Proposed Revision Of Code For Design Of Concrete Chimneys And Research On The Design Method

Chimney is one of the indispensable engineering projects for industry and civil engineering.With the increasing use of pulverized coal-fired boliers in electric power-generating utilities,a huge number of reinforced concrete chimneys have been developed.In recent years,in order to ensure the flue gas emissions in a certain height,and improve the utilization of fuel for chimneys,a lot of ultra-high chimneys come out and some of the thermal power plant's chimney exceeds 240 m.For the tube-in-tube reinforced concrete chimney,the external shell and exhaust pipe are separated,making the outer shell not contact with corrosive flue gas.Therefore,it can ensure the durability of the chimney structure.With this advantage,the tube-in-tube reinforced concrete chimneys have been widely used.In the period time of chimney construction with the rapid development,some problems associated with design calculation and construction have appeared with the development of new heights and new forms for chimneys construction.Many domestic companies already have expanded overseas construction markets,however,there are still agreat gaps between the the chimney design specifications from aboard to domestic.A large number of overseas projects are designed using the American Chimney Design Specification,which is highly recognized by the international experts.Thus,it is necessary to sort out the American specification for the revision of China's regulations.It can be illustrated from the fieldworks that vertical cracks appear on the surface of a large number of chimneys,reducing the overall stiffness and durability of the chimney.The radius of the chimney's shaft has increased because of the use of multi-flue and tube-in-tube chimneys and the circumferential wind bending moments.As a significant factor causing the vertical crack for the chimney shell,it has come to be a new problem in the process of chimney design.In the reference of American "Code Requirements for Reinforced Concrete Chimneys(ACI 307-08)and Commentary",Chinese "Code for Design of Chimneys"(GB 50051-2013)(hereinafter referred to as "the Code")add a new design requirement,i.e.,the formula of circumferential wind bending moments.However,these codes lack detailed analyses in terms of the influencing factors and coefficients of circumferential wind moments.Additionally,during the process of serviceability limit states calculation of reinforced concrete chimneys,the temperature stress is crucial for the structure design.With consideration of the coupled effect of temperature and load,the discontinuity about the windward side steel tension stress can be neglected in the guidance of the Code.In order to meet the requirements of the chimney construction,it is necessary to revise and complete the Code.Based on the above considerations,the specific work and the main conclusions are as follows:(1)In chronological order,the paper outlines the main contents of the reinforced concrete chimneys including 1983,2002,2013 editions of the Chinese Code for design of chimneys and the American Code Requirements for Reinforced concrete Chimneys since 1969.Meanwhile,the project gave a detailed explanation about the cause,adjustments and the related changes in terms of the specification versions.Besides,the Code deletes the flat wall method in the calculation of the shell thermal resistance,reserves the ring method during the calculation of the horizontal earthquake effect and uses the dynamic response spectrum analysis method to replace the simplified calculation method.Additionally,the Code makes corresponding adjustments to across-wind loads.Referring to "Load Codes for the design of building structures"(GB 50009-2012),the American specifications mainly modify the calculation methods with consideration of horizontal earthquake action,basic wind pressure,and across-wind loads.(2)With combination of the engineering projects and numerical analyses,it can be concluded that the value of circumferential wind bending moments from the Code is inferior to the American Standard(ACI 307-08)result.With the method of the finite element software i.e.ABAQUS,a numerical simulation about the circumferential wind bending moments of reinforced concrete chimneys is conducted and some conclusions can be drawn as follows:(i)The circumferential wind bending moments is proportional to the square of radius and reference wind pressure;(ii)Since the gust factor is not taken into account in the reference of the circumferential wind bending moments formula of the Code and the coefficient of the reference wind pressure at the top of the chimney is not multiplied by 1.5,the result on the basis of the Code is less than the finite element result and the result of American code;(iii)the paper suggests that the Code should be revised with consideration of the American regulations and gust factor.Meanwhile,the revision of the Code should also properly amplify the bending moment of the annular wind within a certain range of the top,and adjust the coefficient in the process of calculation formula to correct the circumferential wind bending moments formula.(3)Combined with engineering projects,this paper analyzes the cause about the discontinuity of the tensile stress of the reinforcement under the coupled effect of the load and temperature.The explanations towards the discontinuity are as follows:(i)The values of concrete elastoplastic modulus used for analyses about the large eccentricity compression and small eccentricity compression are inconsistent;(ii)The effect of temperature is not taken into account in the calculation of the tensile strain uneven coefficient of the steel bar.Based on related literature,the tensile strain uneven coefficient formula of the steel bar and the tensile stress formula of the steel bar under the pressure of the small eccentricity can be deduced,reducing the jumping amplitude abouttensile stress of the bar steel.