Punching shear strength of interior slab–column connections strengthened with carbon fiber reinforced polymer sheets

In this paper, punching shear strengthening of flat slabs using Carbon Fiber Reinforced Polymer (CFRP) sheets is studied. Fifteen specimens of reinforced concrete slabs were tested. Thirteen of them were strengthened by CFRP sheets and two specimens were kept as control specimens. Four of these strengthened specimens were tested under cyclic vertical loading. The width of CFRP sheets varied in different specimens. The CFRP sheets were located at the tension side of the slabs in two perpendicular directions. Vertical load was applied downward through a column stub using a hydraulic Jack. In all specimens, no rupture of CFRP sheets was observed. The test results showed that the use of CFRP sheet, in addition to steel reinforcing bars, as flexural reinforcement improves the punching shear strength of slabs. This improvement can be significant for the slabs made of high strength concrete and low steel reinforcement ratio. However, the improvement of punching shear strength due to FRP strengthening reduced under cyclic vertical loading. The test results were compared with the equations proposed by ACI 318 and BS 8110 Codes. The ACI Code underestimates the punching shear strength of slabs and this underestimation becomes more pronounced with the increase in the flexural reinforcement. The BS 8110 Code appropriately accounts for the effect of flexural reinforcement on punching shear strength of slabs. However, for the strengthened slabs, an equivalent reinforcement ratio should be used to include the effect of both steel and CFRP flexural reinforcement.     

钢筋混凝土板柱节点受冲切承载力计算方法分析

钢筋混凝土板柱节点受冲切承载力是目前板柱结构存在的突出问题,对确保此类结构的安全性至关重要。在国内外大量研究的基础上,回顾了国内外学者提出的板柱节点受冲切承载力模型理论的发展过程,重点阐述了桁架模型、临界剪切裂缝理论、塑性理论、切向应变理论和临界截面应变理论等。针对板柱节点受冲切承载力的几种典型计算方法,深入分析了混凝土强度、板有效高度、配筋率、纵筋屈服强度和柱边长等参数对板柱节点受冲切承载力的影响,并利用试验数据分析了各种计算方法的适用性及局限性,提出对板柱节点受冲切承载力计算方法的认识并阐述其发展方向。     

Prediction of punching shear strength of two-way slabs

The punching shear strength of two way slabs without shear reinforcement and without unbalanced moment transfer is estimated using both neural networks and new simplified punching shear equations. An artificial neural network (ANN) was used to predict the punching shear strength of internal slab-column connections. Neural network analysis is conducted using 244 test data available in the literature and experiments conducted by the authors to evaluate the influence of concrete strength, reinforcement ratio and slab effective depth on punching shear strength. A wide range of slab thicknesses (up to 500 mm) and reinforcement ratios were used. In general, the results obtained from the neural network are very close to the experimental data available. The test results were used to develop two new simplified practical punching shear equations. The equations also showed a very good match with available experimental data. Four equations for the punching shear strength prescribed in well-known specifications were evaluated based on the available experimental results. This paper includes a discussion of the parameters of punching shear strength in the American, Canadian, British and European specifications.     

Strain-based strength model for direct punching shear of interior slab-column connections

A strength model was developed to predict the direct punching shear strength of interior slab-column connections without shear reinforcement. At slab-column connections damaged by flexural cracking, it was assumed that the punching shear force was resisted mainly by the compression zone of the critical section. The punching shear strength at the critical section was determined at the intersection between the shear capacity curve and the demand curve. The shear capacity was defined by the material failure criteria of concrete, addressing the effect of the slab flexural stresses on the shear stress capacity of concrete. The shear demand was defined as a shear force required to develop the flexural moment of the slab. For the application of the proposed model to design practice, a simplified strength model was also developed, introducing two modification factors involving in the size effect and non-uniform shear stress distribution. For verification, the proposed strength model was applied to existing test specimens tested by other researchers.     

考虑错动和转动复合影响的钢筋砼板的冲切承载力塑性解

本文用塑性极限方法对钢筋砼板的冲切承载力进行了研究。所采用的冲切破坏机构同时考虑了冲切锥与外围板之间的相对错动位移和转动位移的影响,比较符合实际的冲切破坏机理。得到的冲切承载力计算公式,形式简单,与试验结果较为符合,能反映出弯曲钢筋所作的贡献。     

Effects of openings on the punching shear strength of RC flat‐plate slabs without shear reinforcement

Openings in RC flat‐plate slabs are one of the critical factors that influence the punching shear strength of the slab and determine its thickness in the vicinity of the slab‐column joint. This study experimentally investigates the effects of openings on the punching shear strength of flat‐plate slabs without shear reinforcement. Tests were performed on eight flat‐plate slab specimens considering the layout and number of openings as test variables. The failure characteristics of each specimen are examined, and the effects of the test variables on the punching shear strengths of the test specimens are investigated. The measured punching shear strengths of the test specimens are compared with the predictions of several concrete design codes, including the American Concrete Institute (ACI), Comité Euro‐International du Béton and Fédération Internationale de la Précontrainte (CEB‐FIP) model and fédération internationale du béton (fib) model codes. This indicates that the reductions in punching shear strength due to the existence of openings are generally proportional to the loss of effective critical sections, and an L‐shaped opening layout around the corner of the column may further reduce the punching shear strength and may contribute to the loss of effective critical sections due to the existence of openings. Copyright © 2015 John Wiley & Sons, Ltd.     

钢筋混凝土板冲切锥体比较分析研究

对钢筋混凝土无冲切筋板受冲切承载力计算及冲切计算周长um 的取值进行了比较分析 ,针对同一荷载值不同荷载作用面的冲切承载力计算中出现的不足现象 ,提出一个um 的取值方法 ,以修正不同作用面形状造成的计算结果矛盾现象。     

Durchstanzen von Flachdecken bei hohen Stützenlasten

Sollen Hochbauten eine flexible Raumnutzung erlauben, stellen Systeme aus Stahlbeton‐Flachdecken und Stützen eine wirtschaftliche und effiziente Lösung dar. Die Stützen dieser Bauten werden häufig aus hochfestem Beton vorfabriziert und stark bewehrt, um hohe Normalkräfte mit minimalen Querschnitten aufnehmen zu können. Der oft aus normalfestem Beton bestehende Deckenteil, der zwischen den übereinanderstehenden Stützen liegt, wird dabei ganz beträchtlichen Druckbeanspruchungen ausgesetzt. Zudem erfährt die Deckenplatte um die Stütze eine starke Querkraftbeanspruchung, so dass der Durchstanzwiderstand maßgebend werden kann. Der vorliegende Aufsatz untersucht die Interaktion zwischen den vertikalen Druckbeanspruchungen in der Deckenplatte und dem Durchstanzwiderstand mit Hilfe einer Versuchsreihe an sechs Plattenausschnitten (3,00 x 3,00 x 0,25 m) unter verschiedenen Lastkombinationen. Aufgrund der Versuchsresultate wird ein einfaches Bemessungsmodell vorgestellt. Dieses Modell basiert auf dem Ansatz der DIN 1045‐1 für den Durchstanzwiderstand von Decken ohne Querbewehrung, die einer horizontalen Druckspannung ausgesetzt sind (z. B. Vorspannung). Die mit dem Modell errechneten Resultate stimmen gut mit jenen der Versuche überein – sowohl beim Widerstand wie bei der beobachteten Bruchart. Interaction between column loading and punching shear strength in flat slabs without transverse reinforcement Flat slabs supported by columns are cost efficient solutions for high‐rise buildings. To reduce the cross section of the columns, the columns are typically precast with high strength concrete and include large amounts of longitudinal reinforcement. On the contrary, the slab between columns is typically cast‐in‐place with normal strength concrete. Thus, the slab is subjected in the support region of the columns to large transverse compressive stresses, which may exceed its uniaxial compressive strength. In addition, the flat slab around the column region has to carry large shear stresses, which may potentially lead to punching shear failures. In this paper, the interaction between column loads and shear forces in the support region of a flat slab is investigated by means of an experimental programme consisting of 6 slabs without transverse reinforcement (3.0 x 3.0 x 0.25 m). The flexural reinforcement of the slab was varied as well as the ratio between the column loads and the applied shear forces. On the basis of these results, a theoretical model based on the punching shear approach of DIN 1045‐1 is presented. This model accounts for the compressive stresses in the support region due to column loads and for the potential increase of punching shear strength due to it. A very good agreement between the test results and the theoretical model is obtained both in terms of strength and observed failure modes.     

钢筋砼四边支承矩形板抗冲切性能试验研究

根据４５块在集中荷载作用下无腹筋的边长比不大于２的钢筋砼四边支承矩形板的抗剪试验结果可知，板的破坏形态均为冲切破坏。本文分析了试件的冲切机理和受力特征，讨论了影响受冲切承载力的主要因素，并给出了受冲切承载力的建议计算方法。     

钢筋混凝土异形柱-板连接冲切特性试验研究

针对柱截面形式为异形柱的板柱冲切试验很少以及我国规范未对此明确规定的情况,本文进行了9个柱截面为3种典型异形柱(十字形、T形和L形)-板连接和2个方柱-板连接在未配置抗冲切钢筋时的冲切特性对比试验。阐述了异形柱-板连接的冲切受力性能和破坏特征,并将其受冲切特性与相应的等面积方柱和等有效周长方柱板柱连接试件进行了对比,讨论了异形柱和方柱板柱连接冲切特征的差异。试验结果表明,与相应的方柱-板连接试件相比,异形柱-板连接试件具有较高的抗冲切承载力及延性。根据试验结果提出了符合我国设计实践的异形柱板柱冲切承载力计算方法。并用国外几个主要规范以及我国规范所推荐的冲切承载力计算公式对上述试验板的受冲切承载力进行了验算,相对而言,抗弯钢筋配筋率较高时,我国规范及ACI规范的预测值与试验值相比偏于保守。本文试验结果可供规范修订时参考。     

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.     

配锚栓的再生混凝土双向板破坏机理及有限元分析

对配有锚栓的再生混凝土双向板进行了详细研究,给出了配锚栓板的破坏形态及破坏机理。进一步又采用ABAQUS非线性有限元软件模拟配有锚栓的再生混凝土双向板的冲切破坏,通过合理的选择材料性能参数、几何参数、边界条件等进行配有锚栓的再生混凝土双向板的非线性有限元模拟分析,计算结果符合要求。论文结论可以为提高再生混凝土双向板的抗冲切承载力及延性提供依据和借鉴。     

钢筋局部钢纤维高强混凝土板冲切挠度计算

根据钢筋局部钢纤维高强混凝土冲切板的试验结果,讨论了钢纤维体积率、钢纤维掺加范围和混凝土强度对试验板冲切变形的影响,建立了试验板开裂荷载及荷载—挠度曲线上升段挠度计算公式,并进行了数值计算.结果表明,钢纤维的掺入对钢筋高强混凝土板冲切变形性能的改善比较显著;试验板挠度计算值与试验值吻合较好.     

考虑混凝土贡献的修正变角桁架模型

为提高有腹筋RC（钢筋混凝土）浅梁抗剪承载力的预测准确性和稳定性,提出一种考虑混凝土贡献的修正变角桁架模型。首先,基于变角桁架模型得到箍筋的抗剪贡献值计算公式,并结合塑性下限定理确定斜裂缝倾角的取值|其次,通过混凝土浅梁受压区的劈裂破坏模式得到混凝土项的抗剪贡献值计算公式,并利用尺寸效应系数考虑了混凝土项的脆性破坏特征|最后,将箍筋项和混凝土项提供的抗剪承载力进行叠加,建立有腹筋RC浅梁抗剪承载力计算公式。基于有腹筋RC浅梁抗剪承载力试验数据库ACI-DAfStb对本文、GB50010、ACI及AASHTO规范中抗剪计算公式的预测准确性进行对比评估,结果表明：文中抗剪承载力公式不仅能够体现配筋率、剪跨比和混凝土强度等主要抗剪参数的影响规律,还能考虑尺寸效应和斜裂缝倾角对抗剪承载力的修正|同时,在抗剪承载力预测准确性和稳定性方面,相对规范方法有所较高。     

Berücksichtigung der Längsbewehrung beim Durchstanznachweis nach Eurocode 2 (DIN EN 1992‐1‐1/NA)

Consideration of flexural reinforcement in punching shear provisions according to Eurocode 2 (DIN EN 1992‐1‐1/NA) With introduction of Eurocode 2 in Germany in 2012, the pun ching shear design provisions of flat slabs and column bases were revised compared to DIN 1045‐1. Particularly the consi deration of flexural reinforcement caused some irritations for practical engineers. In this context, this paper presents recent recommendations of DIN technical committee ”NABau“ (DIN: German institute for standardization) for the distribution of minimum flexural reinforcement for wall corners and wall edges as well as for the width and anchorage of flexural reinforcement for the calculation of the flexural reinforcement ratio in flat slabs and column bases.     

Formulation of shear strength of slender RC beams using gene expression programming,part I: Without shear reinforcement

In this study, a new design equation is derived to predict the shear strength of slender reinforced concrete (RC) beams without stirrups using gene expression programming (GEP). The predictor variables included in the analysis are web width, effective depth, concrete compressive strength, amount of longitudinal reinforcement, and shear span to depth ratio. A set of published database containing 1942 experimental test results is used to develop the model. An extra set of test results which is not involved in the modeling process is employed to verify the applicability of the proposed model. Sensitivity and parametric analyses are carried out to determine the contributions of the affecting parameters. The proposed model is effectively capable of estimating the ultimate shear capacity of members without shear steel. The results obtained by GEP are found to be more accurate than those obtained using several building codes. The GEP-based formula is fairly simple and useful for pre-design applications.     

Flachdecken in Elementbauweise. Hinweise zum Durchstanznachweis nach DIN 1045‐1

Flat Slabs built with Semi‐Precast Elements. Advices to Punching Shear Verification according DIN 1045‐1 Flat slabs are increasingly built with precast slabs and insitu topping. The bearing behaviour of these semi‐precast slabs with lattice girders is similar to cast in one concrete slabs. In principle this is also applied for areas where punching failure is endangered. This was shown in full‐scale tests, which were taken as the basis for derivation of design rules. However during design of semi‐precast slabs some specific items have to be considered. The revision of technical approvals for punching shear reinforcements to the new German design standard for concrete and reinforced concrete DIN 1045‐1 took place during the introduction of new punching shear reinforcement for semi‐precast elements. The required verification of punching shear in element slabs are arranged and explained for different types of punching shear reinforcement.     

基于抛物线形库仑-莫尔准则的钢筋混凝土板抗冲切承载力

在刚塑性冲切计算模型的基础上,采用二次抛物线形库仑 莫尔混凝土强度准则,运用虚功原理,建立考虑混凝土板中抗弯钢筋销栓作用的冲切承载力计算式。根据已有板冲切试验研究成果,如冲切角和破坏时刻抗弯钢筋应力等数据,对板冲切承载力计算式进行简化,给出简化计算式。与国内外混凝土板冲切试验数据进行计算比较后发现,简化计算式的计算结果较好地符合实测承载力;抗弯钢筋的销栓作用约占钢筋混凝土板总冲切承载力的6%～11%,而忽略抗弯钢筋的销栓作用会给计算结果带来较大影响。     

Shear strength estimation of reinforced concrete beam–column sub‐assemblages using multiple soft computing techniques

The aim of the present study is to propose innovative predictive models for shear capacity of reinforced concrete (RC) exterior joints in terms of multiple soft computing techniques. Existing models were evaluated and by a preliminary sensitivity analysis, seven parameters including compressive strength of concrete, product of the yield stress and the reinforcement ratio of the joint stirrups, the effective width of the joint panel, cross‐sectional column width, beam tensile longitudinal reinforcement ratio, beam compressive longitudinal reinforcement ratio, and column longitudinal reinforcement ratio were considered. Then, a large data set having the details of experimental programs on shear capacity of exterior RC beam–column joints was provided. The experimental data were utilized in developing the proposed models. After verification of the new models against available database, their efficiency compared with existing models was confirmed. Finally, a sensitivity analysis was performed in order to find the relative importance of each input parameter on the shear strength of RC joints. The results indicated that the beam reinforcement is the most important factor in shear capacity estimation of exterior RC beam–column connections.     

钢筋混凝土板抗冲切的有限元计算研究

采用有限元非线性分析模拟钢筋混凝土板的冲切破坏,可以提供大量板抗冲切的反应信息,有助于研究钢筋混凝土板抗冲切破坏。本文在所选文献试验数据的基础上,通过合理采用试件材料性能参数、边界条件、几何参数等进行钢筋混凝土板抗冲切的有限元非线性分析,计算结果与试验结果吻合较好。为了求得更好的计算精度,还进行选择分离式计算模型、调整材料性能参数以及加载方式等计算手段,通过计算比较,取得较为满意的计算结果。本文可以为钢筋混凝土板抗冲切有限元的模拟计算提供依据与借鉴。