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.     

Limitation of cracking in AAC masonry under the window zone

The article describes the results of tests on the impact of reinforcement on the appearance of cracks and on the manner of damage to masonry under the window zone. Masonry made of Autoclaved Aerated Concrete units with thin layer mortar was tested. Eight unreinforced test specimens (two series) and four series of test specimens (12 specimens) with reinforcement in the bed joints were subject to testing. Two types of reinforcement were used in the tests. It is demonstrated that the reinforcement has a significant impact on the level of cracking stress and propagation of cracks. It was found that when a pillar is correctly designed, cracks should not occur in the masonry under the window zone.     

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.     

Textile reinforcement in the bed joint of basement masonry – First findings from a current research project

The successful structural verification of basement walls under earth pressure loading with light imposed loading is often difficult. This situation is often encountered for external basement walls under terrace doors, stairs, masonry light wells etc., where the theoretically necessary imposed loading is missing. This makes it impossible to resist the acting bending forces from earth pressure using a vertical arch model. In such cases, the earth pressure has to be resisted in a horizontal direction. Since however the bending moment capacity of unreinforced masonry parallel to the bed joint is low, another possibility is to use a textile‐reinforced bed joint with longitudinal fibres of alkali‐resistant glass or carbon fibre. With an appropriately adapted textile reinforcement in the bed joints, the masonry can fulfil the requirements for load‐bearing capacity against earth pressure with horizontal load transfer, even under a small imposed load. Textile reinforcement has the advantage above all of corrosion resistance compared to conventional steel reinforcement, and textiles can also be inserted into thin bed joints. The Chair of Structural Design in the Faculty of Architecture of the TU Dresden is currently carrying out extensive numerical and experimental studies for this purpose. The objective is to develop an optimal configuration of material and textile form for use as bed joint reinforcement. The investigations are concentrating on the tension strength, bonding and durability of the composite material ”textile mortar“. This report should give a brief overview of the state of the work in the currently running research project.     

Textile reinforcement in the bed joint of basement masonry and infill walls subjected to high wind loads

The successful structural verification of basement walls under earth pressure loading with light vertical loading is often difficult. This situation is often encountered for external basement walls under terrace doors, stairs, masonry light wells, etc., where the vertical loading that is theoretically necessary is absent. This makes it impossible to resist the acting flexural forces from earth using a vertical arch model alone. In such cases the basement wall must also resist the earth pressure in a horizontal direction. However, due to the fact the bending moment capacity of unreinforced masonry parallel to the bed joint is low you have the option here of using a textile‐reinforced bed joint with longitudinal fibres of alkali‐resistant glass or carbon fibre. With an appropriately adapted textile reinforcement in the bed joints, the masonry can fulfil the requirements for load‐bearing capacity against earth pressure with a horizontal load transfer, even under a small vertical load. The same applies to infill walls subjected to high wind loads the bending moment capacities of which are also slightly parallel to and vertically to the bed joint and cannot be provably demonstrated on large infill surfaces and strong wind loads. The load‐bearing can also be increased by improving the flexural strength parallel to the bed joint. The Chair of Structural Design in the Faculty of Architecture of the Technical University (TU) Dresden was carrying out extensive numerical and experimental studies for this purpose. In the journal Mauerwerk 01/2018 [1] first findings from small trial series have already been presented. In the meantime, a series of large‐scale tests have additionally been performed to check the promising results of the small‐scale tests with respect to their real applicability. This report should provide a combined insight into the work of the concluded research project.     

New approach for spacing of movement joints in reinforced and unreinforced masonry veneer walls / Neue Bemessungsmethode für die Abstände von Dehnungsfugen in bewehrten und unbewehrten Verblendmauerwerksschalen

The first part of the paper, published in issue 4 [3] and dealing with the spacing of movement joints, described the design method for unreinforced masonry veneer walls. This paper focuses on increasing the spacing of movement joints by applying bed joint reinforcement. The proposed approach enables manufacturers of bed joint reinforcement to provide recommendations for the spacing of movement joints for bed joint reinforced veneer walls dependent upon the allowable crack width, the reinforcement type, the diameter and the vertical spacing of the reinforcement.     

Bond behaviour between textile reinforcement and mortar under tensile loading / Verbundverhalten zwischen textiler Bewehrung und Mörtel unter Zugbeanspruchung

Extensive experimental investigations are currently being carried out on various selected materials covering a wide range of properties to achieve a deeper knowledge about the bond performance of textile reinforced mortar (TRM) for masonry strengthening. The objective of the tests includes investigations of the bonding behaviour between alkali‐resistant glass textile reinforcement and mortar under tensile loading to determine the required anchorage and overlapping lengths of the reinforcement in the mortar‐based material. This article describes the test methods used as well as the results obtained so far. This research will also examine debonding of the mortar‐based reinforcement system and the masonry surface under shear load. The definition of these bond parameters is necessary for the design of textile‐reinforced masonry components, which will be developed in the near future. The research is also intended to contribute to the finding or even designing of matching alkali‐resistant glass textiles specifically for use in masonry.     

Tragfähigkeitserhöhung bei Mauerwerk durch textilglasgitterverstärkte Mörtelfugen

Increase of the vertical load carrying capacity of masonry due to mortar bed joints with textile glass mesh reinforcement From a structural point of view, one of the most important material parameters in the construction sector is the vertical compressive strength of masonry, which consists of the compressive strength of the bricks as well as of the mortar bed. The interaction between the bricks and the mortar beds is the main reason for compression failures of masonry walls. A close analysis of the deformation behavior of the two components shows that different transverse strains in the contact surface between the bricks and the mortar are the main cause for compression failures. However, the load‐bearing capacity of masonry walls can be increased by using some reinforcement in the mortar beds which counteracts lateral expansion. The impact of textile glass mesh reinforcement on the load‐bearing capacity of masonry was analyzed in a test program on masonry columns with different numbers of textile glass mesh reinforced mortar beds. The results of the analyses show that the load‐bearing capacity of the columns rises with an increased ratio of reinforcement, regardless of the type of bricks used. From the ratio of the height of the reinforcement layers to the thickness of the wall it can be deduced that a higher degree of reinforcement has a positive effect on the load‐bearing capacity of the masonry. On this basis, an increase of the strength and load‐bearing capacity of masonry walls is formulated to be on the safe side.     

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.     

New approach for spacing of movement joints in reinforced and unreinforced masonry veneer walls. Part 1: Unreinforced masonry / Neue Bemessungsmethode für die Abstände von Dehnungsfugen in bewehrten und unbewehrten Verblendmauerwerksschalen. Teil 1: Unbewehrtes Mauerwerk

The spacing of movement joints has been subject of many discussions. The current methods for the determination of the spacing of movement joints are based on local traditions and bad experience with cracked veneer walls. This has resulted in various design rules throughout Europe with very stringent limits for spacing of movement joints. According to EC6, one of the solutions for increasing the spacing of movement joints is to introduce bed joint reinforcement, although unfortunately no specific design rules are given. Until now, most scientific research has been focused on numerical simulations without taking time‐dependent effects into account, which is a conservative approach. In this paper, a new approach is described. It is based on Peter Schubert's model and on practical experience with masonry buildings.