experimental analysis of mechanical behavior of lintels in structural

15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
EXPERIMENTAL ANALYSIS OF MECHANICAL BEHAVIOR OF
LINTELS IN STRUCTURAL MASONRY
Rezende, Fabiana Martins de1; Rizzatti, Eduardo2; Mohamad, Gihad2; Roman,
Humberto Ramos3; Bastos, Marcos Scherer4
1
Ms. Candidate, Federal University of Santa Maria, Civil Engineering, [email protected]
Professor, Federal University of Santa Maria, Civil Engineering Department, [email protected];
[email protected]
3
PhD, Professor, Federal University of Santa Catarina, Civil Engineering Department, [email protected]
4
Fourth-year undergraduate, Federal University of Santa Maria, Civil Engineering, [email protected]
2
This study deals with experimental analysis of lintels in structural masonry walls built with
hollow clay blocks. The main purpose of this study is to observe their mechanical behavior,
failure mode and deflections, when subjected to concentrated loading. The experimental
program was based on reinforced lintels made with structural blocks and horizontal
prefabricated bed joint reinforcements, through the use of MURFOR® steel reinforcement.
The test results indicate that the failures occurred by the influence of shear stresses, according
to compressions on the top of some blocks that could be observed.
Keywords: Structural Masonry, Clay Block, Lintel, MURFOR® reinforcement masonry
INTRODUCTION
With the heating market in construction, the companies of these sectors are seeking options to
carry out works fast and economically. One alternative is the structural masonry, construction
system in which the walls make up the structure of the building and also perform the
functions of conventional sealing elements.
The structural concept of the system is based on transferring loads through compressive
stress. The existence of tensile stresses in certain parts is permitted with restrictions to specific
points of the structure with low values, due to the low tensile strength of the components of
the masonry. However, due to the characteristics related to the project design of the building
or attached to some components of the system, tensile stresses with significant values occur at
certain location, which require the use of steel reinforcement to inhibit the cracks. As an
example, in walls provided with openings, the occurrence of tensile stresses in lintels cannot
be avoided.
Several authors studied the performance of reinforced masonry. In some studies, as Gouveia
(2007), Lourenço (2004) and Pfeffermann (2004), is reported the using of a specific
reinforcement in the form of an plane lattice, as MURFOR ®, which is patented and quite
common in Europe. However, in Brazil, there has been little research related to this type of
reinforcement. Therefore, this paper aims to analyze the mechanical behavior of reinforced
lintels with MURFOR ® in the horizontal mortar joint, checking failure mode of the test
samples and displacement occurred.
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
DEFINITION AND APPLICATIONS OF REINFORCEMENT ® MURFOR
According to Timperman (2002), the use of steel reinforcements in masonry in Europe, is
common in areas subject to tensile stresses to improve system performance. It is very
common to use a steel reinforcement, similar to a plane lattice, called MURFOR ®, which is
inserted into the horizontal mortar joint. Murfor ® is a prefabricated reinforcement which was
introduced for the first time in Europe, more than 35 years ago. During this period, this
increase was developed in Belgium at the time of the proliferation of buildings and changes in
construction methods.
Lourenço (2004), in a case study on building a stadium in Portugal (Sporting Club of
Portugal), reports the performance of a masonry wall with more than 10 m long and 4 m high,
reinforced in the horizontal joint mortar. For the author, the introduction of plane lattice
MURFOR ® in the mortar does not lead to changes in the process of traditional masonry
construction and obtain a homogeneous material with appropriate characteristics to limit the
pathologies associated with cracking. It also mentions other advantages such as exemption
from the need to use different blocks to make lintels (block U) and pillars in steel concrete to
stiffen the panels, resulting in higher execution speed and efficiency of construction.
Figure 1: reinforcement Murfor®, Lourenço (2004).
According to the definition of the product manufacturer, Murfor®, is a plane lattice formed
by two longitudinal steel galvanized wires, separated at a distance defined by a sinusoidal
shape wire. The wires have a tensile strength of 600 MPa. Both wires are grooved to ensure a
good adhesion with the mortar. The purpose of the product is to provide a better structural
performance of masonry when subjected to shear and tensile stresses.
The application of horizontal layers of reinforcement in the joints contributes significantly in
reducing the thickness of the cracks. The manufacturer recommends its use in various
situations, such as the presence of concentrated loads, structural deformations, loads (wind,
seismic), drying shrinkage, stress concentrations around openings of doors and windows,
areas with differential settling of the foundation and for the construction of long walls without
stiffeners.
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
Figure 2: example of application of reinforcement MURFOR ® to avoid cracks in the
case of differential settling of foundation, ManufacturerArcelor Mittal.
Figure 3: example of application of reinforcement MURFOR ® to prevent cracks
around openings of windows, ManufacturerArcelor Mittal.
LINTELS
The NBR 15812-1 (2010) defines a lintel as the beam housed on an opening door or window
that has the sole function of transmitting vertical loads to the walls adjacent the opening.
Mamede (2001) performed theoretical simulations of the behavior of openings in walls of
structural masonry analyzing stress flow and quantities to justify the need for lintels over
openings. Adopted in one of the models, the wall with dimensions of 3.60 x2, 72x0, 14 m,
with concrete blocks (fbk=4,5 MPa), modulus of elasticity of the masonry equal to
800·fbk=3600 MPa, coefficient of Poisson equal to 0,20, size of opening 1.20 x 1, 20 m, load
34 kN/m (3400 kgf/m). The opening had lintels bending and stiffened with precast concrete.
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
Figure 4: result of processing finite element modeling: tensions in the horizontal (σx)
and vertical (σy), respectively, in kN/cm2, Mamede (2001).
As benchmarks, Mamede (2001) used, based on BS 5628-1 (1992), the following maximum
permissible shear stress and compression (on the wall adjacent to the will) in masonry
unreinforced: 0,015 kN/cm2 and 2·fmk (0,036 kN/cm2), respectively. Analyzing the simulation
results, observed compressive stress greater than the value adopted as the maximum
permissible in the upper corners of the openings and concluded that in order to absorb these
stresses, the presence of the spar was fundamental and that its length should extend beyond
the opening will .
EXPERIMENTAL TESTS
In order to observe the mechanical behavior of the lintels, five small walls were made,
dimensions 2390 x 790 x 140 mm (length, height and width), with structural ceramic blocks
and reinforcement Murfor ® in the horizontal mortar joints. The walls were built using hollow
ceramic blocks with average size of 289 x 191 x 141 mm (length, height, width), evaluated
according to the requirements of NBR 15270-2; cement mortar, lime and sand, 1:0,5:4
recommended by BS 5628-part 1 and a galvanized lattice with steel wire 4 mm in diameter
and 115 mm in width. Figure 5 shows the arrangement. Table 1 summarizes the test results of
material characterization.
Figure 5: detail of the implementation of the wall.
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
Table 1: characteristic values of the components of the walls.
Material
Characteristic
Block
Medium compressive strength - fbm = 10,0 MPa
Normative
reference
NBR 15270-3
Strength characteristic compression- fbk = 7,14 MPa
NBR 15270-3
Medium compressive strength – fam = 7,12 MPa
NBR 13279
Mortar
Prism
Medium compressive strength – fpm = 4,23 MPa
Lattice
Strength characteristic tensile–fyk= 600 MPa
NBR 15812-2
Manufacturer
catalog
The figure 6 illustrates the procedure for construction of the walls.
Figure 6: the construction of the walls
To determine the failure load, the walls were subjected to concentrated load, in the middle
third of the span, and speed of application loading of 1,0MPa/min, following
recommendations of NBR 12142 (2010). The deflections were measured at mid-span with
analog clock, model Mitutoyo, 0,01 mm resolution. The figure 7 illustrates the test. Table 2
summarizes the results obtained.
Figure 7: test of the walls.
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
Table 2: Experimental results of ultimate load and flexure moment of walls
Wall
Ultimate Load (kN)
Maximum flexure moment (kN.m)
Wall 1
32,5
10,97
Wall 2
22,0
7,42
Wall 3
20,0
6,75
Wall 4
21,0
7,09
Wall 5
19,0
6,41
Figure 8 shows the result of the displacements measured. Section 1 identifies the beginning of
cracking.
Figure 8: value Load x Displacement.
The failure modes of the walls were very similar. On a wall 1, there was failure in the form
of a ladder in the corner with visible loss of adhesion between the mortar joint and blocks,
beyond the rupture of the outer walls of the blocks near the point of load application. There
were no cracks in the mid-span. The figure 9 illustrates the failure mode of wall 1.
Figure 9: detail of failure mode of wall 1.
In wall 2, there was failure in the form of a ladder in the corner with visible loss of adhesion
between the mortar joint and blocks, in addition to the partial rupture of the blocks near
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
the point of load application. There were no cracks in the mid-span. The figure 10 illustrates
the failure mode of wall 2.
Figure10: failure mode of wall 2.
In wall 3, there was failure in the form of a ladder in the corner with visible loss of adhesion
between the mortar joint and blocks and a horizontal fissure in the opposite corner. There
were no cracks in the mid-span. The figure 11 illustrates the failure mode of wall 3.
Figure11: failure mode of wall 3.
In wall 4, spaced cracks were observed in the corners of the mortar joint. At the point of load
application there was cracking in the block. The figure 12 illustrates the failure mode of wall
4.
Figure12: failure mode of wall 4.
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
In wall 5, there was failure in the form of a ladder at both corners with visible loss of adhesion
between the mortar joint and blocks. In one of the points of load application,
crushing occurred in the block. There were no cracks in the mid-span. The figure 13 illustrates
the failure mode of wall 5.
Figure13: failure mode of wall 5.
CONCLUSION
The analysis of failure modes of lintels shows the predominant influence of shear
stresses. The absence of cracks in the mid-span of the specimens indicates little
effect of flexure stress.
The analysis of load versus displacement graph showed a linear elastic behavior until around
80% of ultimate load to failure.
REFERENCES
Associação Brasileira de Normas Técnicas. NBR 12142: Concreto – Determinação da
resistência à tração na flexão de corpos de prova prismáticos. Rio de Janeiro, 2010. 5 p.
Associação Brasileira de Normas Técnicas. NBR 13279: Argamassa para assentamento e
revestimento de paredes e tetos – Determinação da resistência à tração na flexão e à
compressão. Rio de Janeiro, 2005. 9 p.
Associação Brasileira de Normas Técnicas. NBR 15270: Componentes cerâmicos – Parte 2.
Blocos cerâmicos para alvenaria estrutural – Terminologia e requisitos. Rio de Janeiro. 2005.
11p.
Associação Brasileira de Normas Técnicas. NBR 15270: Componentes cerâmicos – Parte 3.
Blocos cerâmicos para alvenaria estrutural e de vedação – Métodos de ensaio. Rio de Janeiro.
2005. 27p.
Associação Brasileira de Normas Técnicas. NBR 15812: Alvenaria estrutural – Blocos
Cerâmicos - Parte 1. Projeto. Rio de Janeiro. 2010. 41p.
Associação Brasileira de Normas Técnicas. NBR 15812: Alvenaria estrutural – Blocos
Cerâmicos - Parte 2. Execução e controle de obras. Rio de Janeiro. 2010. 41p.
British Standards Institution. BSI 5628 – Part 1: code of practice for use of masonry:
structural use of unreinforced masonry. London, 1992. 58 p.
15th International Brick and Block
Masonry Conference
Florianópolis – Brazil – 2012
Gouveia, J. P.; Lourenço, P.B.; Vasconcelos, G. “Soluções construtivas em alvenaria”.
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economical
benefits”.
13th
InternationalBrickandBlockMasonryConference, Amsterdam, 2004.
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(Mestrado em Engenharia de Estruturas). Escola de Engenharia de São Carlos, Universidade
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Brick and Block Masonry Conference, Amsterdam, 2004.
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Paredes de Alvenaria, P.B. Lourenço & H. Sousa (Eds.), Porto, 2002.