constructing with concrete block confined masonry · pdf filethus confined masonry is a shear...
TRANSCRIPT
Presentation Outline
Introduction to confined masonry
General concepts
Comparison with unreinforced masonry and RC frame with infill
Review of confined masonry house designed by Build Change
Why Confined Masonry?
Poor performance of unreinforced masonry and non-ductile reinforced concrete (RC) frame
construction caused unacceptably high human and economic losses in past earthquakes
This prompted a need for developing and/or promoting alternative building technologies
Confined masonry is an opportunity for improved seismic performance both for unreinforced
masonry and reinforced concrete frame construction in low and medium rise buildings.
The goal is to achieve enhanced seismic performance using technologies which require
similar level of construction skills and are economically viable
Development of Confined Masonry
Evolved though an informal process based on its satisfactory
performance in past earthquakes
The first reported use in the reconstruction after the 1908
Messina, Italy earthquake (M 7.2) - death toll 70,000
Practiced in Chile and Columbia since 1930’s and in Mexico
since 1940’s
Currently practiced in several countries/regions with high
seismic risk, including Latin America, Mediterranean Europe,
North Africa, Middle East, Southeast Asia, and the Far East
Development of Confined Masonry
Large scale testing of confined masonry walls have been
conducted in Mexico (UNAM), Peru (PUCP), India (IIT Kanpur),
and other universities
Tests have shown acceptable performance of properly
constructed confined masonry walls
2009 study by Riahi, Elwood & Alcocer (published by ASCE)
reviewed 102 tests and develop a ductility curve that
confirmed that the confined masonry walls are a ductile
system
Standards for Confined Masonry
Provisions included in several
building codes
Mexico, Chile, Peru, Eurocode,
Algeria, China
EERI and IAEE published guidelines
for design and construction of 1-2
story CM buildings
Engineering design guidelines are
currently being developed
CM guidelines for India are currently
being developed
Performance of Confined Masonry
Confined masonry buildings have been subjected to
large earthquakes, most recently in Chile and China
For example, a six-storey confined masonry
building remained undamaged in the August 2007
Pisco, Peru earthquake (M 8.0) while many other
masonry buildings experienced severe damage or
collapse
In the February 2010 Chile earthquake (M 8.8),
there were 521 deaths. Only 24 of those were due
to damage to confined masonry buildings
Performance of Confined Masonry
Performance of confined masonry buildings in the
2008 Great Wenchuan, China earthquake (M 8.0):
In areas of Intensity 7 (31-50% expected damage),
57.6% of CM buildings remained almost intact, 24.2%
lightly damaged (18% actual damage)
In areas of Intensity 8 (51-70% expected damage),
24.3% of CM buildings remained almost intact, 43.6%
lightly damaged (32% actual damage)
In areas of Intensities 10 & 11, there was extensive
damage to CM buildings but few reported collapses
Figures from the China institute of Engineering Mechanics
Key Components of a Confined Masonry Building
Masonry walls made either of burnt clay brick or
concrete block units
Tie-columns: vertical RC confining elements similar
to columns in reinforced concrete frame construction
Tie-columns occur at wall intersections, ends of shear
walls, and wall openings
Tie-columns are typically tied to the walls with
staggered joints (“toothing”) or dowels in the masonry
mortar joints
Tie-beams: horizontal RC confining elements similar
to beams in reinforced concrete frame construction
Tie-beams occur at each floor level and wall openings
Confined Masonry vs. RC Frame with Infill
Both use concrete frames and unreinforced masonry walls. However, the
primary difference is the construction sequence:
Confined Masonry = The masonry wall is built first, then the concrete beams and
columns are placed
RC Frame with Infill = The concrete frame is built first, then the masonry infill is
placed
Confined Masonry vs. RC Frame with Infill
Because the concrete is poured on top of
the masonry wall and is tied to the columns
at the ends of the wall, a bond develops
between the concrete and masonry
This bond transfers the lateral shear loads
from the concrete to the masonry, since no
gaps develop between the two elements
when loaded
Thus confined masonry is a shear wall
system rather than a frame system
Confined Masonry vs. RC Frame with Infill
Because the concrete elements are not the primary
support system, the load on them is much lower &
the ductility requirements are much less
Thus the size of the columns and the amount of
reinforcing steel required is much less than a
comparable RC frame
Meanwhile, the size of the masonry wall is similar
to infill, primarily because the wall thickness is
often dictated by other factors common to both
systems (e.g. out-of-plane forces, insulation,
standard unit sizes)
Confined Masonry vs. Unreinforced Masonry
Both use unreinforced masonry units that are built the same way. The difference
is what happens when the walls are loaded.
When cracks develop in an unreinforced wall (either in-plane or out-of-plane),
the wall quickly loses its ability to resist loads and falls apart
When cracks develop in a confined wall, the confining frame keeps the wall
together, allowing it to continue to resist loads
Confined Masonry vs. Unreinforced Masonry
Analogous to a stack of books on a table:
A stack of books that are not tied together falls apart when the table is shaken.
But when the books are tied together by a string, the stack stays together unless
the string breaks or the pages rip apart
Minimum Criteria for Building a Confined Masonry Building
Geological fault or Ruptured Area
Area susceptible to landslide
Steep slope > 20%
Filled Area
River bank ad water logged area
Maximum
number of
stories Two
Span of wall Largest span of the room should not be more than 3.5 metres
Height of
wall Maximum height of wall should not be more than 2.5 metres
ProportionThe house shall be planned in square, rectangular. External length to
width ratio of the building should not exceed 3
General The foundation trench shall be of uniform width. The foundation bed
shall be on the same level troughout the foundation in the flat area
Depth The depth of footing should not be less than 900mm depending on
the soil sub strata
WidthThe width of footing should not be less than 900 mm in medium soil
condition.
A building shall not be constructed if the site is:
Site Selection1
Shape of house2
Foundation3
Minimum Criteria for Building a Confined Masonry Building
General Provide a reinforced concrete band at plinth level, as shown in detail drawings. The plinth
height should not be less than 300mm from existing ground level.
Depth of beam Depth of plinth beam shall be greater than or equal to 150mm
Width of beam Width of plinth beam shall be greater than or equal to 200mm
ReinforcementMain reinforcement should be 4-10mm dia. Bars. Use 7mm diameter rings at 150mm center to
center. Hook length should be 50mm. Bars shall have a clear cover of 25mm concrete
General Masonry should be laid staggered in order to avoid continuous vertical joints. The wall should
have toothing at the wall-tiecolumn interface to facilitate good connection
Hollow Block unitsThe hollow blocks used shall be of good quality and shall adhere to the Nepal Standards of
block production
Mortar joints and
ratio
Mortar joints should be between 20mm to 10mm in thickness. The mortar shall be 1: 5
(cement:sand) or richer
Span of wallThe length of wall shall be less than or equal to 3.5 metres. The length of wall between
vertical reinforcing members (tie column or vertical grouted reinforcement) shall be less than
or equal to 1.5 metres.
Width The thickness of wall shall be greater than or equal to 150mm
Location Openings are to be located away from inside corners by a clear distance of at least 600mm.
Only one opening is allowed in a pannel
SizeArea of the opening shall not exceed 10% of the area of confined wall panel including the
confining tie columns and beams.
Distance The horizontal distance between two openings shall not be less than 600mm
Lintel level The lintel level should be kept same for all doors and windows
Plinth4
Walls5
Openings6
Minimum Criteria for Building a Confined Masonry Building
LocationTie columns should be placed at each corner and intersection of the walls as
well as on either side of the door.
Size The size of the tie-column shall be equal to the width of the wall
Spacing The spacing of tie-column shall be less than or equal to 3.5 metres.
ReinforcementThe minimum reinforcement to be used is 4-12mm dia. Longitudnal bars and
7mm dia. Stirrups at 150mm c/c
LocationVertical reinforcement in the blocks is placed on either side of the window
Reinforcement Minimum 12mm dia. Bar centred in the Hollow block cell and grouted with
cement mortar
General Horizontal bands should be provided throughout the entire wall
Sill band A continuous sill band shall be provided throughout the entire wall at the
bottom level of the openings. The minimum depth of the band shall be 75mm
Lintel band A continuous lintel band shall be provided throughout the entire wall at the
top level of the openings. The minimum depth of the band shall be 75mm
Floor/Roof bandA continuous floor/roof band shall be provided throughout the entire wall at
the top of the walls at floor/roof level. The minimum depth of the band shall
be 200mm. The minimum width of the band shall be 200mm.
Reinforcement
Main reinforcement should be 4-10mm dia. Bars. Use 7mm diameter rings at
150mm. Hook length should be 50mm. Bars shall have a clear cover of 25mm
concrete
Vertical Tie
columns and
reinforcements
7
Horizontal
band9
Vertical
reinforcement
grouted in
blocks
8
Light roof Use light roof comprising wooden or steel truss covered with CGI sheets
ConnectionAll members of the timber truss or joints should be properly connected as
shown in detail drawings
Cross-tieTrusses should be properly cross tiesd with wooden braces as shown in detail
drawings
TimberWell seasoned Khote salla wood without knots should be used for roofing,
timber treatment such as use of coal tar or any other preservative can
prevent timber from being decayed and attacked by insects
MortarCement sand mortar should not be leaner than 1:5 (cement:sand) for masonry
and 1:6 for plaster
ConcreteConcrete mix for seismic bands should not be leaner than 1:1.5:3
(cement:sand:aggregates)
Reinforcement High strength deformed bars - Fe415
12 Roof Span The center to center span of the truss shall not be greater than 3.5m
RaftersThe size of the rafters shall be greater than 50mm x 100mm and shall be
provided at a spacing of 1138mm or lesser
Purlins
The size of the purlins shall be greater than 50mm x 50mm and shall be
provided at a spacing of 350mm center to center at the porch and at a
spacing of 500mm center to center at the roof
Roof10
Materials11
Minimum Criteria for Building a Confined Masonry Building
Blocks available in the market and their corresponding strengths
District Gross compressive strength (Mpa)
Sindupalchowk 1.97
Kavrepalanchowk 1.66
Gwarko (Kathmandu) 1.75
United Cements (Kathmandu) 5.01
Prefab (Bhaktapur) 7.20
The table shows the gross compressive strength of random block samples collected
from different districts.
Description of the building
Building type: ResidentialNumber of storeys: 2Ground floor storey height: 2.475mFirst floor storey height: 2.475 mWall thickness: 150 mm (without plaster)
Length of the building: 9.66 mWidth of the building: 3.5 mHeight of the building: 6.043m
Hollow Concrete Block (HCB) size: 400 mm x 200mm x 150mm
Design Methodology
- Dead loads were taken as per IS 875 Part I
- Imposed loads were taken as per IS 875 Part II
- The lateral load demand was calculated based on IS 1893:2002 Part I
• Lump mass calculation
• Seismic load calculation based on seismic coefficient method: Z = 0.36; I = 1; R = 2.5
• Calculation of center of mass and center of stiffness
- Check for torsion by calculation of eccentricity due to difference in center of mass and center of stiffness: should be less than 30% of thelength for corresponding direction
- In-Plane Pier analysis was done by manual calculation
• Calculation of Net stress = bending stress + overturning stress + Vertical stress
• Calculation of length of tension zone
• Calculation of rebar for tension
- Out of Plane Pier analysis
• Check height/thickness ratio of the wall
• Stress check of beams at floor and lintel levels:
o Calculation of bending moment due to lateral loads
o Calculation of required depth and reinforcement for the above moment
Design Methodology
- The following stress checks were made based on IITK GSDMA and IS 1905: 1987
• Shear Stress Check:
o Calculation of shear stress due to lateral load demand
o Calculation of permissible shear stress as per IS 1905: 1987
• Compressive stress check
o Calculation of compressive stress based on dead load and live load
o Calculation of permissible compressive stress as per IS 1905: 1987
- To match the demand with the capacity of the masonry, addition of confining reinforced concrete ties were provided as deemed necessary to meet the above stress demands
Quantity and Cost Estimation
Skilled Unskilled Stone Bricks
Hollow
Block Cement Sand
Aggregate
Wood
Ply WoodLocal
Wood for
form work Glass Bar
Binding
Wire Soil Water
CGI
Sheet
Unit Md Md Cum. Nos. Nos. Bags Cum. Cum. Cum. Sq.m Cum. Sq.m kg Kg Cum. lit. Bundle
Upto Plinth Level 39.00 74.00 21.12 0.00 0.00 72.00 9.20 1.14 0.00 2.97 0.07 0.00 274.37 2.74 13.09 2227.93 0.00
Super Structure 220.00 263.00 6.78 0.00 1330.00 160.00 15.14 11.81 2.11 38.31 0.57 3.28 1487.42 14.87 0.00 3597.77 0.00
Roof 36.00 12.00 0.00 0.00 0.00 0.00 0.00 0.00 1.67 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5.00
Total 295.00 349.00 27.90 0.00 1330.00 232.00 24.34 12.95 3.78 41.28 0.64 3.28 1761.79 17.62 13.09 5825.69 5.00
From the District Rate of Sindhupalchowk the Total Building Cost
15% overhead
Total 1,651,796.75
Manpower Material
Nepal Housing Reconstruction Programme
SUMMARY OF QUANTITY
MODEL HOLLOW CONCRETEBLOCK
1,436,345.00
215,451.75
Structural Detail of toothing and mortar grouting
The toothing length is one third of the length of the block.
The window opening is grouted with cement mortar with a
single rebar.