partially reinforced concrete masonry · 2015-04-23 · partially reinforced concrete masonry is...
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PARTIALLY REINFORCED CONCRETE MASONRY
Ahmad A. Hamid
Professor
Drexel University
Philadelphia
PA,USA
ABSTRACT
Gouda M. Ghanem
Research Fellow,Drexel
University, on Leave fr om
Helwan University,
Cairo,Egypt
Partially reinforced concrete masonry is cheeper than reinforced
concrete masonry and can be used in many applications in areas of low or no
seismic action, where benficial properties of reinforced concrete masonry
such as higher strength, improved out-of-plane flexural, in-plane flexural
and shear capacities and lesser variability can be utilized at a reasonable
cost . This paper presents information on an ongoing research program at
Drexel University aimed at evaluating the material properties of partially
reinforced concrete masonry as well as studying the structural behavior o f
wall elements made of this material using small-scale modeling techniques.
Information on the methodology of small-scale modeling techniques, material
properties, and wall behavior of partially reinforced concrete masonry i s
presented. This research will provide desgin recommendations for wall
elements and analytical or empirical formulae for prediction of material
properties and wall capacity.
INTRODUCTION
Masonry is one of the first durable building materiaIs to be
developed. Most of the early applications utilized its compressive strength
and capacity to arching. Resulting masonry structures were invariably
massive as gravity loads stabilized the structures against any lateral
forces due to wind or siesmic action . Although structural use of masonry
was hampered by its long history as a craft based material, greater
understanding of its structural capacity in compression and shear led to
thinner walls and consequent intensive usage in commercial and residential
buildings. Its continued use today i8 primarily due to is economy,
possibility of construction without large capital expenditure by the
builder, fire resistance, aesthetic appearance, durability ease of
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construction, good thermal and sound insulation,weather protection, and
ability to provide structure as well as sub-division of space. Now it is
considered to be an engineering material, one where the properties of that
material are known and its performance can be predicted.
One major shortcoming of plain (unreinforced) masonry is the
inadequate resistance to out-of -plane bending. Further, masonry walls are
often subjected to out-of-plane and in-plane flexure and shear stresses
from wind or seismic forces and also moments due to eccentricity of gravity
loads and continuity of the floor slabs. Reinforced masonry was developed
to overcome this major shortcoming, it is proved to be very effective in
improving structural behavior as well as reducing the inherent variability
of masonry. It is widely used now and most national codes give design
guidan6e.
Partially reinforced masonry can be considered as a by-product of
reinfoced masonry. It has a greater appeal when used with concrete blocks
and the finished product is partially reinforced concrete masonry. It is
defined as masonry where not alI the cavities are grouted and
reinforcement provided is less than 0.002 times the gross cross-sectional
area of the wall or is spaced greater than 4 feet on- centers l !2). Partially
reinforced concrete masonry, which is stronger than hollow concrete masonry
but weaker than reinforced concrete masonry, has a growing demand lll ) in
residential and commercial construction in situation where tension is small
enough to require steel spaced more than 4 feet. It is criticaI, however,
to check the adequacy of the hollow masonry between the reinforcing
elements to resist lateral loads.
The bulck of research information available so far IS ,6,7) on partially
grouted concrete masonry has been produced by investigations on fully
grouted concrete masonry where extent of grouting is varied from fully
grouting to partial grouting. These research studies have helped to
identify partially grouted concrete masonry in its right and ACI-530/ASCEs
cOde l !), is now specifying allowable flexural tension stresses for design
of partially grouted masonry. Yet, partially reinforced concrete masonry
remains to be researched and a better understanding will help to attain the
true potential of this material.
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In-depth studies on this material has only just begun (3,8,9,10,13) and
more research work needs to be done to achieve the full benefits of
partially reinforced concrete masonry.
Researchers have not yet addressed the proplem of developing
analytical methods for prediction of strength of partially reinforced
concrete masonry. Present state of knowledge indicates the need for more
research to enable partially reinforced concrete masonry to achieve its
full potential. Areas needing further study are material properties such as
axial compressive strength, modulus of elasticity, shear strength and
splitting tensile strength, and behavior of elements such as walls under
in-plane compression, in-plane shear and out-of-plane flexure, and walls
with returns. In the following two sections, a brief description of the
ongoing research program at Drexel University in the area of partially
reinforced masonry is presented .
EXPERIMENTAL PROGRAM
Methodo1ogy of Sma11 Sca1e Mode1ing
The main advantages of a physical model over an analytical model is
that it portrays a complate structure loaded to the collapse stage. The
prime motivation to conduct experiments on structures at reduced scales is
to reduce the cost, this reduction comes from: reduction of loading
equipement and associated restraint frames and reduction in cost of test
structure fabrication, preparation and disposal after loading. The major
limitations of using structural models in a design environment are those of
time and expense.
Any structural model must be designed, loaded and interpreted
according to a set of similitude requirements that relate the model to the
prototype structure. These similitude requirements are based upon the
theory of modeling, which can be derived from a dimensional analysis of
physical phenomena involved in the behavior of the structure .
A true or replica modeling (i.e. to produce models which obey all
similitude requirements) has to be used. These modeling techniques are
based on the choice of model materials and their methods of fabrication.
Therefore, the small-scale direct model must be satisfy the similitude
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requirements of not only the masonry units but also the mortar joints,
grout, and reinforcement. For replica modeling, the similitude requirements
can be classified in five distinct categories : loading, geometry, material
properties, prediction equation, and designo Based on the assumption of no
significant time dependent effects which influence the structural behavior,
the pertinent parameters that enter the modeling process in case of static
loading have been developed and are documented in Reference(4) .
The choice of a geometric scale factor for a specific type of model is
governed by the physical capability of the test facility. In addition,
physica l geometry limitations and factors such as scale effects dictate the
limit of the models length factor. The introduction of the larger scale is
principally motivated by a need to increase the facility's capabilities, to
acquire wider knowledge in modeling masonry structures and finally to
minimize the scale effects and increase the accuracy of the detailling of
the modelo In this research program, a scale factor of three was is used to
model the concrete block masonry components and elements.
Scope
The overall research program at Drexel University plan is presented in
the flow chart shown in Figure 1. The program is divided into the following
three phases:
Phase 1- This phase is aimed at modeling the components materiaIs of
concrete masonry (units, mortar, grout, and reinforcement) and conducting
structural performance tests to evaluate the material properties of
partially reinforced concrete masonry. The results of this phase will
enable specification of important material properties needed for structural
use such as flexural tensile strength, compressive strength, modulus of
elasticity, shear strength, and splitting tensile strength. Further,
development of analytical formulae will also be pursued to predict the
above properties in terms of prism strengths of grouted and hollow block
masonry.
The study on modeling material components, flexural tensile strength,
in-plane shear , and splitting tensile strength were completed and the
studies on compressive strength and modulus of elasticity are currently in
progresso
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Phase 2- This phase is aimed at conducting structural performance tests
to study the behavior of wall elements. The results will enable development '
of design recommendations for walls under axial compression, in-plane '
loading and out-of-plane flexure. The following are the test results that
will be measured, observed, and documented during the course of the
proposed testing program:
- Cracking load and crack pattern,
- Yielding and ultimate load,
- Failure Mode,
- Load deformation curve (hyteretic loop),
- Ductility and energy dissipation capability of these walls,
- Maximum masonry strain and maximum strain in reinforcement, and
- Control test data such as masonry prism strength, mortar cube
strength, block strength, grout strength, and steel properties.
Currently, the experimental program of wall behavior for in-plane
loading is ongoing(4l at Drexel University. A total of thirteen 1/3-scale
models of partially reinforced concrete block masonry shear walls, were
constructed and will tested under in-plane lateral loads, with and without
axial precompression. Eleven shear walls will be tested under monotonically
increasing loads, and two shear walls will be tested under fully reversed
cyclically loading.
Five types of parameters are investigated: axial precompression, block
strength, lateral load, and amount and distribution of vertical and
horizontal reinforcement. Table 1 presents and summarized the details of
the model shear wall specimens included in this programo
A typical shear wall panel is 3.1 feet in length (7 blocks) by 3.1
feet in height (13 courses) , representing a li 3-scale of 9.3 x 9.3 feet
full-scale walls. The panels were fabricated with a single wyth of 2 inches
wide blocks representing a 1/3-scale model units of the 6 in .
hollow concrete full-scale b1ocks, as shown in Figure 2.
ANALYTICAL STUDY
nominal
This phase is aimed at conducting analytical analysis using finite
element method to study and analyze the behavior characteristics of
partially reinforced masonry walls. Analytica1 formulas to predict the
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flexural and shear capacity of partially reinforced walls will be
developed. This study is essential for the development of design
methodology and building code requirements.
Availabe formulas for predicting flexural strength of reinforced
masonry assuming plain section remains plane and an equivalent rectangular
stress block(12) will be checked and modified if necessary, to reflect the
difference between reinforced and partially reinforced masonry. Two main
differences between fully reinforced walls and partially reinforced walls
are noted. First, not ali the cells are grouted in case of partially
reinforced walls which requires consideration of an effective width for the
compression block. Secondly, the vertical steel usually is uniformly
distributed in reinforced walls whereas it is not in partially reinforced
walls. This requires defferent treatment of the contribution of tension
steel in flexural strength calculations.
Shear strength of reinforced masonry walls depends on amount and
distribution of vertical and horizontal steel. Available formulas for
predicting nominal shear strength of reinforced walls will be checked and
modified, if necessary, to reflect the unique features of partially
reinforced, namely; the larger spacing of horizontal steel and the shear
capacity of hollow masonry between the reinforced parts of the wall.
ANTICIPATED RESULTS
Several significant indicators can be read from the load deformation
curves obtained experimentally such as initial and subsequent slopes,
cracking load, yield point of the wall, ultimate capacity and the ductility
ratio . These indicators will help determine the wall response, wall
parameters can be varied to improve wall response, and relative suitability
for various applications. Similarly, several significant indicators can be
read from the graphs of the hysteretic loops such as ultimate loads,
ductility ratios and stiffness degradation coefficients. These indicators
will help in determining wall response and resistance under low moderate
seismic loads.
Ultimate ductility factors are required to determine ultimate collapse
conditions using inelastic response spectra. Ultimate ductility is also
essential to apply more sophisticated failure analysis methods such as a
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finite element time-history computation. Past-yield envelopes will be
established from the hyteretic curves. The characteristics o f these
envelopes are essential to determine the applicability of standard analysis
methods such as the inelastic yield spectra and ene rgy balance technique to
partially reinforced concrete masonry walls.
The analysis of the above results is expected to produce design
recommendations for walls under in-plane compression, in-plane shear, out
of-plane flexure, and design information on walls with returns. It is also
expected that limits within which partially reinforced concrete masonry can
be used optimally, between hollow concrete block masonry at one extreme and
reinforced concrete masonry at the other, for various applications can be
identified during this phase of the study.
Finally, the outlined research program is expected to provide
comprehensive design information on partially reinforced concrete ma s onry
which will enable structural designers to utilize the full potential o f
this material. The final goal is envisaged to be the achievement of greater
economy in concrete masonry structures by encouraging the judicious use o f
hollow block, partially reinforced or reinforced concrete masonry to
satisfy optimally the requirements of any particular application.
BIBLIOGRAPHY
1) ACI/ASCE Standareds (ACI 530-88/ASCE 5-88), .. Building Code Requireme nts
for Masonry Structures" ACI and ASCE, 1988 .
2) Amrhein, J.E ... The Future of Masonry" , Proceedings of the 4th North
American Masonry Conference, Los angeles, California, August 1987.
3) Elnawawy, O.A. and Hamid,A . A. , .. Flexural Strength of Partially Grouted
Concrete Block' Masonry Using Small-Scale Model Wall Elements", Report
NO. STL-02/89, Departement of Civil and Architectural Engineering,
Drexel University, Pila, PA 19104.
4) Ghanem,G ... Behavioral Characteristics of partially Reinforced Masonry
Shear Walls Using 1/3-Scale Models" Ph.D. Thesis in-progress o
5) Hamid,A.A . and Drysdale, R.G.," Flexural Tensile Strength of Concrete
Block Masonry", Journal of Structural Engineering, VOL. 114, NO. 1,
January 1988.
6) Hamid,A . A., Abboud, B . E . , Farah, M.W., Hatem, M.K, and Harris, H. G.,
"Response of Reinforced Block Masonry Wal l s to Out-of Plane Static
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Loads", Report NO. 3.2(a), U.S.Japan Coordinated Program for Masonry
Building Research, Departement of Civil and Architectural. Eng., Drexel
University, Piladelphia, PA 19104, 1984.
7) Hamid,A.A., Assis,G.F., and Harris, H.G.,"Materia1 Mode1s for Grouted
Block Masonry", Report NO. 1.2(a)-1, U.S.Japan Coordinated Program for
Masonry Building Research, Departement of Civil and Architectural. Eng.,
Drexel University, Piladelphia, PA 19104, 1988.
8) Hamid,A.A.,Elnawawy,O.A., and Farah,M.,"Strengthening of Existing Hollow
Block Masonry Walls", Proceedings of the 8th International Brick/Block
Masonry Conference, Dublin, Ireland, 1988.
9) Hamid,A.A.and Chandrakeerthy, S .R., "Compressive Strength of Partially
Grouted Concrete Block Masonry Using Small-Scale Model Wall Elements"
10)Hosny,A.H. and Fouad,H.A.,"Behavior of Partially Grouted Concrete Block
Masonry Walls Under Concentrated Loads", M.SC.Thesis, Ain Shams
University, Cairo, Egypt,1988.
11) 'Schneider, R. R. and Dickey, W. L., "Reinforced Masonry Design", Prentice
Hall,Inc., Eaglewood Cliffs, New Jersey,1987.
12)Uniform Building Code,"Masonry Codes and Specifications", Commentary to
Chapter 24, The Masonry Society, Boulder, Colarado, Ju1y 1989.
13)Vekey,R.C. "The Effectiveness of Concrete Grout for Reinforced
Masonry", Proceedings Of the 8th International Brick/Block Masonry
Conference, Dublin, Ireland, 1988.
Table 1. TEST SPEClMENS OF SHEAR WALLS (PRASE 2)
r------------------r--Vertical Steel
Wall Designation
m
Reinf. P/lo(1)
----------3#5 0.12
3#5 0. 12
3#5 0.12
3#5 0.12
3#5 0.12
3#5 0. 12
3'# 5 + 3 # 4 0.21
3#5+3#4 0.21
3#5+3#4 0.21
3#5+3#4 0.21
----------- -4#4 0.12
4#4 0 .12
4#4+2#5 0.20
(1) Based on gross area
(2) Monotonic loading
(3) Cyclic loading
Horizontal Steel Masonry Strength Axial Stress
-l---_____ ~------------ --... I P1 P2 P3 Ph%(1) Reinf. f m
( psi) (psi)
- - -----t-----r--------3#5 0. 12 I 2350 100
3#3
3#3
3#3
3#3
3#4
3#5+3#4
3#5+3#4
3#4
3#4
0. 05
0. 05
0.05
0.05
0.09
0.21
0 .21
0.09
0.09
2350
2350
2350
2350
2350
2350
2350
2350
o 100
200
1400 I 100
100
100
100
100
100
---- ---t -t---------4#4 0.12
-------t 4#4 0. 12
4#4 0 .12
2350
2350
2350
100
100
100 __________ ---l ________________ -l-____________ _ _
Lateral Load
M (2) c(3)
M
M
M
M
M
M
M
M
M
M
M
c
c
W -...J m
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Experimental program (Phase I)
Materi a I Propert i es
I I I I Axial In-Plane Compression Joint Splitting Flexural and Modu lus of Shear
Tension Tension
Elasticity
Experimenta l program (Phase 2) Wa ll Behavior
Ax i al compression In-P lane Loading Out-of-Plane Flexure
I Analytical Study I I I
I I Analytica l formulae for Design recommendations for predicting the flexural and elements under in-plane, shear strength of part ially out-of-plane loading , and wal reinforced masonry walls w ith returns
Figure 1 - Flow Chert Df the Reseerch PleR
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4
+ 40 In.
~I ~ 1 I - - - I
I
I I I I I I I I I I I I I
I I I I I I I I J I I I I
I I I I I I I 1 .88
I I I I I I
I I I I I I I c
I I I I I I I I I I I I I
I I I I I I I I I I I I I
I -- -- - I lf)
-1 3 1 .... ~ ____ 37_ln_. ---~.l 3 r-H T
Figure 2 - Typicol Mosonry WolI Ponel