No abstract

Many advantages of concrete-filled steel tube columns have led to their use in a variety of distinctive and important structures in recent years, including high-rise buildings, piers, stairs and other structures. The exterior steel wall of these columns is uncoated, which is the main disadvantage. When these columns are subjected to external pressure on the external steel wall, such as collision, explosion, fire or other accidents, the confinement of concrete will be lost owing to the weakness generated in the steel wall, resulting in an abrupt loss in column strength. In this study, a new upgraded section using internal steel mesh is designed and introduced, with this steel mesh and the external steel wall acting as a double-skin steel tube to protect the concrete core from being destroyed if the external steel wall is destroyed and the column's strength drops suddenly. Under axial and cyclic loads, a comparison of the behavior of this innovative section (with internal steel mesh) with the most commonly used sections of concrete-filled steel columns was conducted. Following the verification of the finite element modeling, various other studies were carried out. The results of the analyses clearly show that the suggested CFT section has increased strength, improved resistance to progressive collapse, and energy absorption capacity under axial and cyclic loading, particularly abrupt loads such as fire or explosion, and thus its use in construction is recommended.

The present study set out to investigate the nonlinear seismic response of the dam-reservoir-rock foundation system, taking into consideration the effects of change in the material properties of discontinuous foundation. To this end, it is important to provide the proper modeling of truncated boundary conditions at the far-end of rock foundation and reservoir fluid domain and to correctly apply the in situ stresses for rock foundation. The nonlinear seismic response of an arch dam mainly depends on the opening and sliding of the dam body's contraction joints and foundation discontinuities, failure of the jointed rock and concrete materials, etc. In this paper, a time domain dynamic analysis of the 3D dam-reservoir-foundation interaction problem was performed by developing a nonlinear Finite Element program. The results of the analysis of Karun-4 Dam revealed the essential role of modeling discontinuities and boundary conditions of rock foundation under seismic excitation. Keywords Concrete arch dam Á Non-homogeneous and discontinuous foundation Á Nonlinear finite element dynamic analysis Á In situ stress Á Boundary conditions

The access tunnel of the Urmia Lake for water transfer and restoration project (Kani Sib) is located in the south of West Azerbaijan Province, Iran. Part of this tunnel is located on weak and very loose soil, which in some areas, cannot be stabilized, despite the use of step drilling, and may lead to ceiling collapse, face collapse and even deformation in support system. In these cases, it is necessary to adopt the pre-support method of the umbrella arch. Tunnel stability analysis is one of the important factors in tunnel design and support system. Indeed, the type of support system is chosen according to the required stability and permitted displacement for the tunnel. In the present article, first the permitted displacement for the tunnel is calculated by Sakurai correlation. Then, the ground reaction curve is plotted using the numerical method of finite difference, namely, FLAC3D software, and the convergenceconfinement method (CCM) is used to determine the acting instant for the support system. Finally, the safety level of the proposed support system is investigated considering different safety factors. The results of this study indicate that the Sakurai displacement correlation is more reliable than the other graphs presented. The results derived from numerical modeling are verified accurately against visual observation and instrumentation results. A suitable umbrella arch pre-support system with Lattice and Shotcrete support system is recommended. The umbrella arch pre-support system encompasses forepoling pipes of 90 mm diameter with 9 m in length and 2.5 m in overlap length.

In most cases, concrete arch dams in the presence of suitable abutments, have high bearing capacity and more appropriate safety regarding the cost aspects, when compared to the other types of dams. However, according to the dam failure statistics, site specific conditions and abutment instability are the main factors of concrete dam's failure. In this paper, the effects of two important factors on earthquake response of high arch dams are considered. These factors are: effects of contraction joints opening between the dam monoliths and appropriate rock foundation boundary conditions. Nonlinear seismic response of dam reservoir foundation system includes dam-canyon interaction, dam body contraction joint opening, discontinuities (sliding planes) of foundation rock and failure of the jointed rock and concrete materials. Therefore, a finite element program for nonlinear dynamic analysis of 3D dam reservoir foundation system was developed. Karun 4 Dam as a case study was analyzed and the results revealed the essential role of modeling discontinuities and boundary conditions of rock foundation under seismic excitation. Also, The results demonstrate that the contraction joint openings during strong earthquakes are substantial and greatly change the arch to cantilever stress distribution in the dam body.