Structural Masonry

An Experimental/ Numerical Basis for Practical Design Rules (CUR Report 171)

CUR Centre for Civil Engineering Research and Codes

• Publisher: CRC Press
• Year: 2022
• Print ISBN: 9789054106807
• eBook ISBN: 9781000150636
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This text provides a basis for a standardized approach to structural masonry, using an integration of experimental and computational techniques. Accurate displacement-controlled materials experiments have produced an extensive database of strength, stiffness and softening properties for tension, compression and shear, and this data has been transferred into numerical models for simulating the deformational behaviour of masonry structures. The models have been implemented into finite and distinct element codes and have subsequently been verified against shear wall experiments and analytical solutions for masonry parts.

CHAPTER 1 Introduction

DOI: 10.1201/9781003077961-1

1.1 GENERAL

Building in unit bond has been known since ancient times. Innumerable variations have occurred throughout the centuries, influenced by the local supply of raw materials and the country’s culture. Strong points in favour of masonry are undoubtedly the aesthetic appearance, the durability and the simplicity of the stacking technique. Supported by slogans like ‘Unit is Beautiful’, for clay units the aesthetic motives have gradually gained the upper hand over the structural aspects. In contrast to earlier days the load bearing structure of modern buildings is provided by concrete or steel, while clay units are mainly used as facade cladding. Using calcium silicate products is a different matter. This material has primarily a load bearing structural function and has become popular during the last decade owing to the ample supply of blocks and elements combined with mechanical handling at the building site.

The decrease in the market share of clay unit in the load bearing sector can to a considerable extent be attributed to the fact that the technical-scientific development of design rules has lagged behind compared to concrete and steel. This lack of a good basis for design and calculation hinders innovative applications and causes insufficient understanding of damage symptoms. Also in the case of calcium silicate unit many structural questions have remained unanswered and the traditional rules of thumb are inadequate as soon as new applications outside the usual field of experience are discovered. The gravity of the situation has been shown, among other things, by the TNO report ‘Safety of masonry constructions - Probabilistic approach’ [1]. It showed that traditional calculation methods do not fit in with the actual behaviour of masonry structures. Some examples are the problems concerning bending strength of facades and cavity walls, crack spacing and movement joint spacing under restrained shrinkage or thermal cooling and the failure behaviour of new types of structures such as diaphragm walls in clay unit or glued pier-wall connections in calcium silicate unit.

The above gaps in knowledge have been recorded systematically by the CUR PC55 ‘Structural Masonry’ pre-advisory committee [2]. This was the basis for the CUR PAC4 programme advisory committee of the same name on which a final advice concerning a research programme was drawn up [3]. In that advice this committee states (quotation): ‘… knowledge of masonry might be present in a few subsectors but due to the unstructured form of description it cannot or can hardly be used to dimension and calculate structures. There is no unity in the principles of mechanics, no unity in the test methods and no unity in the approach to the problem. …’. Traditional research in the field of masonry mainly took place (and still takes place) through ad hoc tests of structures, so that the result is only valid for that specific structure, with that specific type of unit and joint, those specific boundary conditions and those specific loads. Extrapolation of the results to other situations is not or only to a very limited extent possible. A more general approach is needed. The CUR PA33 ‘Computational masonry mechanics’ pre-advisory committee [4] has in this respect pointed out the new possibilities of numerical simulation techniques. Especially the non-linear finite element method seems to be appropriate, with the salient detail of the natural presence of ‘finite elements’ in masonry.

Another aspect that is of importance for the future of structural masonry is the increasing internationalisation of the building process. The originally mainly local character of building in units has resulted in a diversity of approaches. National regulations differ in volume and quality to a large extent. In this field the Netherlands does not keep up with countries such as Germany and Great Britain. The recently formulated Eurocode no 6, ‘Common unified rules for masonry structures’[5], is a compromise in which the achievements of the countries with a clear tradition in masonry have been integrated. Defence of the typically Dutch building methods in this European circuit is of major importance. This applies, for example, to our slender cavity walls which differ to a large extent from the building practice with solid walls in surrounding countries. This stimulates the Netherlands to increase its research efforts.

The above tendencies have been recognised by the Royal Dutch Association of Unit Manufacturers (KNB) and have led to the CUR ‘Structural Masonry P research project during 1989-1993. As from 1990 the project has also been supported by the calcium silicate (sandlime unit) industry (CVK/RCK).

1.2 PURPOSE AND FRAMEWORK OF THE RESEARCH

Starting point in the research was the above final advice from the CUR PAC4 committee [3]. Following this advice the main purpose of the research can be defined as follows:

To create a bases for a general approach to structural masonry. This includes closing the gap between theory, mechanics models and materials testing on the one hand, and the structural practice on the other hand, which should result in a clearer insight and a scientific basis for the behaviour of masonry structures.

As far as the research method is concerned a combined experimental/numerical approach has been chosen. This method has been recommended by the CUR PA33 pre-advisory committee [4] and is considered to be the most promising to achieve a general approach to structural masonry. The outlines of the method are:

1. a) Execution of material experiments. Through systematic testing a fundamental knowledge will be developed concerning tensile strength, shear strength, compressive strength and the deformational capacity of masonry as a synthesis of the basic components unit and mortar.
2. b) Formulation of material models. The acquired material properties are transformed into constitutive models for unit, joint, interface and composite. The modelshave to correctly represent the non-linear behaviour of the material through crack formation.
3. c) Implementation of the models in finite element method soft ware. In this project the multi-purpose program DIANA has been chosen because of the ample possibilities in the non-linear field and because of the good access to TNO Building and Construction Research and both Universities of Technology. Besides, partial studies are carried out with the special purpose program UDEC.
4. d) Verification by means of construction experiments. A limited number of experiments on parts of structures, piers for example, are necessary to verify whether the numerical models are sufficiently reliable. In case numerical results and experimental results do not correspond sufficiently, an adjustment of the models or parameters should take place.
5. e) Execution of practice-oriented case studies. With the help of new material data and numerical models the behaviour of masonry structures can be simulated. The purpose of which is a clearer insight. The ultimate purpose is the development, foundation and improvement of calculation rules, auxiliary tables and diagrams, as well as the propagation of the obtained knowledge to the building practice.

This report contains the results of the above process. Chapter 2 shows an outline of the materials testing carried out for tension, compression and shear, whereby not only strength criteria are dealt with but also the softening behaviour after having reached the ultimate strength. This softening behaviour is essential with unit-like materials and determines the way in which crack formation propagates within a structure. Subsequently Chapter 3 describes the way in which experimental observations are transformed into cracking and friction models within DIANA. A distinction is made between micro and macro. Micro-models model units and joints separately, whereas macro-models describe the overall behaviour of masonry-as-composite. Given the fundamental purpose of the research the accent has been laying on the micro-models. Chapter 4 deals with evaluation and verification studies for masonry components that are in tension and piers under shear. The numerical results are discussed and judged on the basis of expectations derived from manual calculations and experimental macro-data. Chapter 5 describes the numerical approach with UDEC. With this program also pier studies have been carried out. The distinct element method within UDEC is compared with the finite element method within DIANA.

Subsequently the step towards applications is made in Chapters 6 and 7. Two practice-oriented case studies are discussed: the failure behaviour of pier-wall connections and the crack formation of walls under restrained shrinkage. The case studies illustrate the way in which practical calculation rules for respectively stability and dilatation’s (movement joints) are given a basis or are improved via the research carried out. Other applications of numerical simulations have been published separately, among which the formulation of design models for masonry diaphragm walls [6], the assessment of existing test methods for cohesion (tensile and bending) [7] and the evaluation of existing test methods as well as the development of a new test method for shear [8]. Finally Chapter 8 deals with the conclusions.

CHAPTER 2 Testing of materials

DOI: 10.1201/9781003077961-2

2.1 GENERAL

This chapter describes tests by which various material parameters have been determined. Within this research distinction has been made between the components masonry consists of. This level of research is often indicated with micro- or detail level. The tests concerned half-unit masonry with mortar joints.

The main purpose of the tests is to determine parameters which are necessary in finite element method programs to describe the behaviour of materials.

A second purpose of the tests is to determine possible links between the various parameters so that in the future relatively simple tests will be sufficient to determine normative parameters.

A relatively soft mud clay unit, a wire cut unit and a calcium silicate unit, all of standard quality were used. With the compression tests another stronger wire cut unit was used as well. These units were used in combination with calcareous and cement-rich mortars. As a result a wide range of possible combinations of materials was tested, providing insight into the magnitude and possible variation of the material parameters which were determined by means of the various tests.

Deformation-controlled compression, tension and shear tests were carried out. With this way of testing the load is applied in such a way that during a test, deformation of a specimen increases with a prescribed speed over a chosen measuring length; this in contrast to load-controlled tests whereby the load increases with a prescribed speed. The great advantage of such deformation-controlled tests is the possibility to continue registration of the behaviour after the maximum force has occurred.

The next section deals with the materials that were used and the way in w...

Table of contents

• Cover Page
• Half-Title Page
• Title Page
• Copyright Page
• Contents
• Preface
• Summary
• Symbols
• 1. Introduction
• 2. Testing of Materials
• 3 Numerical Models in Diana
• 4 Evaluation and Verification Studies with Diana
• 5 Numerical Studies with Udec
• 6 Case Study Cracking Behaviour of Walls Under Restrained Shrinkage
• 7 Case Study Pier-Main Wall Connections
• 8 Concluding Remarks
• Appendix: Influence of Wall Length on Stress Distribution With Restrained Shrinkage
• References