigou substation office building (nearby the third area of Que'ergou village of Que'ergou town).(d), (e), (f)Taxi River Shimenzi Reservois Sluice house

Analysis of the causes: The load-bearing wall of the brick masonry structure is made of brittle material, so the structure thus built is also fragile (Liu Qin et al., 2015), and the weight of the roof is great. This type of housing built in earlier stages did not adopt any reasonable anti-seismic structural measures and the position of door/window openings was not rational, resulting in the external walls having to bear transverse thrusting generated by the horizontal shaking of the whole roof during the earthquake, and cracks extended to stress-concentrating locations such as door/window openings and wall corners. The bottom frame structure houses are isolated and high, having an obvious whipping effect under the earthquake load, resulting in penetrating shear cracks appearing in the upper part of brick masonry walls (Fig. 5(d), (e)).

2.3.2 Cracks at the Door/Window Openings

The damage is mainly in the form of diagonal cracks and vertical cracks in the door and window openings (Fig. 6(a), (b), (c), (d), (e)), distributed mainly in the intensity Ⅷ and Ⅶ regions. In the Ⅳ region, they are mainly widened cracks of previous existing ones (Fig. 6(f)).

Fig. 6 Cracks of doors and windows of brick-concrete buildings (a)Fengyuan coal mine production scheduling office building (Three-storey brick-concrete sturcture).(b), (c)The same window outside the house and inside the house photo of (a).(d), (e)106 coal mine 35kV substation.(f)House in Lianfeng village of Dafeng town.(g)Hutubi River Dalabai Power Station.(h)Baiyangshu coal mine accommodation room.(i)Xiaoganggou coal mine office building

Analysis of the causes: The earthquake caused wall deformation, leading to local stress concentration, thus producing cracks at the corner of the openings. For houses designed asymmetrically, many diagonal and vertical cracks appeared in the walls between windows because of too many open windows and narrow walls between windows (Fig. 6(a)).

2.3.3 Cracks on the Walls

This type of crack consists mainly of vertical cracks (Fig. 6(g)), transverse cracks (Fig. 6(h)) and diagonal cracks (Fig. 6(i)) in the walls of houses.

Analysis of the causes: Precast concrete beams are simply laid on the top of brick walls without padding under the beam. Under an earthquake, the wall under the beams is subject to concentrated load, resulting in vertical cracks on the walls. The walls between windows are too narrow, in addition, the whole house tilted backwards, impacted by foundation settlement and transverse cracks were generated under the earthquake force. In some houses with a high floor height, diagonal shear cracks are likely to occur under a horizontal earthquake load.

2.3.4 Damage to the Roof Structural Members, such as Lintels, Precast Slabs, etc.

The earthquake produced minor vertical cracks in the lintel of the house(Fig. 7(a)), and cracks in the roof precast slab (Fig. 7(b)) and in the joints between the roof and wall (Fig. 7(c)).

Fig. 7 Damage of wall, roof and non load bearing of brick-concrete buildings (a)The self-built houses in Que'ergou town.(b)House in Xiagebi village of Dongwan town.(c)House in Taxihe village of Baojiadian town.(d)Fengyuan coal mine administrative office building.(e)Hongxin Baiyanggou Coal Mine Building.(f)Kangning Community House of Manas County(Multi layer)

Analysis of the reasons: Some of the houses in the quake-disaster area were built by the local residents themselves according to their own experience, with a lack of relevant instructions and guidance about seismic design, and the rooms are wide and deep. It can be seen that under the seismic load, vertical cracks and significant downward bending deformation occurred in the concrete beams. Precast slab is not mortar bedded, so the bonding with the walls is not strong enough; the bond strength between the cast-in-place roof and the walls is insufficeint, resulting in cracks occurring in the junctions under the action of earthquake force.

2.3.5 Damage to Parapets and Other Non-load-bearing Structural Members

The major form of earthquake damage in the intensity Ⅷ region is diagonal cracks on parapet walls and the penetrating transverse cracks at the junction between the roof and parapet (Fig. 7(d)), breakage of decorative ceramic tiles on the external wall and window glass (Fig. 7(e)); in the intensity Ⅵ region, the damage is mainly in the form of cracks appearing in the insulating layer outside the houses (Fig. 7(f)), peeling of indoor walls, and cracking of the corners of plastic/ steel window frames and deformation and leakage of heating pipes.

Analysis of the causes: Because the parapet is too high and poorly bonded, there is an obvious whipping effect. Waterproofing for the roof was not done effectively, the rain erosion of the foot of parapet decreased the strength of the materials, producing cracks in the junction between the roof and parapet during the earthquake. There was not enough bonding strength between the decorative ceramic tiles and wall, walls were deformed under the seismic load, resulting in stress concentrating in the window openings, causing breakage of window glass.

2.4 Frame Structure Houses

The damage to this type of housing occurred mostly in public housing, some in residential houses. The earthquake did not cause damage to the main structure of the frame, but to a certain number of filling walls, meanwhile, a large number of indoor facilities and equipment collapsed, causing loss to a certain extent. The characteristics of major seismic damage to the frame structure houses are as follows.

2.4.1 Cracking of Filling Walls

This type of damage occurred mainly in the intensityⅦ region and seldom in Ⅵ region. In the Ⅶ region, the damages appear mainly in the form of X-shape shear cracks in the filling walls (Fig. 8(a), (b)), and there is no damage to the beams and columns(Fig. 8(c)), shear or diagonal cracks are found in the door/window openings (Fig. 8(d), (e)). In intensity Ⅵ region, the major form of damage is diagonal cracking of the walls (Fig. 8(f)).

Fig. 8 Crake of filling wall and beam column binding sites of frame structure buildings (a), (b), (c)Shenhua Tiandian Mining Co., Ltd.office building.(d)Town office building of Que'ergou town.(e)106 coal mine office building.(f)Govermment Office Building in Dongwan Town of Shawan County.(g)106 coal mine office building.(h)Social service station in Baojiadian town.(i)No.9 Middle School Teaching Building in Shihezi City

Analysis of the causes: Filling walls are mostly built with hollow bricks or concrete blocks. The lateral stiffness of a filling wall is higher than that of a frame; a frame structure is a flexible structure, and the deformation of a frame column is larger than that of the filling wall. The horizontal seismic force acts on both the filling wall and the frame. Since the stiffness strength of the filling wall is greater in the early stage, it absorbs a larger amount of seismic load, but due to the lower shear resistance and deformability of the wall, and the lack of effective bonding between the wall and frame, shear failure is likely to occur in the reciprocating deformation of the wall (Sun Jing et al., 2014; Xu Kaiming, 2012). Earthquake damage to the filling wall is similar to that of masonry structure. Under the earthquake force, stress concentrates in the corners of door and window openings, causing the damage. Diagonal cracks are likely to occur in some buildings with higher floors (Fig. 8(f)).

2.4.2 Cracks at the Junctions between Wall and Beam and Column

This kind of damage appeared both in intensity Ⅶ and Ⅵ regions. There is less distribution of these cracks in houses in the intensity Ⅶ region, and most of the houses are used by factories and mines (Fig. 8(g)). But in Ⅵ region, there is a wide distribution of the cracks occurring at the joints between wall and beam and column (Fig. 8(h), (i)).

Analysis of the causes: The filling wall and frame column are built with materials of different mechanical properties. Due to insufficient bonding in construction, deformation between them is disharmonic, thus producing cracks under the earthquake.

2.4.3 The Ground Subsidence and Damage of Decoration Engineering and Utensils

Affected by the foundations of individual houses (the canteen of Shenhua Tiandian Mine Co., Ltd.), ground subsidence occurred, resulting in fracturing of floor tiles (Fig. 9(a)), cracking and dropping off of ceramic tiles used in the decoration of interior walls of frame structure houses (Fig. 9(b)), collapse of the accessory walls (Fig. 9(c)), caving in of the ceilings (Fig. 9(d)), breaking of window glass (Fig. 9(e)), and fracturing of the dining-tables under the seismic force (Fig. 9(f)).

Fig. 9 The ground subsidence and damage of the decoration engineering of frame structure buildings

Analysis of the causes: The ground was improperly treated, causing the foundation to sink. The decoration ceramic tiles were poorly bonded to the accessory walls and the ceilings were loose, therefore, damage occurred under the strong ground motion.

3 FINDINGS AND SUGGESTIONS

(1) The population and houses are mostly concentrated in the intensity Ⅵ region of this earthquake. Relative to the Ⅵ region, there is scattered distribution of civil structures, old brick-timber and brick masonry structure houses in the intensity Ⅷ region. Partial collapse and X-shape penetrating cracks occurred in the walls of civil structure houses in the high seismic intensity region. Damage to the old brick-timber structure houses is mainly in the form of cracks at the door and window openings; penetrating shear cracks appeared in the walls of old brick masonry houses. The damage to the frame structure houses is mainly in the form of cracks of filling walls and damage to indoor decoration engineering.

(2) The implementation of quake-proof comfortable housing project has played an important role in protecting the lives and property of people in the earthquake-stricken area. In the recent earthquakes in Xinjiang(e.g. the June 30, 2012 MS6.6 Xinyuan-Hejing earthquake, the February 12, 2014 MS7.3 Yutian earthquake, and the July 3, 2015 MS6.5 Pishan earthquake), prominent effects were achieved with the quake-proof comfortable housing in reducing casualties and property losses (Chang Xiangde et al., 2012; Li Shuai et al., 2014; Liu Jun et al., 2016). There has been no damage caused by the earthquake to quake-proof comfortable houses built since 2011 according to the new higher standards, indicating that these houses have good seismic performance (Fig. 10(a), (b), (c)). The coverage rate of quake-proof comfortable houses reaches 80% in the stricken area of this earthquake, playing an important role in reducing casualties and investing funds for earthquake relief.

Fig. 10 Buildings constructed under "Urban and Rural Anti-earthquake Project" (a)Que'ergou town (On the left side of the house built under quake-proof and comfortable housing project, intact without damage; on the right side is the old civil structure houses, the front wall is cracked and the Bearing wall is mostly cracked).(b)Huositielieke village of Que'ergou town.(c)Nu'erjia village of Ashili town

In Fig. 10(a), the house on the left, built under the comfortable housing project, is intact without damage, and the house on the right is an old civil structure house, of which the front longitudinal wall tilted forwards and the gable wall cracked, and most of the indoor load-bearing walls cracked

(3) The site has an obvious effect on the seismic damage of houses in this earthquake. Earthquake damage to rigid structure houses in near-field area is obviously larger than that of frame flexible structure houses in the far field. The damage of filling walls is widely distributed.

(4) The frame structure houses have good seismic performance, the main structures of this type of houses remained intact in this earthquake, but the filling walls were damaged as a general case. Therefore, it is recommended that in the future design and construction, we should take into account the compatible deformation of the wall with the main structure besides its partition function when selecting the materials for the filling wall, thus reducing the losses resulting from damage to the filling wall.

(5) The simple structure houses and self-built masonry structure houses in the rural areas did not take necessary anti-seismic measures, so the integrity of these houses is poor, and some of the houses were damaged to various extents due to ground subsidence. Threfore, it is recommended to strengthen planning in site selection for new construction in the rural area, give publicity to knowledge of earthquake preparedness and disaster reduction, and provide technical guidance and service for house construction.

(6) In the process of post-earthquake restoration and reconstruction, the construction of quake-proof comfortable housing should be further increased to realize full coverage. For the old remaining buildings used for specific functions, it is recommended that the government should encourage the building of better seismic performance auxiliary houses through methods such as guiding social capital input, etc.

ACKNOWLEDGEMENTS

We are grateful to the on-spot investigation team of the MS6.2 Hutubi, Xinjiang earthquake who provided the house survey data for this study, and to the staff of the on-spot earthquake emergency relief team for their diligent work.

This paper has been published in Chinese in the Journal of Technology for Earthquake Disasters Prevention, Volume 12, Number 1, 2017.

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