Theoretical investigations on load‐bearing capacity of RC flat‐plate framed structures subject to middle column loss

This paper presents theoretical studies on load‐bearing capacity of reinforced concrete flat‐plate framed structures. The existing methods for determining load‐bearing capacity of simply supported slabs are first reviewed, and their limitations are presented. An energy‐based refined method is then proposed to enhance the accuracy of the existing methods by considering the contributions to internal energy dissipation due to the extension of reinforcing bars along yield lines, the additional resultant bending moment from membrane forces, and the sectional bending moment along yield lines of slabs. The refined method for simply supported slabs is extended for fixed supported slabs, and thus, the load‐bearing capacity of reinforced concrete flat‐plate structures subject to a middle column loss is analytically determined. The performance of the proposed method is validated against test results and also verified against finite element analyses. Parametric studies are conducted to investigate the effect of reinforcement ratio, slab thickness and aspect ratio on the stiffness, and yield‐line resistance of structures. It is found that for the fixed supported slabs, it is reasonable to assume negative yield lines along the slab edges to consider the effect of obvious concrete crushing along the edge. Square slabs have higher ultimate loads than rectangular slabs, due to a longer horizontal yield line in the middle in the rectangular slab, which has detrimental effect on the sectional ultimate bending moment. The numerical results show that the reinforcement ratio has little effect on the initial bending stiffness and yield‐line resistance of slabs for a given slab thickness and aspect ratio. The initial stiffness and yield‐line resistance increase as the slab thickness increases. For the same reinforcement ratio and slab thickness, a larger aspect ratio leads to a lower initial bending stiffness, yield‐line resistance, and stiffness in tensile membrane action stage, due to a longer yield line along which tensile membrane forces have a detrimental effect on the sectional bending moment.     

双向面内约束混凝土板火灾试验及数值分析

为研究混凝土板在双向面内约束作用下的火灾行为,对4块混凝土矩形板进行恒载-升温条件下的火灾试验,获得试验板沿板厚的温度场分布、钢筋温度、板平面内外变形、板角约束力和破坏模式等规律。在此基础上,编程对试验板的温度场、变形行为和薄膜机理进行数值分析。结果表明：对于双向面内约束板,其裂缝模式与简支板和单向约束板不同;面内约束作用致使混凝土板后期变形及变形速率增大;不同面内约束下混凝土板具有不同的薄膜机理;相比单向面内约束,双向约束作用更不利于混凝土板受拉薄膜效应发展。     

Strengthening of one-way spanning RC slabs with cutouts using FRP composites

This paper reports the results of tests on fibre reinforced polymer (FRP) strengthened one-way spanning reinforced concrete (RC) slabs with central cutouts. Four wide slabs with cutouts were tested in addition to two narrow slabs without cutouts. Different positions of applied line loads for the slabs with cutouts resulted in different slab bending action and hence different FRP behaviour for the strengthened slabs. All FRP-strengthened slabs achieved a higher load-carrying capacity than their unstrengthened control counterparts. In addition, all strengthened slabs failed by debonding initiating at intermediate cracks (IC debonding) and in the case of the slabs with cutouts, the critical cracks were diagonal and originated from the corners of the cutout. The ability of the FRP to redistribute stresses around the cutout, the failure mechanisms, as well as pre- and post-debonding behaviour of the strengthened slabs was therefore assessed for different load application positions. Strains on the FRP, concrete and internal steel reinforcement, as well as deflections at different positions on the slab surfaces are also reported. An analytical model, which is based on the ultimate moment of resistance about critical crack lines, is also reported and it its predictions are found to correlate well with the experimental results. The analytical model is able to capture the different slab bending actions in addition to the debonding failure of the strengthened slabs.     

混凝土连续板火灾后力学性能试验及理论分析

为研究受火跨位置和数量对混凝土连续灾后板力学行为影响,对一块常温连续板和五块混凝土灾后连续板开展力学性能试验,分析灾后板裂缝、变形、混凝土和钢筋应变以及灾后破坏模式等规律。基于塑性铰线理论,提出混凝土双向板破坏模式和椭圆方程,推导板块内力平衡方程,建立灾后板剩余承载力计算方法,并与其他方法计算结果进行对比分析。结果表明,相比常温板屈服线延性破坏模式,灾后板易出现脆性破坏,即加载点或内支座处冲切破坏,特别是爆裂严重工况。相比混凝土或钢筋应变破坏准则,变形破坏准则l/50更适用于确定灾后板剩余极限承载力。此外,本文方法可用于确定灾后板剩余极限承载力,计算结果偏于保守。对于灾后板,应分别采用弯曲理论和冲切理论进行剩余承载力分析,两者最小值可作为剩余极限承载力。     

Repair and structural performance of initially cracked reinforced concrete slabs

Crack is one of the most common defects observed in reinforced concrete slabs and beams. Major cracks in concrete structures may occur due to overloading, corrosion of reinforcement or differential settlement of support. To restore the structural capacity of the distressed elements, retrofitting and/or strengthening are needed. There are different techniques available for retrofitting and strengthening of different reinforced concrete structural elements reported in the literature. This paper investigates the structural behaviour of cracked reinforced concrete one-way slab, which is repaired using different techniques.Five different techniques are used for the purpose of repair in the cracked concrete slab namely; cement grout, epoxy injection, ferrocement layer, carbon fibre strip and section enlargement. The slabs were loaded to failure stage and the structural response of each slab specimens have been predicted in terms of deflection, variation of strain in concrete and steel, collapse loads and the failure modes.The efficiency of different repair and strengthening techniques and their effects on the structural behaviour of cracked one-way reinforced concrete slab had been analyzed. It was observed that the type of repair technique used will affect the load carrying capacity of the slab and will lead to a redistribution of the strains and hence stresses in both concrete and steel reinforcement. All repair techniques are found to be able to restore or enhance the structural capacity of cracked concrete slabs.     

Crushing and flexural strength of slab-column joints

In multi-storey buildings, columns are usually not continuous through the slabs to enhance ease of construction. Consequently, in slab-column joints, slabs have to carry column loads in addition to the shear and bending moments due to loads applied to the slab. In most cases, when high strength concrete is used for the columns and normal strength concrete for the slabs, compression stresses at the support areas of the inner columns exceed the uniaxial compressive strength of the concrete of the slab. Due to this reason, most current details for such regions reinforce the concrete of the slab between columns to ensure load transfer. Typically, this is achieved by linking top and bottom columns with reinforcement. Sometimes, it is also needed to incorporate special load transfer devices. This latter solution is however relatively complicated and expensive.In this paper, the crushing and flexural strength of slab-column joints is investigated accounting for an increase of the compressive strength of the failure region (concrete between columns) due to confinement stresses provided by the flexural reinforcement of the slab. The results of an experimental programme on 6 full-scale slabs (250 mm thick) are presented showing that flexural reinforcement of a slab significantly increases the crushing strength of slab-column joints. This allows ensuring load transfer without incorporating special devices or even without linking top and bottom column reinforcement for a wide range of cases leading potentially to more economic designs. An analytical approach, grounded on the theory of plasticity, is also presented allowing one to determine a failure criterion for such regions. This approach, which can also be used for design purposes, leads to an excellent correlation with test results.     

Experimental investigation on thermal and mechanical behaviour of composite floors exposed to standard fire

This paper presents experimental investigations on the thermal and mechanical behavior of composite floors subjected to ISO standard fire. Four 5.2 m×3.7 m composite slabs are tested with different combinations of the presence of one unprotected secondary beam, direction of ribs, and location of the reinforcement. The experimental results show that the highest temperature in the reinforcements occurs during the cooling phase (30–50 °C increment after 10-min cooling). The temperature at the unexposed side of the slabs is below 100 °C up to 100-min heating, compared to the predicted fire resistance close to 90 mins from EC4. For the slabs without secondary beams, the cracks first occur around the boundaries of the slab, while for the slabs supported by one unprotected secondary beam, concrete cracks first occur on the top of the slab above the beam due to the negative bending moment, and later on develop around boundaries. Debonding is observed between the steel deck and concrete slab. The secondary beam significantly impacts the deformation shape of tested slabs. Although a large deflection, 1/20 of the span length, is reached in the tests, the composite slabs can still provide sufficient load-bearing capacity due to membrane action. The occurrence of tensile membrane action is confirmed by the measured tensile stress in the reinforcement and compressive stress in the concrete. A comparison between measured and predicted fire resistance of the slabs indicates that EC4 calculations might be used for the composite slabs beyond the specified geometry limit, and the prediction is conservative.     

The tensile membrane action of unrestrained composite slabs simulated under fire conditions

Following a major fire test programme on a full-scale, steel-framed building it was found that the composite flooring system, comprising lightweight concrete, anti-crack mesh reinforcement and steel deck, had a greater inherent fire resistance than suggested by current codified design methods. It was felt that this was due to tensile membrane action occurring in the slab at large displacements. This led to an independent test being conducted at the Building Research Establishment where a 9.5 m×6.5 m composite slab, with nominal horizontal restraint to its edges, was tested to failure. To simulate the behaviour of the slab in fire, the steel deck was removed, leaving the concrete and anti-crack reinforcement, before load was applied. Tensile membrane action was shown to occur, with the failure load being approximately double that calculated using the classic yield line theory.     

钢-混凝土组合楼板实用抗火设计方法

利用钢-混凝土组合楼盖抗火性能非线性有限元分析程序对不同荷载大小、不同压型钢板厚度、不同配筋的压型钢板-混凝土组合楼板进行了抗火性能分析,找出了火灾影响下组合楼板结构性能的因素,给出了计算组合楼板耐火时间的拟合公式,得到了一些适用于压型钢板-混凝土组合楼板抗火设计的结论和建议。     

Fire tests on bonded post-tensioned concrete slabs

The results from eight fire tests conducted on bonded post-tensioned one-way spanning concrete slabs are presented in this paper. The fire tests were augmented with two additional tests at ambient temperature, carried out to failure on slabs with identical geometry and prestressing tendons. The different structural response between using plastic and metallic ducts, Limestone and Thames Gravel aggregates, and different axial restraint conditions to longitudinal thermal expansion, have been highlighted. Slabs with Thames Gravel aggregates were shown to have a much higher deflection compared to slabs with Limestone aggregates, with restrained slabs having a lower vertical deflection compared to equivalent unrestrained slabs. In all the fire tests, cracks directly inline and parallel to the tendons occurred due to thermal stresses at relatively low tendon temperatures, which were not observed in the ambient tests. It is shown that the use of plastic ducts resulted in slightly higher tendon temperatures due to the ease at which water migrated from the grout once the duct had melted. The fire tests have shown that the fire resistance specified in current codes of practice are generally conservative for bonded post-tensioned one-way spanning concrete slabs. The tests have provided detailed experimental data in the form of temperature distributions within the slab, vertical and horizontal displacements and strains in the tendons, which will allow validation of future computer models to predict the behaviour of bonded post-tensioned concrete slabs under fire conditions.     

不同预应力筋布筋型式双向板的受力性能及承载力计算

预应力混凝土双向板已广泛应用于工程建设 ,迄今为止 ,工程中的双向板大量采用的预应力筋布筋型式是双向均匀布置 ,关于预应力筋布筋型式对四边简支双向板受力性能及承载力影响的研究还未见报道。因此 ,本文着重对板中央承受局部荷载的双向均匀布置预应力筋、双向中密边稀布置预应力筋、双向中间密布预应力筋的三块双向板的受力性能进行了试验研究 ,并采用塑性铰线理论对三块试验板的承载力进行了分析和计算。试验研究和计算分析表明 ,双向中间密布预应力筋试验板的承载力最高 ,双向中密边稀布置预应力筋试验板的承载力次之 ,双向均匀布置预应力筋试验板的承载力最低 ,塑性铰线理论可应用于四边支承预应力双向板的承载力计算。     

GFRP筋地下连续墙混凝土板受弯性能试验研究及有限元分析

为了解GFRP筋地下连续墙的受弯性能,通过GFRP筋混凝土板和钢筋混凝土板的对比受弯试验,分析了两者的受力-变形过程和破坏形态,对比了两者的挠度、开裂荷载、极限荷载以及混凝土应变。结果表明：GFRP筋混凝土板的受力-变形曲线大致可划分为开裂前和开裂后两个阶段,其破坏表现为脆性;混凝土开裂前两种板的截面应变变化规律均基本符合平截面假定,但开裂后GFRP筋混凝土板的挠度增长速率远大于钢筋混凝土板,且该速率基本不变;两种板的开裂荷载较为接近,而GFRP筋混凝土板的极限荷载为钢筋混凝土板的1.2倍。在试验基础上,建立了GFRP筋混凝土板的有限元模型,通过参数分析表明,GFRP筋混凝土板的抗弯刚度在开裂后随配筋率的增大而增大。图13表6参8     

Non-linear structural modelling of a fire test subject to high restraint

The computer code VULCAN has been developed for the three-dimensional structural analysis of composite and steel-framed buildings in fire. In this paper, the main features of the program are outlined, with particular emphasis on the most recent development to the layered procedure for modelling of concrete floor slabs. This development has introduced geometric non-linearity into the modelling of slabs, whose layer structure already allowed temperature distributions and change of material properties through the thickness, as well as modelling the effect of the ribs at the bottom of composite decking slabs. The capabilities of the model are firstly tested at ambient temperature for a uniformly loaded ribbed reinforced concrete slab with simply supported edges, and this is followed by a very detailed modelling of the Cardington restrained beam fire test. In both cases the development of membrane action is demonstrated and the structural behaviour is compared with the geometrically linear case. A number of studies are carried out to demonstrate the influence of the major floor slab details on the behaviour of the structure in fire conditions. These studies provide evidence that when exposed steel temperatures are less than 400°C the concrete slab has little influence, other than to play a part in generating thermal curvature to composite beams. For temperatures higher than about 500°C the effect of the slab progressively becomes much greater, and it is very important to model concrete slabs correctly. The influence of membrane action cannot be ignored, particularly when the fire compartment is subject to high restraint because it is surrounded by cool, stiff structure. At very high temperatures the floor slab becomes the main load-bearing element and the floor loads above the fire compartment are carried by the membrane forces developed in the slab, with tension being carried mainly by the steel anti-cracking mesh or reinforcing bars. However, the effect of the very high in-plane restraint to thermal expansion in the particular Cardington test considered is to enhance the peripheral zone of compressive membrane force and to reduce the extent of the central area of tensile force compared with more usual cases.     

Flachdecken aus Stahlfaserbeton

Steel Fibre reinforced concrete Flat Slabs Steel fibre reinforced concrete is a proven and reliable material for slabs on grade of industrial floor systems. Since several years, steel fibre reinforced concrete with additional reinforcement bars is used for free suspended pile supported industrial floors. The load carrying behaviour under service loading conditions is similar to that of elevated flat slabs. Recently, concretes with high fluidity allow higher fibre dosages than before. A full scale loading test on a flat slab of 340 m² at TREFILARBED in Bissen, Luxemburg has proven that elevated slab structures made of steel fibre reinforced concrete with 100 kg/m³ fibre content can compete with traditional reinforced concrete slabs by means of load carrying capacity as well as concerning cost effectiveness. This paper deals with the development of free suspended SFRC slabs based on the performed full scale tests. Formulas for the ultimate limit state design according to the yield line theory are presented. Design criteria at serviceability limit state are provided.     

Zeitabhängiges Verhalten von Makrokunststofffaserbeton und dessen Einfluss auf die Bemessung von Industriefußböden

Time Dependent Behaviour of Macro Synthetic Fibre. Reinforced Concrete and its Influence on the Design of Industrial Slabs on Ground Material tests and tests on structural elements with synthetic macro fibre reinforced concrete are presented. The experimental program included the investigation of the load carrying capacity and the deformation behaviour under short‐term as well as under long‐term loading conditions. Long‐term load tests were carried out on pre‐cracked fibrereinforced concrete beams to measure the load level above which creep failure can occur. At service load level creep coefficients for a time period of up to three years were determined. Centre point load tests were carried out on concrete slabson‐ground. Two slabs were reinforced with steel mesh and two with synthetic macro fibres only. The tests were performed under short‐term and long‐term loading conditions over a period of 18 month. The current main application of synthetic macro fibre reinforced concrete is slab‐on‐ground. Slabs‐on‐ground can be designed under the assumption of elastic subgrade reaction. The concrete slab itself can be calculated either by elastic theory for uncracked concrete or by yield line theory taking fibre reinforcement into account. Based on the test results of the experimental programme, design recommendations for permanently loaded slabs‐on‐ground are presented.     

预应力混凝土空心叠合板试验研究

针对现有的一般预应力空心叠合板楼盖板厚较大,自重较大等缺点,提出了一种新形式的预应力混凝土空心叠合板,通过对3块简支板,3块简支叠合板和1块两跨连续叠合板进行静力荷载试验,分析了这种板在静力荷载作用下的裂缝、承载力、挠度等特点,研究了其开裂荷载和极限承载力较高的原因。试验结果表明这种预应力空心叠合板具有良好的抗裂性能和较高的极限承载能力,建立了开裂弯矩、极限承载力、挠度的计算方法,为这种叠合板的设计和工程应用提供了依据。     

钢筋混凝土缺角板受弯性能试验研究

为了研究混凝土缺角板的受弯性能,设计制作了角部不同开孔大小的7块钢筋混凝土四边简支板,并进行了静力受弯性能试验。结果表明：缺角板跨中区域钢筋先于板四周区域钢筋屈服,随后屈服范围不断扩展并往四角延伸,最终板顶缺角处沿与板边大致成45°方向的混凝土被压碎;开孔系数(板角所开孔洞边长与跨度的比值)为0.05、0.08、0.10、0.12、0.15和0.20的缺角板极限荷载较矩形板的极限荷载分别降低5.98%、10.35%、21.37%、23.63%、38.50%和49.29%,开裂荷载、屈服荷载基本随开孔系数的增大而降低。根据钢筋混凝土板塑性极限分析的塑性铰线法,对6块不同开孔系数混凝土缺角板提出了4种塑性铰线模式,并利用虚功原理建立缺角板极限荷载计算公式,极限荷载计算值与试验值吻合较好,验证了所提出的塑性铰线模式的合理性,及利用虚功原理建立缺角板极限荷载计算公式的可行性。引入一个与开孔系数相关的系数对塑性铰线模式进行简化,简化公式得出的计算值与试验值吻合较好。     

Modelling of unbonded post-tensioned concrete slabs under fire conditions

This paper investigates the structural behaviour of unbonded post-tensioned one-way spanning concrete slabs in fire conditions. The slabs were simply supported and reinforced with 15.7 mm nominal diameter seven-wire mono-strand tendons. A nonlinear finite element model for the analysis of post-tensioned unbonded concrete slabs at elevated temperatures was developed. The mechanical and thermal material nonlinearities of the concrete, prestressing tendon and anchorages have been carefully inserted into the model. The interface between the tendon and surrounding concrete was also modelled, allowing the tendon to retain its profile shape during the deformation of the slab. The temperature distribution throughout the slab, time–deflection behaviour, time–longitudinal expansion, time–stress behaviour in the tendon, and the failure modes were predicted by the model and verified against test data. The study has shown that the coefficients of thermal expansion currently used in the European Code for calcareous and siliceous concrete can lead to inaccurate predictions of the structural behaviour. A parametric study was conducted to investigate the effects on the global structural behaviour due to the change in the aggregate type, load ratio and boundary conditions. It was shown that by varying the boundary conditions the fire resistance was greatly affected. Although changing the aggregate type and load ratio affected the time-displacement response, the fire resistance defined by failure of the slab was not affected due to the splitting mode of failure above the tendon locations not being affected by these parameters. Comparison with the codes shows that the UK code BS8110 is generally unconservative, whereas the Eurocode EN1992-1-2 provides reasonable design rules.     

钢筋混凝土单向梁板子结构抗连续倒塌试验研究

梁和板组成的楼盖系统是框架结构的主要抗连续倒塌构件。为了分析各类结构参数对钢筋混凝土楼盖系统抗连续倒塌性能的影响,该文首先根据《混凝土结构设计规范》GB 50010—2010设计制作了8个钢筋混凝土单向梁板子结构缩尺试件,这些试件具有不同的截面尺寸和配筋率。然后通过竖向加载试验研究这些试件在中柱破坏后的材料变形/损伤和抗连续倒塌承载力。试验结果表明：带楼板的子结构试件的承载能力明显高于相同截面的梁试件的承载能力;试件在梁机制阶段的承载能力主要由截面尺寸和钢筋面积所决定,而悬链线机制阶段的承载能力主要由截面中连续钢筋面积所决定;楼板的宽度、厚度和板内配筋以及梁高对梁机制下的承载力有较大的提高,其中板宽在大于一定值后影响变得不显著;只有楼板宽度和楼板配筋率对悬链线机制下的承载力有显著影响;梁内抗震配筋对缩尺试件在两个阶段的抗连续倒塌承载力影响都不大。     

单、双向钢筋混凝土板的受力性能分析

对工程设计常用参数下的单双向钢筋混凝土板的受力性能进行分析。通过对比说明,按常用工程设计经验中四边支承的钢筋混凝土板配筋按单向或双向板进行的设计计算时,其两者受力性能之间的差异,为实际工程设计提供参考。