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Confined masonry
Confined masonrySDC
Lesson prepared by the Swiss Agency for Development and Cooperation for thetrainings at the Housing Reconstruction Centres Ballakot and Battagram
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Confined masonry
DefinitionConfined Masonry is a construction system
where the walls are built first, and the
columns and beams are poured inafterwards to enclose (confine) the wall.
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Confined masonry
Concept
The walls are tieddown to the foundation.
The ties work like a string around a parcel.
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Confined masonry
Difference with RC frames
Confined Masonry
Walls first
Concrete later
Reinforced Concrete Frame
Concrete first
Walls later
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Confined masonry
Difference with RC frames
Confined Masonry
Walls ensure rigidity
Reinforced Concrete Frame
Column beam connectionsensure rigidity
Straightforwardtransmission of efforts
Complicatedtransmission of efforts
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Confined masonry
Difference with RC frames
Confined Masonry
Walls are anchored to theframes through indentation.
(Additional re-bars willimprove anchoring)
Reinforced Concrete Frame
Walls are usually notanchored to the frames and
will fall out.
Later anchoring of walls canbe difficult
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Confined masonry
Difference with RC frames
Confined Masonry Reinforced Concrete Frame
All efforts go through thebeam-column connection.
If they are badly done,they break.
Concrete goes intothe indentation of
the brick wall
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Confined masonry
Summing up
Confined Masonry
Its the walls that do the work.
Thats why they are calledload bearing walls.
Reinforced Concrete Frame
Its the columns and beams that
do the work.
Walls are only infills.
From here on well talk about
load bearing walls made withthe confined masonry system.
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Confined masonry
(the walls shown here are not confined)
Walls
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Confined masonry
Walls: why do we need them
A building needs walls for
1. Protection against climate, view, intruders, etc.
+When we talk about load bearing walls, they will
2. Carry the roof and/or upper floor (vertical loads)
+
When we talk about earthquakes, they will also3. Carry the lateral pushes (horizontal loads)
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Confined masonry
Load bearing walls
They can be made out of:
Bricks Stone
Cement blocks
Most importantly, they must be strong. Water the bricks or blocks before use
Fill all joints (horizontal and vertical with mortar) Use the right mixture for the mortar (1 bucket of
cement for 5 buckets of clean and rough sand)
+ cement or cement/lime mortar
+ reinforced concrete confinements
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Confined masonry
Load bearing walls
All load bearing walls must be confined.
or
All confined walls are load bearing.
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Confined masonry
Earthquake resistance
When we talk earthquake resistance, we areparticularly interested in the capacity of a wall towithstand horizontal loads.
Confined walls work also for horizontal loads.
Horizontal load
Horizontal load
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Confined masonry
Horizontal loads
In an earthquake the groundmoves sideward. Thats likepushing the roof. The walls musthelp to resist this horizontal load.
A serious earthquake (0.35 g) hasthe same effect on a house astilting it at an angle of 20.5degrees. Thats why walls mustbe strong.
20.5
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Confined masonry
Distribution of wallsThe earthquake resistance of ahouse is much better, if least two
strong walls without openings(called shear walls) are placed ineach direction.
These shear wallsmust beplaced as far away from each
other as possible.
Special casewith walls inthe centre
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Confined masonry
Distribution of walls
The shear wallsmust be placed
symmetrically and as far awayfrom each other as possible toavoid rotation.
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Confined masonry
Shear walls
Shear walls cannot have any openings and no slits forpipes!
Shear walls should ideally be as long as they are high.
Minimum length should be 2/3 of their height.
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Confined masonry
Shear walls
Shear walls should not be higher than 8 feet.
If the wall is higher, an intermediate bond beam must beintroduced at 8 feet.
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Confined masonry
Shear walls
The total surface on the ground of
all shear walls (L x B) in the samedirection must be between 1.0%and 1.4% of the roof surface (with2 floors, add up each floor),according to the soil.
Example:
If you roof surface is 400 sqft, thetotal shear wall surface touchingthe ground must be 4 to 5.6 sqft(i.e. all shear walls added up)
1.4%Loose sandsoft clay
Soft of loose
1.2%Hard clayishsand
Intermediate
1.0%RockGravel
Hard
Min. wallsurface onground
DescriptionType of soil
Blondet 2005
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Confined masonry
Shear wallsExample: Total surface of shear walls on the ground =
1.0 to 1.4% of roof surface in this direction.
The same calculation must be repeated for the otherdirection.
Blondet 2005
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Confined masonry
Construction details
Foundation
Plinth beam
Walls
Columns
Bond beam
Roof slab
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Confined masonry
Construction details
Step by step 1. Column reinforcements
2. Foundations
3. Plinth beam4. Walls
5. Doors and windows
6. Bond beam
7. Roof slab8. Extensions
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Confined masonry
Column reinforcements
General rules
1
3
Alternateposition of hooks
Bend hooks at45 degrees
All rebars must becovered by at least1 inch of concrete
Always use steel barswith ridges (smoothrebars may only beused for stirrups)
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Confined masonry
ColumnreinforcementsPrepare columns
reinforcements:
Through plinth and bondbeams place stirrups at 4intervals.
Keep this interval for 2
feet in the lower and upperpart of the column.
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Confined masonry
Plinth beam
Prepare the reinforcement of the plinth beam
Dont forget:stirrup ends bentat 45 degrees!
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Confined masonry
1 1
1
1 Distance blocks(height 1 )
Plinth beamPlace plinth beam reinforcementon foundation using distanceblocks or spacers.
The distance blocks must also beput on both sides to keep thedistance to the formwork.
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Confined masonry
Plinth beamConnect beams to columns usingstraight rebars and pins
Overlapping 50 cm
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Confined masonry
Brick wallsUse only bricks with frogs orvertical holes
Water the bricks before use
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Confined masonry
Brick wallsUse the Flamish rather than the English bond. It has an equalnumber of joints in each layer, thus ensures more regular mortar
joints (i.e. all of the same thickness).
Flamish Bond English Bond
6 joints
3 joints
6 joints
3 joints
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Confined masonry
Brick wallsEnd walls in zig-zag towardscolumns
Avoid continuous joints
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Confined masonry
Brick wallsAll joints (horizontal and vertical)must be between 3/8 and .
Join walls carefully by alternatingbrick connection.
1st layer
2nd layer
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Confined masonry
Brick wallsDont build higher than 4 feet perday!
Protect fresh walls in hot climatesagainst drying out.
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Confined masonry
Improved wall anchoringTo improve connection betweencolumns and walls, use stitches
made of rebar anchors ( 1/3 or8mm).
Place anchors every 1 foot.
Place the anchor bars in a bed ofconcrete the height of a brick layer.
Anchors must end with a hook.
Link anchors with small stirrups.Stitches
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Confined masonry
Improved wall anchoring Add horizontal through bands at sill and lintel level, even if youdont have a window. These bands will secure the wall against beingknocked out during an earthquake.
Seismic bandat lintel level
Seismic bandat sill level
Seismic bandrebars 3/8,
h = brick course
Stitches
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Confined masonry
Bond beam
Prepare the reinforcement of the bond beam
Dont forget:stirrup ends bentat 45 degrees!
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Confined masonry
Bond beam
Connect bond beams to columns with straight rebars andpins.
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Confined masonry
Roof slab
For spans up to 11 feet
@ 10
3/8 @ 8 3/8 @ 8
6
2
Distanceblocks 1
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Confined masonry
Roof slab
Example of reinforcement bar distribution for a span of 11 feet.
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Confined masonry
Roof slab
A roof pulled over the wall
edge is another good method(protects the walls againstrain).
But you still have to place abond beam over the walls asshown on the previouspicture.
Make sure that the slabrebars go correctly throughthe bond beam.
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Confined masonry
Roof slab
Water the slab and keep it wet for one week.
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Confined masonry
Column roof slab connection
If you want the column rebarsto stick out of your roof for
future extensions, they mustbe:
Long enough (at least 2feet).
Cast in concrete (toensure correctanchoring).
Otherwise its better to bendthem into the bond beambefore casting the slab!
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Confined masonry
Roof terrace
If you want to make a roof terrace, theterrace walls must be anchored like allother walls.
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Confined masonry
Roof terrace
Avoid putting your terrace wall on acantilevered cornice, as it will fall onyour head during an earthquake.
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Confined masonry
Slope roof
You can use the column reinforcement bars sticking out of the roofslab to anchor the beams of a slope roof.
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Confined masonry
Retaining walls
If you build against aslope, you must makeretaining walls at least2.5 feet away from thebuilding.
Retaining walls keepaway the pressure of theslope and of the water.
If you make retainingwalls in dry stonemasonry or situ theymust have an inclinationof 1 to 5.
The walls of your house must not be in direct contact with the ground!
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Confined masonry
Retaining walls
To lead away rain water(including all water thathas rained on the slopeabove your house), makea drainage canal aroundthe house.
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Confined masonry
Gaps between buildingsTo avoid damage among twobuildings, or an extension to the
original building, a gap of 1/100thof the height is compulsory.
This gap must be absolutelyempty and clean (no mortarrests).
Example: Height of the building
12 feet (3.6m), the gap must be1 (36mm).
Take into account any futuresecond floor!
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Confined masonry
They will brake away during anearthquake, thats all.
Gaps between buildingsClose gaps with bricks placed infront of them.
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Confined masonry
Find the error
No distance blocks
or spacers in theroof slab.
These rebarsexposed on thesurface will rustquickly andbecome
ineffective.
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Confined masonry
Find the error
Stirrups are notbent at a 45
degree angle.
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Confined masonry
Find the error
The overlapping
length isinsufficient
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Confined masonry
Find the error
Bond beam mustbe cast together
with the roof slab.
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Confined masonry
Find the error
Bond beam must
be cast togetherwith the roof slab.
Depending on thedirection (i.e. in thedirection of theshorter span), thehorizontal
reinforcement barsmust end in ahook.
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Confined masonry
Find the error
Slab is castagainst another
house. There is nomovement joint toprotect your houseagainst thehammering of yourneighbours houseduring a quake.
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Find the error
These distance blocks orspacers dont have a wire toattach them to the rebars. Theycan move while pouring theconcrete.
You should prepare them asshown below, each block with awire:
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Confined masonry
Find the error
Concrete has not been vibratedcorrectly to fill every gap.
You should use a vibrator when
casting concrete, or at leasthammer against formwork tomake sure that the concrete fillsevery hole.
Dont add more water to theconcrete to make it more fluent.It will become weaker.
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Find the error
Wall is to highwithoutintermediateseismic band.
An opportunityhas been missed:for little moremoney a muchstronger wallcould have been
made.
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Confined masonry
Further reading Blondet M. (2005), Construction and Maintenance of Masonry Houses, Pontifica
Universidad Catolica del Peru, Lima,www.eeri.org/lfe/clearinghouse/kashmir/resources.html
City (year n/a), Confined Masonry Construction, City University London,
www.staff.city.ac.uk/earthquakes/MasonryBrick/ConfinedBrickMasonryP.htm Earthquake Hazard Centre, Newsletter, Victoria University of Wellington, New
Zealand, www.vuw.ac.nz/architecture/research/ehc/
AIS 2001), Manual de construccion, evaluacion y rehabilitacion sismoresistente en viviendas e mamposteria, Asociation Colombiana de IngenieriaSismica, www.desenredando.org/public/libros/2001/cersrvm/index.html
NICEE (2004), Guidelines for Earthquake Resistant Non-EngineeredConstruction, Chapter IV: Buildings in Fired-Brick and other Masonry Units,National Information Centre of Earthquake Engineering, IIT Kanput,www.iitk.ac.in/nicee/iaee_english/Chapter4.pdf
IITK-BMTPC, Earthquake tips 1-24, Indian Institute of Technology Kanpur (IITK) andB ildi M t i l d T h l P ti C il (BMTPC)