Post on 06-May-2015
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transcript
Prof. A. R. SanthakumarVisiting Professor
IIT Madras
Design Principle
• Steel reinforcement can be placed to enhance load resistance
• It also makes the wall ductile
• Steel may be placed in grouted cavities in hallow blocks are inserted in specially made holes in bricks
• It can also be provided in specially made bonds
Basis of Design
• Masonry is designed for a specific action
• It is also common to have reinforcement for one action and unreinforced for another action. Example for shear and compression
• Design is carried out using the same principle as RCC
• Equilibrium and strain compatibility should be considered
Type of reinforcements
• Main reinforcements for compression, bending and shear
• Secondary reinforcement for shrinkage and temperature
• Reinforcement should either be grouted or enclosed in lean concrete
• AS 3600 and AS 3700
Recommendation of codes
• There are no recommendations in IS
• Australian code gives detailed recommendations
• AS 3700
• Limit State method is followed with suitable reduction factors for shear and to avoid brittle buckling failure
Monadnock Building, Chicago, 1891, Burnham and Root, architects
Concept of Shear Wall System
Hanalei Hotel, San Diego
Types of Masonry Construction
2Thickness 15” min
Thickness
(a) Hollow concrete block prisms
Concrete Masonry Compression-test prisms
High-Rise Concept in Block Masonry
Holiday Inn Motel, a Round Bearing-wall Multi-storey Structure
2 d
ays
at 1
6m =
30m
5 days at 8m=40m
Plan
N
4m 8m 3m 2.25m 2.5m 2.25m 1m 3m 4m
30m East wall elevation
Diaphragm level
7.5m
4m
1.5m
1.5m
One Storey Commercial Building
BEARING WALLS
BEARING WALLS
BEARING WALLS
TRANSVERSE SHEAR WALLS
Some Examples of Two-directional Bearing/shear wall layouts
BEARING WALLS
BEARING WALLS
TRANSVERSE SHEAR WALLS
Examples of multi-directional bearing/shear wall layouts
t
h
Conventional load bearing wall
Continuous support
t
Deep wall beams
t
h
Deep wall beam
Columns or rootings
WALL SPAN
300 or 450 PLAN
100
mm
100
mm
400
WALL SPAN
WA
LL H
EIG
HT
P
anel
Wal
l R
einf
orci
ng S
teel
Required Embedment
400 or 600 Dia.
ELEVATION
100
PA
NE
L W
ALL
R
einf
orci
ng S
teel
W
ALL
HE
IGH
T
SECTION
Brick pier-and-panel garden walls
WALL PLAN
(A) BRICK WALL
DE
PT
H
1.3m
LENGTH 6m
TO
FR
OS
T
DE
PT
H
MIN
IMU
M
MA
XIM
UM
HE
IGH
T 1
.6m
100
WALL SECTION t
Serpentine walls
WALL SECTION
GRADE IN BLOCK OR CAST-IN-PLACE CONC. BELOW GRADE
300
450
150
TO
FR
OS
T D
EP
TH
MIN
IMU
M
MA
XIM
UM
HE
IGH
T 1
5 x
t
t
WALL SECTION
SHORT RADIUS AT FREE END
1m RAD
3m
RA
D
3m R
AD
PILASTER AT FREE END
WALL PLAN
1m
DE
PT
H
LENGTH 6m
(B) CONCRETE BLOCK WALL
Lateral load design of masonry walls and their behavior
There are three types’ failure modes that define seismic behavior of structural masonry walls when subjected to in-plane seismic loads. The mechanism depends on the geometry of the wall (height/ width ratio) and quality of materials, and the type of load transfer.
Sliding shear failure
• In the situation of low vertical load and poor quality mortar, seismic loads frequently cause shearing of wall causing sliding of the upper part of the wall at one of the horizontal mortar joints.
Shear failure
• It is a typical mode of failure of masonry wall subjected to seismic loads, and it takes place where the principal tensile stresses, developed in the wall under a combination of vertical and horizontal loads exceeds the tensile strength of masonry.
flexural mode of failure (flexural compression).
With the improved shear resistance and high moment/shear ratio, crushing of compresses zones at the ends of the wall usually take place,
1. There are several different types of structural systems employed to resist the lateral forces which carry the loads from the various floor levels to the foundation.
2. The vertical structural elements used to transfer lateral forces are 1. Shear walls 2.braced frames 3.moment resisting space frames, 4.combination of above.
3. The horizontal structural elements which distribute these forces to the vertical resisting elements are the floor and roof diaphragms.
Floor and roof diaphragms
Buildings Resist Horizontal Earthquake Forces
1. Horizontal Parts:Roof & Floor Structures Diaphragms
2. Vertical PartsSpan horizontal elements Shearwalls
House Element Resist Horizontal Forces
Two-story building
Arrows on left of figure are the seismic forces based on the weight of the building.
Arrow at the roof: represents the seismic force from both the roof weight and one-half of the weight of the walls between the second floor and roofline (F1).
Arrow at the second floor: represents the seismic force of half the second floor weight and one-half of the weight of the first and second story walls (F2).
Arrow at the first floor: represents the force at the first floor that is similarly calculated (F3).
Arrow at the foundation level: Sum of all these forces that must be transmitted safely into the ground. This is why the foundation and cripple wall are so important (FSum=F1+F2+F3)
No Shear Wall at Garage
House Elements Resist Gravity
•What part of a building resists the horizontal earthquake forces?
•Both horizontal and vertical parts of the building resist horizontal earthquake forces.
•Horizontal parts: roof and the floor structures. These parts are called diaphragms.
•Vertical parts that span between the horizontal elements. These walls are called shear walls.
Seismic Force Distribution
The diaphragms are classified into three groups of relative flexibilities:
rigid, flexible, and semi rigid.
It is assumed to tribute the horizontal forces to the vertical resisting elements in direct proportion to the relative rigidities of those elements.
This premise stems from the fact that under a symmetrical loading, the rigid diaphragm, which in it self does not deform appreciably will cause each vertical element to deflect the same amount.
Rigid diaphragms are capable of transferring lateral and torsional forces to the walls.
Rigid diaphragm
It may be likened to a series spans extending between very rigid supports, (i.e. vertical resisting elements).
It is assumed here that the relative stiffness of these non yielding supports is very great compared to that of the diaphragm, which therefore deflects as a beam.
This beam, having no appreciable continuity across the supports, thus develops no negative moment over them which would affect the distribution of lateral load
Flexible diaphragm:
These exhibits significant deflection under load, and also have sufficient stiffness to distribute a portion of their load to the vertical elements in direct proportion to the rigidities of those elements.
Semi rigid diaphragm
Horizontal forces at any floor or roof level may be transferred to the foundation through the strength and rigidity of the side walls, called as shear walls.
The design strength of shear walls is often governed by flexure.
However, in low walls, the governing design criterion may be shear, Masonry shear walls can be described not only in terms of types of masonry used , but also as load- bearing , non load bearing , reinforced or unreinforced, solid or perforated rectangular or flanged and cantilevered or coupled.
Vertical stability elements
Moment Shear
Deflection of walls due to bending and shear deformations
P
P
Ph P
h
c=m+v
mm
c AG
Ph
IE
Ph 2.1
3
3
L
h
L
h
tE
p
m
c 343
Rigidity of the pier =Rc =
c1
L
h
L
hp
tEm
343=
Δf
PPh/2
Ph/2
PP
Moment
Shear
Deflection of walls due to bending and shear deformations
Rigidity of the pier = Rf =
L
h
L
hp
tEm
33
Effect of aspect ratio on deflection due to shear
Aspect ratioh/L
Percentage deflection due to shear
Cantilever wall Fixed end wall
0.25 92 98
1 43 75
2 16 43
4 5 16
8 1 4.5
1For squat walls (h/L < 0.25), rigidities based on shear deformations are reasonably accurate.2For (0.25<h/L<4) intermediate cantilever walls both deflections components should be include ‘d’ in the calculation of relative rigidities.For high (h/L) the effect of shear deformation is very small and rigidity based on flexural stiffness is reasonably accurate.
SEISMIC RESISTANCE OF RAT-TRAP BOND WALL AND
FILLER SLAB SYSTEM
FIGURE 2 TYPICAL CROSS SECTION
The Building System
Rat – Trap Bond Masonry
Typical Corner Joint
Method of Construction
Advantages
Validation
Experimental setup
EXPERIMENTAL SET-UP
225230
150230
920230
150
230
660
150
230
# #
#
##
#
#
#
#
#
One layer brick on edge
15 MB 300
15 MB 300 15 MB 300
16
2
59
7
4 83
10
1.2
6
5.97
10
8
4.3
WALLUNDERTEST
ELEVATION END VIEW
1. STRAILS
2. DEFORMAIONS
3. LOADS
4. FAILURE PATTERN
Data for 1 cubic metre
Earthquake Resistance
Test Procedure
Load vs Moment
Load Application
Specimen Details
S.No.
Name Load (N) Moment (N mm)
Failure Between
1 450M1 650 292500 Brick and Concrete surface at the bottom level
2 450M2 18431 310500 Brick and Concrete surface at the bottom level
3 340M3 1440 597600 II and III level Bricks
4 340M4 6143 601750 II and III level Bricks
5 450M5 4733 647400 I and II level Bricks
6 450M6 7973 9337500 I and II level Bricks
7 340M7 90000 0 Vertical cracks on all four sides
Practical Case
Practical Case
Safe Moment(Nmm)
Axial Load (N)
Safe Span of the Cantilever (L1)(mm)
1000 200000 574.9891
2000 325000 759.8557
3000 450000 911.9664
4000 575000 1044.287
5000 685000 1149.326
6000 775000 1229.175
7000 885000 1320.751
8000 940000 1364.414
Safe Cantilever Spans for Limiting Tension in Brickwork