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.     

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.     

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.     

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.     

Reinforced masonry beams under shear load – Proposals for future designs / Bewehrte schubbeanspruchte Mauerwerksbalken – Vorschläge für zukünftige Bemessungen

This article is written against the backdrop of the work of the European standardisation committees on the amendment of EN 1996‐1‐1 [N 4] which will also exert an influence on the design of reinforced masonry in Germany. This paper focusses on the design approaches of DIN EN 1996‐1‐1 for untensioned reinforced masonry beams under shear load in the ultimate limit state (ULS). Proposals are made to discuss their revision. The contents of E DIN 1053‐3 [N 3] and of the final draft of the guideline ”Flat Lintels” [7] are taken into account.     

Shear tests on full scale storey‐height specimens constructed with thermally insulating clay unit masonry with thin‐layer mortar

The paper presents results of a series of 6 in‐plane shear tests on storey‐height clay unit masonry panels [1] with thin‐layer mortar, carried out in addition to previous test campaigns [2], [3], and [4]. The walls were constructed with unfilled thermally insulating clay units with a thermal conductivity of λ = 0.09 W/(m · K). The current design rules for clay unit masonry according to DIN EN 1996‐1‐1/NA [5] are conservative compared to the presented test results for thermally insulating clay unit masonry.     

Partial rebuilding of the Einsiedelstein Viaduct / Teilerneuerung der Talbrücke Einsiedelstein1)

As part of the six‐lane widening of the Autobahn A1, the existing Einsiedelstein viaduct had to be partially rebuilt. The arch viaduct was originally built in 1938. The design used finite element calculations carried out assuming non‐linear material behaviour of the fill concrete and the arch masonry for the loadings of the DIN Specialist Report “Actions on Bridges”. The calculation methods used enable realistic modelling of the load bearing behaviour of historic masonry structures at the limit states of load bearing and serviceability. The results of the calculation served as the basis for the selection of sustainable measures for the partial rebuilding of the arch viaduct.     

Flexural tensile tests with vertically perforated clay unit masonry with thin layer mortar / Biegezugversuche an Planziegelmauerwerk

As part of the EU project, INSYSME – INnovative SYStems for earthquake resistant Masonry Enclosures in reinforced concrete buildings – to optimise infill masonry the German project partners carried out an initial part of the project on flexural strength testing of high‐tech clay block masonry in accordance with DIN EN 1052‐2. In this a wide range of modern products was considered which at present is regulated in Germany by means of general building authority approvals. The test results show that the specifications for flexural tensile strength of high‐tech clay block masonry in DIN EN 1996 are very conservative in most cases.     

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.     

Minimum vertical load on masonry walls – a realistic view / Mindestauflast auf Mauerwerkswänden – eine realitätsnahe Betrachtung

To transfer bending moments in building components consisting of a material without tensile strength always requires a simultaneously acting normal force. Accordingly, masonry walls exposed to horizontal loads (e.g. wind) require a minimum vertical load, so that the resultant stress at the mid‐height of the wall remains the same within the cross‐section. As part of the A2 amendment to DIN EN 1996‐3/NA, this verification of walls subjected to low vertical loads, such as outer walls on the top floor exposed to high wind load was implemented in the National Annex. Part 3 of DIN EN 1996‐3 includes a similar standard regulation for verification of the minimum vertical load, which is based on an arch effect within the wall cross‐section. Based on this technical background and taking into account the main influencing parameters, a verification model is presented here which realistically describes the load‐bearing behaviour of unreinforced masonry walls subjected primarily to bending. Apart from the bending moments due to wind load, an initial eccentricity of the wall as well as second order effects due to wall deformations also have to be taken into account. In addition, a simple approximation equation is provided for the practical determination of the required minimum vertical load.     

Bauakustische Bemessung von Mehrgeschossbauten mit monolithischen Ziegelaußenwänden

Acoustic design of multi‐storey buildings with external walls of monolithic clay masonry For masonry buildings with monolithic, highly insulated walls of clay units, no acoustic design according to standard was practically possible under Supplement 1 to DIN 4109:1989. Therefore a design procedure regulated by approvals was introduced in 2010, with which acoustic calculations for a building could be performed with a high security of forecasting. This procedure has been taken up in the completely revised series of standards DIN 4109:2016/2018 “Sound insulation in buildings”. The basis for the application of this method is knowledge of the individual sound insulation quantities and joint sound insulation quantities for the relevant clay masonry products or product combinations. In order to simplify performance of the verification for clay masonry buildings, the clay masonry industry provides the program “Modul Schall 4.0” (Acoustic module 4.0), in which the decisive acoustic parameters of external wall products from numerous clay masonry unit producers are stored in a database. In this report, experience of application of the design procedure for clay masonry buildings is presented. There is good agreement between forecasts and tests on completed buildings.     

Shear tests on highly thermal insulating clay unit masonry walls with thin layer mortar

The paper presents the results of a series of 6 shear tests on full scale highly thermal insulating clay unit masonry walls. The walls consisted of units with large voids filled with mineral wool with a thermal conductivity of λ = 0,07 W/(m · K). The aim of the investigations was the verification of the in‐plane‐shear resistance of this type of thermal insulating clay unit masonry in addition to the tests reported in [1]. The current design rules for clay unit masonry in DIN EN 1996‐1‐1/NA are rather conservative compared to the test results for thermal insulating units.     

Extended conditions of application of DIN EN 1996‐3/NA for high walls

According to Eurocode 6, unreinforced masonry walls can be designed using different verification methods, whereby the simplified calculation methods are contained in Part 3 of DIN EN 1996 [1]. If the associated application limits and boundary conditions are fulfilled, a large part of the usual problems occurring in masonry construction can be dealt with without great effort. A limiting condition for the application of the simplified calculation methods is a maximum clear wall height of h = 2.75 m or h = 12 ? t. Changes in user requirements for modern buildings with masonry walls nowadays often require greater wall heights, wherefore a verification according to the general rules from DIN EN 1996‐1‐1/NA [2] is necessary. This means a considerably higher effort for the structural engineer. A considerable amount of calculations was done to verify whether the results of the simplified calculation methods are also valid for greater wall heights. The results were transferred into a consistent standardization proposal with regard to extended application limits of DIN EN 1996‐3/NA, which is contained in a new draft Amendment A3 for the National Application Document for Germany.     

Numerical investigation of lateral earth pressure for unreinforced masonry walls

For the design of unreinforced masonry walls under lateral earth pressure according to DIN EN 1996‐3 [1], the active earth pressure is used, which is less than the earth pressure at rest. For the consideration of active earth pressure, a sufficient deflection of the wall is needed. It is unknown whether the deflections in reality are large enough to justify a reduction of the active earth pressure. Therefore a numerical model has been developed which considers the load‐bearing behaviour of masonry walls, with several boundary conditions being considered to estimate the effective earth pressure.     

Wann kommt der EC 6 in Deutschland und was bedeutet das für die nationalen Bemessungsnormen?

Entsprechend der zwischen dem DIN und der europäischen Normungsorganisation CEN getroffenen Vereinbarungen sollen im Jahr 2010 alle europäischen Bemessungsnormen mit ihren nationalen Anhängen zur Verfügung stehen und diesen widersprechende nationale Normen zurückgezogen werden. Der Beitrag erläutert, wie der derzeitige Stand im Mauerwerksbau ist, ob und wie lange noch die deutschen Bemessungsnormen im Mauerwerksbau weiter angewendet werden können und was das für die überarbeitete DIN 1053‐1 bedeutet. Abschließend wird dem Wunsch der Praxis entsprechend zum Ausdruck gebracht, dass nur eine bauaufsichtliche Einführung als komplettes Dokument in der Liste der Technischen Baubestimmungen sinnvoll ist. When will be the EC 6 established for use in Germany and what means it for the national design codes? The European Design Codes with their National Annexes should be available in the year 2010 and contradictory national design standards withdrawn according to the agreement between DIN and the European Standardisation Organisation CEN. The contribution explains how the current state in masonry construction is, and how long the German design codes for masonry can be applied. Also the effect on the revised DIN 1053‐1 is described. Closing it will be expressed corresponding to the wish of practise that only a complete document should be introduced in the list of technically regulations for construction by the building authority.     

Extension of the conditions for application of the simplified calculation methods according to DIN EN 1996‐3/NA for the design of unreinforced masonry walls

Dedicated to University Prof. Dr.‐Ing. Carl‐Alexander Graubner for his 60th birthday The simplified calculation methods for unreinforced masonry structures given in DIN EN 1996‐3/NA are an easily applicable design standard for an efficient and fast verification of the resistance of mainly vertically loaded masonry walls. However, the design rules are not based on mechanical models. Instead, they are empirical approaches for a simplified estimation of the load bearing capacity. For this reason, the range of application of DIN EN 1996‐3/NA is limited by several conditions to ensure a sufficient safety of this design procedure. With regard to extending the conditions for application, extensive comparative calculations were carried out. Thereby, considering clearly defined boundary conditions, the load bearing capacity according to DIN EN 1996‐3/NA was compared to that according to DIN EN 1996‐1‐1/NA. It was the aim of this comparison to identify load bearing reserves of the simplified calculation methods to point out potential for an extension regarding the maximum permissible clear wall height and the slab span. As a result, it can be stated, that an increase of the maximum wall height up to 6.0 m and the maximum slab span of 7.0 m is possible in certain cases.     

Schubtragfähigkeit von Mauerwerk aus Wärmedämmziegeln

Der Beitrag berichtet über zentrische und exzentrische Schubversuche an Mauerwerk aus Wärmedämmziegeln. Die Ergebnisse werden mit den aktuell gültigen Bemessungsansätzen in DIN 1053‐1 verglichen. Es wird gezeigt, dass die Versuchsergebnisse immer eine ausreichende Sicherheit gegenüber den Bemessungswerten nach DIN 1053‐1 aufweisen. Auch für Mauerwerk aus Wärmedämmziegeln mit allgemeiner bauaufsichtlicher Zulassung können somit die vereinfachten Bemessungsregeln der DIN 4149 ohne Abminderungen angewendet werden. In plane shear resistance of thermal insulating vertically perforated clay brick masonry. In plane shear tests on thermal insulating vertically perforated clay brick masonry are presented. The results are compared with the design rules from DIN 1053‐1. The test results exceeded the design predictions of DIN 1053‐1 by a significant safety margin. The simplified design rules from the German earthquake standard DIN 4149 can be applied to thermal insulating clay unit masonry with technical approvals without any reduction.     

Experimental determination of the load‐bearing capacity of a reinforced perforated brick facade

For a new multi‐storey car park over the Central Bus Station (ZOB) in Kiel, a perforated clinker brick veneer facade not conforming to standards was planned. The design and technical characteristics of the facade have already been described in the article by Medzech and Schrade in this issue [1]. This article deals with the experimental investigations carried out to obtain a project‐related one‐off approval (ZiE). These experiments contain in particular large tests on storey‐height wall sections, which were subjected to eccentric compressive loading and partly to horizontal loads representing wind action. Supplementary small tests on unreinforced and reinforced masonry served to determine the bending capacity, the anchoring capacity of the reinforcement and the load‐bearing capacity of the wall anchors in the masonry. Due to the special facade construction with special bricks for the project, wall anchors, reinforcement bar couplers and unique test set‐ups had to be developed for the specific project.     

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.     

Schubtragfähigkeit von Wänden aus Kalksand‐Planelementen mit geringem Überbindemaß — Experiment und rechnerische Simulation

Der Querkraftwiderstand V_R von Mauerwerkswänden, die in ihrer Ebene durch Wind‐ oder Erdbebeneinwirkungen beansprucht werden, hängt auch vom Überbindemaß ü bzw. vom Verhältnis des Überbindemaßes zur Steinhöhe ü/h_st ab. Das nach Norm derzeit zulässige Überbindemaß von ü ≥ 0,4 h_st kann bei der Verwendung von Planelementen in der Praxis nicht immer eingehalten werden. Für diese Fälle ist in den allgemeinen bauaufsichtlichen Zulassungen (abZ) des Deutschen Instituts für Bautechnik (DIBt) für Kalksand‐Planelemente im Bereich 0,4 > ü ≥ 0,2 h_st bzw. 12,5 cm bisher ein verminderter zulässiger Rechenwert der charakteristischen Schubfestigkeit f_vk von 60 % des Wertes nach DIN 1053‐100:2007‐09 (ü ≥ 0,4 h_st ) anzuwenden. Diese, auf Ergebnissen alter Versuche mit überholten Prüfanordnungen beruhenden, hohen Tragfähigkeitseinbußen waren im Zuge der Aufnahme von Planelementen in die Bemessungsnormen für Mauerwerk mit neuem Schubbemessungskonzept zu überprüfen. Daher wurden umfangreiche experimentelle und theoretische Untersuchungen an 17,5 cm dicken, 2,50 m hohen Schubwänden aus Kalksand‐Planelementmauerwerk mit Dünnbettmörtel und unvermörtelten Stoßfugen durchgeführt. Ziel war es, den Einfluss geringer Überbindemaße ü/h_st < 0,4 auf die Schubtragfähigkeit dieser Wände, insbesondere bei statisch‐zyklischen Horizontalverformungen in Wandebene, quantitativ zu bestimmen. Als Versuchsparameter wurden die Wandauflast (σ = 0,5/1,0/1,43 N/mm²), das Überbindemaß (ü/h_st = 0,2/0,4) und die Einspannung am Wandkopf und ‐fuß variiert. Die Untersuchungen ergaben im Bereich der normativ bemessungs relevanten geringen Überbindemaße 0,2 ″ ü/h_st ″ 0,4 keine signifikanten Traglasteinbußen. Bei erweiterten theoretischen FE‐Analysen für das baupraktisch übliche Spektrum vorhandener Überbindemaße 0,2 ″ ü/h_st ″ 1,0 von Wänden mit Auflastspannungen von 0,5 N/mm² bzw. 1,0 N/mm² wurde eine Traglastminderung von maximal 12 bis 16 % berechnet. Der Abtrag der Horizontallast vom Wandkopf zum Wandfuß erfolgt über ein schräges Druckspannungsfeld. Im Überbindebereich der Elemente auftretende Spannungskonzentrationen können zu örtlich begrenzten Rissbildungen führen, ohne dass die Tragfähigkeit der Wand hierdurch beeinträchtigt wird. Diese ist erst dann erschöpft, wenn insbesondere am Wandfuß die vom gerissenen Mauerwerk übertragbaren, schrägen Druckspannungen nicht mehr aufgenommen werden können. Shear load bearing capacity of masonry walls made of calcium silicate element units with a low overlap length — Experimental and numerical simulation analysis. The shear force resistance capacity V_R of masonry walls subjected, in their plane, to loads from winds or earthquakes, amongst other things, depends on the overlap length of the units ü or on the ratio of the overlap length and the height of the unit ü/h_st . The currently permissible overlap length, according to German design standards and norms, of ü ≥ 0.4 h_st can not always be adhered to in building practice, when using element units. In such cases and according to general technical approval code (abZ) of the German Institute for Building Technology (DIBt) for calcium silicate element units within the range 0.4 > ü ≥ 0.2 h_st or 12.5 cm, a reduced permissible calculation value of the characteristic shear strength f_vk of 60 % of the value according to DIN 1053‐100:2007‐09 (ü ≥ 0.4 h_st ) has been used to date. This high loss of load bearing capacity, based on results of older experiments with ob solete test setups, was to be tested in the course of the inclusion of element units in the calculation standards for masonry walls, with a new shear calculation concept. As a result, extensive experimental and theoretical tests were carried out on 17.5 cm thick and 2.50 m high shear walls made of masonry calcium silicate element units and ungrouted butt joints. The objective was to quantatively determine the influence of low overlap length ü/h_st < 0.4 on the shear resistance of these masonry walls and in particular with static‐cyclical horizontal displacement at the wall top. The vertical load pressure (σ = 0.5/1.0/1.43 N/mm²), the overlap length (ü/h_st = 0.2/0.4) and the fixing at the top and bottom of the wall were varied and used as experimental parameters. Within the range of normative calculation with low overlap lengths 0.2 ″ ü/h_st ″ 0.4, the investigations showed no significant load bearing capacity loss. With more extensive theoretical Finite Element analyses for the normal building practice spectrum of overlap lengths 0.2 ″ ü/h_st ″ 1.0 for walls with a vertical load of 0.5 N/mm² or 1.0 N/mm², a reduction of load bearing capacity of maximum 12 % to 16 % was calculated. The transmission of the horizontal shear load from the top of the wall to the bottom of the wall takes place through a diagonal compression stress field. Cracks may occur in the overlap area of the element units as a result of stress concentration, which are limited to the overlap area and do not cause any impairment to the load bearing capacity of the wall. This capacity is only then exhausted when the trans missible, diagonal compression stress can no longer be absorbed, especially at the bottom of the wall with cracked masonry.