Gründerzeitliche Mauerwerksbauten unter Erdbebeneinwirkung – Tragverhalten im Widerspruch zur aktuell angewandten Nachbemessung

Historic masonry buildings under earthquakes – Load‐bearing behaviour in contradiction to the currently applied methods of analysis The stability of historic masonry buildings must be guaranteed not only under normal conditions, but also during natural disasters. The seismic assessment of the masonry buildings of the Gründerzeit (1840–1918) in Vienna is a central topic in the qualitative and constructive assessment. Although masonry construction has been used for many centuries, the realistic evaluation of the load‐bearing behaviour is still a complex challenge. The methods of analysis according to current regulations are only insufficiently able to reflect the real load‐bearing behaviour and the possible activation of global failure mechanisms. As a result, the simplified verification is often difficult to calculate for many historic buildings, and questionable reinforcement measures are taken to compensate, even though the buildings have already experienced several earthquakes and survived most of them without damage. The present work deals with the approaches of current methods of analysis and aims to identify problem points and to compare them with time history analysis, which is supported by a powerful material model based on test series. It is shown that the conventional analysis for the historic masonry buildings without consideration of the interaction and load transfer effects as well as the characteristic construction methods only partially reflect the real load‐bearing behaviour. The work is intended to be a contribution to the technical expert discussions on the seismic safety of historic buildings and to stimulate the discussion on the formulation of realistic methods of analysis.     

Shear resistance verification of late 19th century masonry according to EC 6 – State of the art

A large part of buildings in Central European cities like Vienna was built in the Gründerzeit period between about 1840 and 1918 [1]. These buildings were constructed according to traditional rules. Current urban development requires historic buildings to be structurally adapted, which requires retroactive analysis of the masonry walls; in Austria according to ÖNORM EN 1998‐3 [2] and ÖNORM EN 1996‐3 (EC 6) [3]. Here, special focus is on the load transfer of horizontal earthquake loads, i. e. the shear strength of masonry walls. This paper describes the verification of historic masonry in detail and discusses individual components. Initial shear strength, load‐influenced friction and the length of the compressed part of the wall are first determined using results from experimental testing and relevant literature and then compared to the approaches in EC 6. Based on this analysis, recommendations are provided to make theoretical approaches more realistic.     

Investigation of the seismic out‐of‐plane behaviour of unreinforced masonry walls

The structural stability of unreinforced masonry (URM) walls has to be guaranteed not only under static (permanent and live) loads but also under earthquake loads. Loads transverse to the plane (out‐of‐plane) often have a decisive influence on the load‐bearing capacity. In practical applications, simplified methods from codes, guidelines and literature are often used to analyse and evaluate the out‐of‐plane capacity of load‐bearing and non‐load‐bearing URM walls. The results of these simplified methods can be significantly conservative and inaccurate since essential influencing effects are neglected. For many existing buildings, the simplified methods underestimate the capacity, which leads to cost‐intensive retrofitting and strengthening measures or complete replacement by other wall systems. In order to realistically estimate the out‐of‐plane capacity, parameters such as wall geometry, boundary conditions, vertical loads and especially dynamic effects (e.g. inertia forces) have to be taken into account. In this paper, non‐linear time history simulations are presented to investigate the influence of these effects. The numerically determined maximum acceptable earthquake acceleration is compared with results from simplified analysis models. The comparison shows that the out‐of‐plane capacity is significantly higher than the values predicted by simplified models. Finally, several initial experimental seismic tests conducted on the shaking table of the TU Kaiserslautern are presented, together with the planned extensive experimental test program on the out‐of‐plane capacity of masonry walls.     

Buckling of masonry walls – A new proposal for the Eurocode 6

The buckling of masonry wall depends on the deformation behaviour and can be described with the modulus of elasticity depending on masonry strength. The reduction factor solution considering buckling is described by the Gaussian bell‐shaped curve in Eurocode 6, Part 1‐1 Annex G and was calibrated for masonry with a modulus of elasticity between 700 and 1000 f_k, which describes the masonry currently used in most European countries. In case of historic masonry or where deformability is needed in new construction, the modulus of elasticity can actually be lower. In those cases the approximation procedure according to Annex G of the Eurocode 6 delivers results which do not represent the real behaviour and leads to uneconomical results. The present paper proposes a new empirical method, which can be applied over the whole practical range of the elastic modulus of masonry. The new proposed method has been verified with experimental data and shows a very good fit.     

Bond behaviour of a textile‐reinforced mortar for AAC masonry

The bond behaviour of a textile reinforced mortar (TRM) applied to autoclaved aerated concrete (AAC) masonry has been evaluated experimentally. The TRM is composed of a glass‐fibre mesh combined with a cementitious mortar and is intended to strengthen AAC masonry walls subjected to out‐of‐plane bending during an earthquake. The main components have been characterized with preliminary tests. Then, pull‐off and shear bond tests have been performed to determine the bonding properties of the TRM applied to the AAC substrate. Three types of AAC blocks have been used, which differ in the bulk density and compressive strength, to evaluate possible variation in the bond strength. The results of the experimental campaign have shown a good performance of the strengthening system. In most cases, the bonding between TRM and masonry was maintained up to tensile failure of the dry textile. As expected, the masonry samples realized using AAC blocks with a higher bulk density showed better performances. The paper presents and discusses main test results, providing background data for future recommendations for the use of the analysed strengthening system in AAC masonry structures.     

Design aids for unreinforced masonry under bending compression – Part 2: Application of design aids for structural design according to DIN EN 1996‐1‐1/NA

The semi‐probabilistic safety concept of divided safety factors for action and resistance of DIN EN 1990 [1] in combination with the structural design codes DIN EN 1996‐1‐1 [2] and DIN EN 1996‐1‐1/NA [3] include the requirement that acting normal forces N_Ed may not exceed the normal force resistances N_Rd for the structural design of masonry under bending compression. According to [3], fully plastic material behaviour can be assumed and the stress block used as the material law for masonry. Building on this, design aids and their theoretical basis were presented in Part 1 of this scientific paper [4], which are comparable with the ω tables (called the ? table here) and the general design diagram for massive construction. The application of the design aids is described in this second part of this scientific paper through calculation examples and the connection with the calculation approaches of [3] is made clear. The relation to the reduction factor ?_m, which covers effects of 2^nd order theory, is also obtained. With known values of the load eccentricities according to 1^st and 2^nd order theory, the design task becomes the analysis of the loadbearing capacity of the masonry section at half wall height. Knowing ?_m, the load eccentricity e₂ and the additional moment according to 2^nd order theory can subsequently be determined, which does not ensue from the calculation equations of [3]. With the general design diagram, the values of compression zone height and the assumed load eccentricities of the acting normal forces, which result from the reset rule for masonry sections with high load eccentricities, can be directly read off, greatly improving the clarity of this procedure.     

Experimentelle und analytische Untersuchung des out‐of‐plane‐Verhaltens von unbewehrten Mauerwerkswänden unter Erdbebenbelastung

Experimental and analytical investigation of the seismic out‐of‐plane behavior of unreinforced masonry walls In addition to the vertical and horizontal load‐bearing in‐plane, masonry must also withstand out‐of‐plane loads that occur in earthquake scenarios. The out‐of‐plane behavior of unreinforced masonry walls depends on a variety of parameters and is very complex due to the strong non‐linearity. Current design methods in German codes and various international codes have not been explicitly developed for out‐of‐plane behavior and contain considerable conservatism. In the present work, shaking‐table experiments with heat‐insulating masonry walls have been conducted to investigate the out‐of‐plane behavior of vertical spanning unreinforced masonry walls. As shown in previous numerical investigations, important parameters are neglected in existing design and analysis models and the out‐of‐plane capacity is underestimated significantly. In the conducted experiments the results of these numerical investigations are verified. Furthermore, the development of an analytical design model to determine the force‐displacement relationship and the out‐of‐plane load‐bearing capacity considering all significant parameters is presented.     

Minimum reinforcement of beam‐type reinforced masonry constructions – proposals for future regulations

The minimum reinforcement of reinforced masonry under bending should according to DIN EN 1996‐1‐1:2013‐02 [N 1], Section 8.2.3(1), be not less than ρ_min = 0.05 % of the effective masonry cross‐section for building elements, in which the reinforcement makes a contribution to the loadbearing capacity of the section, with the effective masonry cross‐section being the product of the effective width (b_ef) and the usable height d of the building element. In order to limit cracking and increase the ductility of the element, the reinforcement area should according to [N 1], Section 8.2.3(3), be not less than 0.03 % of the gross cross‐sectional area (of a wall). Other regulations ([1], [N 2], [N 3], [N 4], [N 5], [N 6], [N 7]) also prescribe minimum reinforcements in order to avoid brittle behaviour of the building element when the first crack forms or to limit cracking. In this specialist article, the figures given in [N 1] for the minimum reinforcement of reinforced masonry beams, like flat lintels or prefabricated lintels, are checked. The work concentrates on avoiding brittle failure when the first crack forms. In addition to geometrical requirements, the amount of minimum reinforcement depends on the tensile strength of the masonry f_t,m. Values of ρ_min vary considerably depending on the magnitude of the tensile strength of the masonry that can be assumed. For lintels over openings in facing brickwork facades, the height of any capping or soldier courses under the reinforcement layer also has an enlarging influence on the value of ρ_min. With regard to future regulations in standards or Allgemeine bauaufsichtliche Zulassungen (national technical approvals), it is recommended not to give lump sum values for ρ_min but to undertake a calculation like for reinforced concrete, using the algorithms given in this article.     

Simplified design concept for slender masonry walls / Vereinfachter Stabilitätsnachweis knickgefährdeter Mauerwerkswände

The verification of safety against buckling of unreinforced masonry walls according to the accurate design procedure of EN 1996‐1‐1 Appendix G is based on semi‐empirical approaches, which do not always realistically describe the load‐bearing behaviour. This statement is also supported by an objection of the country Denmark concerning the load capacity function which is regulated in Appendix G. Using new findings about the effects of non‐linear material behaviour in case of stability failure this article investigates fundamental questions about the buckling behaviour of masonry walls and transfers these into a simple practical structural design proposal. As a result, the load capacity function can be considerably simplified, the influence of creep can be integrated and the number of input parameters can be reduced.     

Prediction of stiffness,force and drift capacity of modern in‐plane loaded URM walls

Unreinforced masonry (URM) walls show a limited horizontal in‐plane deformation capacity, which can lead to an unfavorable seismic response. To predict this response, the walls' effective stiffness, shear force and drift capacity are required. While mechanics‐based models for the force capacity are well established, such approaches are largely lacking for the effective stiffness and the drift capacity. The mostly empirical code equations for the two latter parameters lead to often unsatisfactory and, in the case of drift capacities, sometimes unconservative predictions when compared to test results. This article summarises recently developed simple closed‐form equations for the effective stiffness, the shear force and the drift capacity. Furthermore, it compares said formulations and currently used code equations to a database of shear compression tests. It shows that the novel models capture the effective stiffness and the drift capacity more accurately than current code equations. The shear force capacity is predicted with a similar reliability, yet using a very simple formulation.     

Tragfähigkeitstafeln für unbewehrtes Mauerwerk nach DIN EN 1996‐3/NA:2019‐12

Load‐bearing capacity tables for unreinforced masonry according to DIN EN 1996‐3/NA:2019‐12 Practical design aids are important tools in the day‐to‐day business of structural design. The design of primarily vertically loaded masonry walls in usual building construction can be carried out with the help of so‐called load‐bearing capacity tables. A table value is read off exclusively as a function of the geometric conditions, which – multiplied by the masonry compressive strength – results in the load‐bearing capacity of the wall for cold design and in case of fire. By comparing the acting and resisting force, the verification of structural design can be provided in a simple and yet economical form. The bearing capacity tables based on the simplified calculation methods according to DIN EN 1996‐3/NA:2019‐12 [1], [2] and DIN EN 1996‐1‐2/NA:2013‐06 [3], [4] are presented in this paper. Compared to the previous edition of Part 3 of Eurocode 6, the extended scope of application is taken into account, as well as the normative changes to the construction method with partially supported slabs.     

Reinforced bending load‐stressed masonry – Suggestions for future designs / Bewehrtes biegedruckbeanspruchtes Mauerwerk – Vorschläge für zukünftige Bemessungen

European standardization bodies are currently working on the amendment to EN 1996‐1‐1, which will also affect the evaluation of reinforced masonry in Germany. For that reason, discussion suggestions are being made here for revisions to lay the groundwork for building materials evaluations and especially, evaluations of bending load‐stressed masonry walls or beams at their serviceability limit state (SLS) for load‐bearing capacities. Information already presented in E DIN 1053‐3:2008‐03 [N3] is being incorporated as well. Characteristic values for the compressive strength of the masonry parallel to the bed joints f_k,∥ are essential for the design of reinforced masonry, although they are currently not included in national application documents for Germany. For the time being, they can be mathematically calculated using conversion factors for the characteristic compressive strength values vertical to the bed joints f_k or by using the declared axial compressive strengths of the masonry units. The ultimate strains for masonry in general should be set consistently at ?_mu = ∣–0.002∣ as several masonry types do not exhibit higher compressive strain values. The use of steel strains higher than ?_su = 0.005 does not change any measurement results. Varying stress‐strain curves of the constitutive equations on masonry under compressive strain (parabolic, parabolic‐rectangular, tension block) lead to differing values of recordable bending moments despite having the same mechanical reinforcement percentage at higher normal forces. Therefore, clear guidelines should be made for the type of applicable constitutive equation for masonry walls under compressive strain. With the introduction of a tension block, the number values of the reduction factors λ for the compression zone height x, which is dependent on limit strains, and where applicable, reduced compressive strength, need to be determined, as with reinforced concrete construction. A modification of the bending moment based on the second order theory according to [N4] is presented for the calculation of reinforced masonry walls in danger of buckling. The use of reduction factors for the load capacity of the masonry cross section, such as for unreinforced masonry, does not appear to be appropriate as buckling safety evidence because here, the design task is the determination of a required reinforcement cross section.     

Modelling strategies for horizontally loaded infill masonry / Modellierungsansätze für horizontal beanspruchtes Ausfachungsmauerwerk

The realistic modelling of seismically loaded infill masonry is a complicated task due to the complex interaction of the infill with the surrounding frame. Practical approaches, that take into consideration the non‐linear behaviour of the infill and the frame, are especially lacking at the moment. Within the framework of the European project INSYSME (Innovative Systems for Earthquake Resistant Masonry Enclosures in R.C. Buildings [1]), the behaviour of infill masonry in reinforced concrete frames is being investigated experimentally to derive different strategies for the modelling of infill masonry. In addition, systems for the improvement of the seismic behaviour will be developed. In the following, some initial results of the different modelling strategies are presented.     

Deflection limitation of beam‐type reinforced masonry constructions – proposals for future requirements

Deflection limitation of reinforced masonry building elements under bending is undertaken according in DIN EN 1996‐1‐1:2013‐02, Section 5.5.2.6, Table 5.2 [N 4], by limiting the span l _ef or the ratio of l_ef to the effective depth d, for example l_ef/d ≤ 20 for simply supported beams. A further requirement in DIN EN 1996‐1‐1:2013‐02, Section 7.3, states that reinforced masonry elements should not deflect excessively under serviceability loading conditions. For reinforced masonry with dimensions, which are within the limits stated in clause 5.5.2.6 [N 4], acceptable vertical deflection of a beam can normally be assumed. In this scientific paper, the figures stated in [N 4] for the limitation of the bending slenderness l _ef/d of reinforced masonry beams like masonry or prefabricated lintels are checked by calculation with the ”ζ procedure“ from reinforced concrete theory. The suitability of this procedure was first demonstrated by comparing calculated and experimentally obtained values. It was determined that maintenance of the bending slenderness ratio l_ef/d ≤ 20 for the tested calcium silicate masonry lintels does not always lead to deflection values w/l_ef ≤ 1/250. For prefabricated straight (flat arch) calcium silicate lintels and horizontal aerated concrete lintels with limit slendernesses of l_ef/d ≤ 15 and calcium silicate masonry lintels with l_ef/d ≤ 10, w/l_ef ≤ 1/250 was fulfilled. With regard to future requirements for the tested reinforced masonry constructions, a method is proposed for the calculation of the limit slenderness ratio l_ef/d, which leads to maintenance of w/lef ≤ 1/250. Furthermore, the presented ”ζ procedure“ enables reliable calculation of deflection figures at the serviceability limit state considering long‐term effects.     

Current developments in fire protection – Changeover to design and classification according to the Eurocode / Aktuelle Entwicklungen im Brandschutz – Umstellung auf die Bemessung und Klassifizierung nach europäischen Normen

The European requirements for fire safety design and testing of structural masonry members are already the governing requirements in many cases. In principle, both the European and the German classification may be used according to the Bauregelliste. However, the latter may only be used when European classification of a member or construction material is not possible because the appropriate European standards do not exist. The European standards do not differ fundamentally from the German standard DIN 4102‐2. One significant difference is that according to the DIN 4102‐2, it was required to carry out two tests with the most unfavourable result governing, while according to the European standard, only one test is required. According to the EN Standard, the tests for fire resistance and the reaction to fire are carried out separately. There are other differences related to the pressure in the furnace as well as the use of plate thermocouples instead of jacketed thermocouples. Fire safety design of masonry is carried out in accordance with EC 6‐1‐2 and the National Annex. Only the members not regulated in the EC 6‐1‐2, e.g. pre‐cast masonry members, non‐load‐bearing walls, lintels, connections and joints, should be designed and checked according to the revised DIN 4102‐4.     

Non‐loadbearing partitions of unfired clay masonry – Fire behaviour tests

Unfired clay masonry is the most frequently used construction type for residential buildings worldwide, but the long tradition of building with unfired clay masonry in Germany came to an end with the onset of industrialization. The research project EGsL ”Unfired clay masonry: design and construction principles for a widespread use in residential building taking into account climatic conditions in temperate zones with Germany as example location“ is devoted to the preparation of basic principles based on the current state of knowledge about unfired clay as a building material in order to filter out design and construction principles for residential buildings of modern unfired clay masonry. It is assumed that unfired clay has a much better performance capability than is currently expected from the material. The greatest suspicion about the structural safety of unfired clay buildings is based on the water susceptibility of unfired clay, since unfired clay loses its strength under the action of water. In order to improve confidence in the structural stability of residential buildings of unfired clay masonry, a display at the trade fair BAU 2017 showed the basis for an example application of important constructional joints of a theoretical building of unfired clay masonry. As a follow‐up to this, the EGsL research project now intends to demonstrate the fire protection behaviour of unfired clay internal walls in order to ensure the structural stability of unfired clay buildings. The article reports on a first fire test on non‐loadbearing clay masonry walls and describes an example application of non‐loadbearing clay walls in the new Zinzendorf Gymnasium in Herrnhut.     

Analysis of steel‐reinforced masonry walls regarding maximum shear loads / Traglastberechnung von vertikal bewehrten Mauerwerksscheiben

Loadings on masonry for the earthquake case pose particular challenges for the material. In order to improve the load‐bearing and deformation behaviour, masonry building elements can be strengthened with reinforcement. This article presents an analytical model for the calculation of the load‐bearing capacity of vertically reinforced masonry panels. The masonry is modelled as a homogeneous and anisotropic material and failure conditions are based on the plastic theory. Using uniaxially loaded stress fields and considering the structural constraints, a lower load‐bearing threshold can be given. In order to verify the model, three shear tests on reinforced sand‐lime block masonry were recalculated regarding their load‐bearing capacity. The test panels each contained vertical steel reinforcement in the blocks. The blocks were laid in thin bed mortar.     

Numerical model for dynamic analysis of masonry‐infilled steel and concrete frames

A numerical model for nonlinear dynamic analysis of planar masonry‐infilled concrete and steel frames strengthened with composites is briefly presented. The model is quite simple and it can simulate main nonlinear effects of these structures. Besides modelling of nonlinear behavior of concrete, steel, masonry, plaster and soil, it can simulate nonlinearities at contact surfaces, changes in structural geometry and construction of a structure over different time phases. The model is based on a relatively small number of material parameters and intended for practical application primarily. The model is verified by using the data of the performed shake‐table tests on masonry‐infilled steel and concrete frames. Numerical results show fairly good agreement with the experimental results. This shows the potential reliability of the developed numerical model for the analysis of planar masonry structures. However, further verifications of the model and corresponding computational software are most welcome.