【作者】 闫凯；

【导师】 王焕定； 郑文忠；

【作者基本信息】 哈尔滨工业大学 ， 工程力学， 2012， 博士

【摘要】 建筑火灾是发生频度较高的灾种。既有底部框架砖房在我国既有建筑中约占15%，《建筑抗震设计规范》（GB50011-2010）中关于底部框架砖房的设计条款有一定篇幅，故仍会继续建造底部框架砖房。一般作为商业用房使用的底部框架层含有大量可燃物，火灾荷载相对较大；上部楼层为砖砌体结构，底部框架承受的永久荷载在全部荷载中所占比例较纯框架大，当设计轴压比相同时，底部框架柱承担的实际荷载较纯框架柱大。由于梁柱截面选择和配筋构造与抗震构造等级相关，非抗震设防区和6度抗震设防区的底部框架砖房，底部框架柱长期处于高轴压比、低配筋率的状态，底部框架可能是此类建筑的抗火薄弱层，火灾下易因底部框架抗力不足而发生整体坍塌。7度和8度抗震设防区的底部框架砖房，尽管底部框架梁柱截面尺寸相对较大，配筋相对多些，但能否确保底部框架砖房在火灾下的安全，也需要开展研究。既有底部框架砖房和新建底部框架砖房的抗火性能与火灾安全问题，是工程界十分关注的热点问题之一。本文针对底部框架砖房抗火问题开展以下研究工作：（1）考虑到粘土砖砌体线膨胀系数为5×10-6/℃，混凝土的线膨胀系数为1×10-5/℃，粘土砖砌体线膨胀系数仅为混凝土线膨胀系数的一半；同时考虑到底部框架发生火灾，其托梁顶的温度一般不超过160℃，砖砌体温度会更低，且离托梁距离越大，受火灾影响越小。进行底部框架砖房在底部框架层发生火灾时的受火分析时，假定上部砖砌体不受火灾影响。底部框架层发生火灾时上部砖砌体与框架梁（墙梁的托梁）间的荷载传递机制，是底部框架砖房受火分析必须解决的问题之一。采用Hill屈服准则和Rankine屈服准则，合理考虑压应力对砖砌体抗剪强度的影响，通过在砖砌体两个材料主轴方向上定义不同的材料属性，构建了考虑材料各向异性硬化和软化的砖砌体弹塑性整体化模型，采用Newton-Raphson方法和隐式Euler积分回映算法，实现了对塑性屈服面非线性方程组的求解和应力更新。通过对试验结果和计算结果的对比分析，得出与各向同性砖砌体材料模型相比，考虑材料各向异性的砖砌体弹塑性模型能够更为准确的分析砖砌体在平面应力下的弹塑性性能。为包括底部框架层发生火灾时上部砖砌体与框架梁（墙梁的托梁）间的荷载传递机制在内的底部框架砖房抗火计算分析提供了模型。（2）实现对火灾下底部框架砖房的全过程分析，是把握该类建筑抗火性能的关键。分析确定了高温下钢筋和混凝土的热力学参数，基于热传导理论建立了底部框架温度场分析模型，编制了混凝土瞬态热应变和高温徐变的计算程序及钢筋高温蠕变的计算程序，合理考虑了高温下钢筋与混凝土间粘结滑移性能，结合ABAQUS有限元软件的顺序耦合热力分析平台，引入考虑材料各向异性的砖砌体弹塑性模型，建立了底部框架砖房抗火分析精细化模型，实现了火灾和外荷载共同作用下底部框架从起火到坍塌的全过程分析。通过已有钢筋混凝土构件和结构抗火试验对该精细化模型进行验证，结果表明钢筋混凝土构件和结构抗火分析时，考虑混凝土的瞬态热应变、高温徐变以及钢筋的高温蠕变是有必要的，火灾下底部框架温度场分析模型和力学性能分析精细化模型具有较好的准确性。（3）基于底部框架砖房抗火分析精细化模型，通过变参数，考察了火灾下框架梁（墙梁的托梁）顶面竖向荷载分布的变化、底部框架所承受荷载效应的变化及底部框架的破坏过程。获得了荷载水平、梁柱截面尺寸和配筋、保护层厚度和上部砖砌体房屋层数等参数影响的底部框架从起火到坍塌的发展过程和规律。结果表明，火灾下高温引起的框架梁（墙梁的托梁）轴向膨胀变形和向下挠曲变形对墙拱传力机制不利，使框架梁（墙梁的托梁）内力显著增大，加速边柱顶端外侧纵向钢筋受拉屈服，内侧混凝土被压碎。边柱上端损伤后中柱混凝土被压溃，砖墙的大拱效应使中柱承担的荷载减小。（4）底部框架砖房的底部框架设计时应同时满足抗火与抗震的双重要求。基于底部框架砖房抗火性能参数分析结果，对不同耐火极限相应的底部框架柱轴压比、梁柱截面尺寸和配筋率与不同抗震等级相应的底部框架柱轴压比限值、梁柱最小截面尺寸和配筋率进行了对比分析，结果表明，非抗震设防区的底部框架柱截面尺寸大于500mm×500mm，边柱纵向钢筋配筋率大于1.5%，以满足耐火等级大于3.0h；抗震等级为三级的底部框架边柱纵向钢筋配筋率需大于0.8%、1.0%和1.2%，以满足耐火等级大于2.0h、2.5h和3.0h；抗震等级为一级和二级的底部框架耐火极限均大于3.0h。基于柱轴压比、梁柱截面尺寸与配筋、保护层厚度等关键参数对底部框架抗火性能的影响规律，提出了底部框架梁柱截面选择、配筋构造、保护层厚度取值等抗火设计建议，以使底部框架抗火设计与抗震设计相协调。（5）底部框架层发生火灾后，根据受火损伤程度不同，一些受火损伤后的底部框架加固后可继续使用。抗震设防区火灾后受火损伤的底部框架砖房需进行抗震加固计算。针对以往水平荷载下底部框架的内力计算时，上部砖砌体材料假设为各向同性，以及框架梁内力计算未考虑其与上部砖砌体共同工作的问题，开展了780个常温时水平荷载下该类房屋的底部框架计算分析，获得了梁柱线刚度比、托梁高跨比等对托梁内力、柱反弯点和柱剪力分配的影响规律，提出了考虑上部砖砌体和构造柱影响的水平荷载下底部框架的内力计算方法。为火灾后底部框架砖房安全性评估及对火灾后受火损伤的底部框架合理进行抗震加固提供了参考依据。更多还原

【Abstract】 Building fire is a high frequency disaster. The existing bottom frames of multi-story masonry buildings take a certain proportion of existing buildings in our country.Code for seismic design of buildings (GB50011-2010) has a certain amount ofclauses for the bottom frames of multi-story masonry buildings design, so manybuildings of this structural type will still to be constructed. The fire load of thebottom frames is large for the reason that the bottom floor is commonly used asmercantile occupancy. The upper floors of the bottom frames are masonry structures,the ratio of the constant load to the total loads is larger for bottom frames, socolumns of bottom frames endure more actual load than that of bare frames, whendesigned axial compression ratios are equal for them. For the reason that sectionselection and reinforcement design of structural members are related to seismicclasses, in non-seismic fortification zone and6degree seismic fortification zone,columns of bottom frames are in high axial compressive ratio and low steel ratio stateduring their inservice time, bottom frames are the unsubstantial floors of this type ofbuildings in fire, these buildings are prone to collapse for the resistance of bottomframes is inadequate. In7and8degree seismic fortification zone, though dimensionsof structural members and reinforced steel ratios are respectively large, the safety ofthese buildings exposed to fire is necessary to be studied. Fire resistance and firesafety of the bottom frames of multi-story masonry buildings is one of the hot issuesin civil engineer. The following works are carried out:(1) Considering the liner expansion coefficient of masonry is5×10-6/℃, whichis only half of that of concrete,1×10-5/℃. The temperature of the top of joist is lowerthan160℃, the temperature of upper masonry is lower than that of the top of joist,and the effect of temperature on masonry decreases with the distance betweenmasonry and joist increases. In the process of fire resistance analysis of bottomframes, masonry is considered as temperature-independent material. The mechanicalmodel of load transmitting between upper masonry and joist during the bottomframes exposed to fire is one of the problems in analysis of fire resistance of thebottom frames. Hill yield criterion and Rankine yield criterion are adopted, the effectof pressure on shear strength is introduced reasonably, the anisotropic hardening and softening macro material model is put forward for elasto-plastic analysis of masonry,by means of defining different material properties in two material principal axis ofmasonry. Newton-Raphson method and implicit Euler backward return mappingalgorithm are employed to solve nonlinear equations of yield surfaces and to updatestresses. From the comparison anlysis between experiment results and calculationresults, the proposed anisotropic material model shows better performance thanisotropic material models in elasto-plastic analysis of masonry in plane stress states.The proposed anisotropic material model provides the practicability for fireresistance analysis of the bottom frames of multi-story masonry buildings, whichcontains the mechanical model of load transmitting between upper masonry and joistduring the bottom frames exposed to fire.(2) The key issue for the fire resistance of the bottom frames of multi-storymasonry buildings is to realize the entire process analysis of the bottom frames ofmulti-story masonry buildings exposed to fire. After the determinations of materialthermal and mechanical parameters at elevated temperatures, temperature fieldanalysis model of the bottom frames is set up based on heat conduction theory.Calculation subroutine of concrete creep, transient thermal strain and steel creep atelevated temperatures is written. Bond-slip behavior between concrete and steel baris also adopted reasonably. The fine model for mechanical behavior analysis of thebottom frames of multi-story masonry buildings exposed to fire is established basedon sequential coupling temperature and stress analysis platform in ABAQUS andconcrete plastic finite element theory. By introducing the anisotropic masonry model,the fine models can be used to analyze the entire process from fire breaking out tothe frames collapse on condition that the vertical load is invariable. The experimentalresults comparison shows that it is necessary to introduce concrete transient thermalstrain and creep, and steel creep at elevated temperatures in fire resistance analysis ofconcrete structures. The temperature field analysis model and fine mechanicalbehavior analysis model have good accuracy.(3) The changes of vertical stress distribution on the top of joist and load effectof bottom frames, the failure process of bottom frames are studied by parametricanalysis, using the established fine finite element models. A parametric study isconducted to investigate the effects on the entire process from fire breaking out to thebuilding collapses when the bottom floors exposed to fire due to the change in loadratio, sectional dimension, reinforcement ratio of columns and beams, and number of masonry storeys. Axial expansion deformations and flexural deflections of joistcaused by high temperature produce adverse effect on arch mechanism fortransmitting the vertical loads of masonry structures, which increases the inner forceof joist, and accelerates longitudinal reinforcement outside of the top of edge columnyield for tensile, then concrete inside of the top of edge columns yields for pressure.Concrete of the middle columns crushes after the damage of the top of outsidecolumns.(4) The bottom frames should be designed to satisfy to seimic design and firesafety requirements. Comparative analysis is carried out between the effect of axialcompression ratio, section dimensions and reinforcement ratio of joists and columnson fire resistance of the bottom frames, and criterion on axial compression ratio,section dimensions and reinforcement ratio of the bottom frames in current seismiccode. The result shows that the sectional dimension of columns should be lager than500mm×500mm, and longitudinal reinforcement ratio of edge column is lager than1.5%, so that the fire resistance of the bottom frames in non-seismic region is aboveto3.0h. The longitudinal reinforcement ratios should be larger than0.8%,1.0%and1.2%, so that the fire resistances of the bottom frames in seismic grade three is aboveto2.0h,2.5h and3.0h respectively. The fire resistances of the bottom fames designedwith seismic grade one and two are larger than3.0h. Based on the effect laws of keyparameters on fire resistance of the bottom frames, including axial compressive ratioof columns, sectional dimension and reinforcement ratio of columns and beams, andthickness of concrete cover, the details of fire safety design for the bottom frames tomatch different fire resistance construction degrees are put forward, such as sectionaldimension, reinforcement ratio, and thickness of concrete cover of columns andbeams, so that the fire safety design of the bottom frames is coordinate with seimicdesign.(5) Many damaged bottom frames can be used continuely after reinforcement, onthe basis of different damage degrees of the bottom frames after fire. Seismicreinforcement calculation is required for the damaged bottom frames of multi-storymasonry buildings after fire in earthquake fortification zones. Focus on the problemthat masonry is assume as isotropic material in force analysis of the bottom frames,and no effect of upper masonry introduced in calculating the force of joist in theprocess of the bottom frames force calculation on condition that the buildings underhorizontal load, which is treated as pure frame beam,780bottom frames are analyzedby finite element method considering the effect of upper masonry, taking the masonry as anisotropic material. The effects of stiffness ratio of beams to columnsand depth-span ratio of the joists on the force of joists, the inflection point ofcolumns and shear force distribution in bottom columns are obtained. Calculationmethod for the force of the bottom frames with the effect of brick masonry andconstructional column under horizontal load is proposed. The calculation methodprovides reasonable reference for safety evaluation of the bottom frames after fire andseismic reinforcement design of the damaged bottom frames after fire.更多还原