The design of elements under blast load came to the attention of scientists after World War II. Extensive research has been done in the last fifty years on the behavior of structures against blast loads and their reaction. Due to the intensity and magnitude of blast loads, it is important to carry out the most appropriate scheme in terms of economics and safety for a structure. If the design is not done correctly and with the right criteria selected, it may impose exorbitant costs on the execution of a structure. The type of blast in this research is blast in the open air and unrestricted surface. A surface blast is a blast that occurs at a surface or at a very close distance to the ground. In this research, different methods of strengthening the column of concrete bridges under blast load will be investigated. The considered strengthening methods include changing the arrangement of the rebars on the cross-sectional area, increasing the amount of cross-sectional rebars, using FRP rebars and increasing or changing the geometric shape of the column cross-sectional area,Then the most appropriate, economical and safest strengthening scheme is selected and a suitable strengthening model for the above system is presented. The structural models studied in this research are investigated at an identical blast level (1000 kg TNT at 3 meters from the structure). The software used in analyzing the studied models is SAP2000 software. The results showed that some strengthening methods, such as increasing the amount of cross-sectional rebars, increase structural responses. Also, among the strengthening methods selected in the research, replacing FRP rebars instead of regular rebars in the reinforced concrete bridge columns is the most effective and the best method of strengthening these structures against the blast loads.

Examining the thermal behavior of concrete lining of tunnels against thermal loads is of special practical importance. The purpose of this research is to investigate the behavior of fiber concrete lining tunnels with rectangular, semicircular and horseshoe sections under the effect of thermal load in a specific time history. In this regard, numerical simulations have been carried out using Abaqus finite element software. Stress-time and deformation-time graphs in fiber concrete coating under the effect of thermal loads have been investigated. Fiber concrete in this research, which is considered as cover in tunnel sections, is of steel fiber concrete type with a volume percentage of 0. 5 to 1. 5. Also, in addition to the common design loads, the sections are subjected to thermal loading conditions. According to the obtained results, the strength of the investigated sections depends on the amount of steel fibers in the concrete in addition to the appearance. Investigations show that the cross-section with horseshoe shape has the best performance against thermal load and the increase of fibers up to 1. 5 volume percent of concrete compared to tunnel linings with a lower volume percentage of fibers leads to stability in a longer period of time.

In this research, at first, different types of trenches, headquarters and concrete hospitals during the holy defense were investigated from the perspective of commanders and specialists of holy defense. In order to investigate in addition to library studies, the performance of these structures has been investigated through interviews and questionnaire distribution among veterans in the field of sacred defense engineering and after aggregating the opinions, general analysis of the role of these structures, their technical disadvantages and advantages has been presented and then some strategies have been presented to promote these types of structures for today's use. One of the main disadvantages of them is the heavyness of prefabricated concrete parts and one of the solutions provided for today's use is to increase resistance to bearing impact loads. In a numerical analysis, the effect of increasing steel fibers on concrete slabs under impact load in Matlab programming environment has been investigated. The results show that the use of steel fibers up to 2% increases the ductility and strength of concrete to a great extent and is recommended for use in war zones.