Research On The Application Of Infrared Thermography In The Rapid Post-earthquake Damage Assessment Of Buildings

On the basis of the rapid safety appraisal of buildings in seismic field carried out by the seismic department during the period of earthquake emergency response, this paper, introducing the infrared nondestructive test technology, one supplementary means of the rapid safety appraisal of buildings nowadays, employing image processing, mathematical statistics and so on, discusses the infrared images of the damaged buildings in the earthquake and analyzes their relationship with all kinds of influence factors. Based on the typical damages of buildings in seismic field in Wenchuan Earthquake, emphasis is laid on the infrared images of the buildings of frame structure, brick and concrete structure or simple structure, mainly judging from: 1) the national standard of safety assessment of buildings in seismic fields (Post-earthquake field works Part 2: Safety assessment of buildings, GB18208-2001). 2) Post-earthquake field works Part 3: Code for field survey. 3) Field Damage Evaluation Standard, 20080702.This paper introduces evaluation systems for earthquake damage and losses of buildings, the characteristics and duties of seismic field work at the present in our country, and highlights the typical damage of buildings of different structure together with the features of the infrared and visible images.A correlation analysis has been done after a post process of the images so as to study the relevancy between the thermal images and the factors concerned. This research designs a table for each infrared image, recording the capture time, location, weather, angle, distance together with the structure types, materials, damage level of the buildings. What the correlation analysis reveals are as follows:⑴The study focuses on the controversial divisions of the damage degree. In the practical earthquake field work, the divergences exist mainly in the buildings of moderate damage, so the images of moderate damage are mainly collected. In fact, conclusions of all the working groups are almost the same on the judge of buildings of destruction and well preservation, according to the severely destroyed and damaged cases such as the collapse of walls, the crush and spalling of the concrete at the end of columns and the main bars compressed like the lantern, people can estimate the level of damage quickly and accurately by the eye, so INDT is not necessary for auxiliary judgment. Similarly, we know from the field work that a small number of wood structural houses are enclosed by wooden walls, and this kind of houses are usually severely damaged to collapse or slightly damaged by dropping of enclosing walls, at the same time people can estimate the degree of damage quickly and accurately by the eye, so INDT is also unnecessary for auxiliary judgment.In the study 7803 infrared images have been processed, including 54 of well preservation, 1934 of slight damage, 3436 of moderate damage, 2192 of severely damaged and 191 of destruction.The correlation analysis shows that among the destruction degrees of well preservation, slight damage and moderate damage, the effect of the images enhanced as the damage degree increases, and as for images of moderate damage, severe damage and destruction, the effect of the images is inconspicuous as the damage degree increases.⑵For the diversity of main stressed members in different types of buildings, the national standard (Post-earthquake field works Part 2: Safety assessment of buildings)specifies that the focus of detection should vary according to the variety of structures. The statistics of the data of structure type show that the sequence of the representation effect is bottom-frame structure, masonry structure, frame structure and simple structure.⑶The image gives a better presentation in the noon than in the morning, and even better in the afternoon. In practice, the afternoon should be made full use of.⑷A good presentation can be given in the sunny days, worse in the rainy days and worst in the cloudy days. At the same time, the effect of the rainy days on the damaged part of the buildings should be made the best of in the light of the advantages of the infrared nondestructive test in water leakage detection.⑸The presentation of the infrared images is good in the case of direct sunlight, worse in the case of the slanting sunlight, and worst in the case of no sunlight.Therefore, when images are captured, all the factors, time, weather, sun direction, location and others should be taken into consideration. ⑹The presentation of the infrared images is related to the distance they are obtained, different distances leading to different coverage and different presentation as well. The statistics reveal that generally the images are better used with the increase of the distance. However, a close-range image is needed if the damage is short, small and deep.⑺The presentation of the infrared images is also related to the angle they are obtained. The image should be captured slantingly against the light and the higher part of the damage so as to make the heat accumulation clear so that features of the damaged part can be highlighted. Unfortunately, slant capture easily causes the positional deviation of the image due to a bigger angle.⑻Owing to the limit of the technology and the precision of the instrument, the thinner cracks are not distinct in the infrared images even if they are very deep. On the contrary, resulting from the heat change between the inside and the outside, the through cracks with a certain width can be obviously identified in the infrared images, the images obtained from the outside in particular.⑼The correlation analysis of classified statistics according to the decoration layer shows that the effect of images is correlated to the properties, color, and surface roughness of the materials. Among all the factors, the heat absorption of the material is directly influenced by the surface color, so the influence of surface color should not be ignored. The diversity of color appearing on the same material will interfere the interpretation of the images, and INDT is also not suitable when there are many kinds of decoration layers and complex background colors.