Research On Continuous Reinforcement Technology Of Cold-formed Thin-walled C-shaped Steel Simply Supported Purlins

Cold formed thin-walled members have been widely used in industrial and civil buildings.The typical representative is the light portal rigid frame workshop.As a cold-formed thin-walled steel member,purlin plays an extremely important role in the roof system of light steel structure workshop.In recent years,the state has vigorously promoted solar photovoltaic power generation,and the roofs of old light steel structure plants have been widely used.The partial coefficients of dead load and live load in the unified standard for reliability design of building structures gb50068-2018 implemented on April 1,2019 are changed to 1.3 and 1.5(the old standard is 1.2 and 1.4).The general code for engineering structures gb55001-2021implemented on January 1,2022 stipulates that the live load of inaccessible roof is 0.5k N/m~2(the original portal frame design is generally taken as 0.3k N/m~2).Considering the increased weight of photovoltaic panels and supports,the general bearing capacity of purlins is insufficient,Reinforcement is required.At present,the commonly used reinforcement methods in China include increasing the section size of purlins,densifying purlins,etc.these reinforcement methods have low construction efficiency and large amount of steel consumption,which is uneconomical.As a continuous reinforcement method of purlins,simply supported to continuous purlins have less steel consumption,convenient construction and good economy.At present,there is a lack of relevant standards and specifications for the design of cold-formed thin-walled C-section steel continuous purlins,The research at home and abroad mainly focuses on the mechanical performance and connection of C-type continuous purlin,and there is less research on the mechanical performance and connection of locally strengthened C-type continuous purlin.In order to gradually improve the relevant research on local strengthening of C-shaped continuous purlins,for the reinforcement of cold-formed thin-walled C-shaped steel simply supported purlins,the reinforcement method of simply supported to continuous is adopted,that is,the connectors are nested at the support of adjacent purlins to make them continuous.The optimal reinforcement scheme is selected by using the methods of theoretical calculation,finite element analysis and experimental verification The connection mode and the specification and size of embedded connectors are studied.Finally,an engineering example is verified,calculated and analyzed to provide a basis for the application of C-type continuous purlin in practical engineering.The main contents are as follows:Firstly,some related theories of cold-formed thin-walled C-section steel purlin design are introduced,and then the three reinforcement methods of increasing purlin section size,densifying purlin and simply supported to continuous are calculated and compared through PKPM software toolbox.From the mechanical performance,economy and construction operability,it is concluded that the simply supported to continuous reinforcement method is the best reinforcement scheme.Secondly,in view of the current situation that China's relevant codes and standards do not clearly specify the nested lap length of C-shaped steel continuous purlin,the embedded C-shaped continuous purlin model is established by using the finite element software ABAQUS.The nested connectors at the support are effectively connected with the original purlin by bolts.Considering the purlin span of 6m,7m,8m and 9m and the lap length coefficient?Take 2%-20%for modeling analysis,and discuss the continuity of purlin?,According to the continuity of purlin?The reasonable lap length of embedded continuous reinforcement of cold-formed thin-walled C-steel purlin is obtained.The results show that the lap length of cold-formed thin-wall embedded C-type continuous purlin should not be less than 10%of the span of simply supported purlin.With the increase of nested lap length,the purlin continuity increases continuously.When the lap length is 10%of the purlin span,the purlin continuity reaches about 90%.Thirdly,the key of simply supported variable continuity lies in the nested lap at the support.The connection mode of nested lap at the support of adjacent purlin and the specification and size of nested lap are also important factors affecting the mechanical performance of C-type continuous purlin.Through the finite element software ABAQUS,the calculation and comparative analysis of bolt connection and structural adhesive connection under the embedded reinforcement scheme of continuous purlin are carried out.In addition,change the dimensions of embedded nested connectors to ensure the continuity at the support?On the premise of nearly 100%,the influence of nested lap connectors with different specifications and sizes on the lap length is studied.The results show that the continuity and stiffness of purlin embedded reinforcement are better than that of bolt connection.Considering the construction process and economy of the two connection methods,bolt connection can be preferred for purlin continuous reinforcement;Ensure the continuity at the support?On the premise of close to 100%,the required lap length of connectors with different specifications and sizes is different.With the continuous increase of connector specifications and sizes,the required nested lap length decreases;In order to ensure the convenience of the construction of nested connectors,the height of the web of the connector should not be the same as that of the original purlin.The web height of the nested connector of cold-formed thin-walled C-section steel continuous purlin should not be less than 70%of the web height of the original purlin,and the flange width should not be less than 60%of the flange width of the original purlin.Finally,taking a practical project as an example,simply supported purlins are connected by adding connectors at the support to form continuous purlins.ABAQUS finite element numerical simulation analysis and experimental research are carried out respectively by using embedded and outsourcing reinforcement schemes.The changes of purlin deflection and stress under the two continuous reinforcement schemes are calculated and analyzed,and compared with simply supported C-shaped steel purlins,In order to verify the rationality of the above numerical simulation conclusions.The results show that through these two continuous reinforcement schemes,the overall stiffness of simply supported purlin is significantly improved and the bearing capacity meets the design requirements;Using the continuous reinforcement scheme of outsourcing connectors at the support,the overall stiffness is greatly improved,and the reinforcement effect is good,but the construction is troublesome.The vertical displacement calculated by finite element simulation is less than the test value,which is due to the influence of the fastening degree of bolt connection,and the limitation of the lateral displacement of the upper flange can only approximate the stress skin effect of the roof slab;The continuous reinforcement scheme of simply supported purlins has little impact on the end span purlins.It is recommended to densify or increase the section size of purlins for reinforcement of end span purlins.