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Confined masonry

Confined masonrySDC

Lesson prepared by the Swiss Agency for Development and Cooperation for the trainings 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 in

afterwards to enclose (confine) the wall.

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Confined masonry

Concept

The walls are tied down to the foundation.

The ties work like a string around a parcel.

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Difference with RC framesConfined Masonry

• Walls first

• Concrete later

Reinforced Concrete Frame• Concrete first

• Walls later

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Confined masonry

Difference with RC framesConfined Masonry

Walls ensure rigidity

Reinforced Concrete FrameColumn – beam connections ensure rigidity

Straightforward transmission of efforts

Complicated transmission of efforts

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Difference with RC framesConfined Masonry

Walls are anchored to the frames through indentation.

(Additional re-bars will improve anchoring)

Reinforced Concrete FrameWalls are usually not anchored to the frames and will fall out.

Later anchoring of walls can be difficult

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Difference with RC framesConfined Masonry Reinforced Concrete Frame

All efforts go through the beam-column connection.

If they are badly done, they break.

Concrete goes into the indentation of

the brick wall

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Summing upConfined Masonry

It’s the walls that do the work.

That’s why they are called load bearing walls.

Reinforced Concrete Frame

It’s the columns and beams that do the work.

Walls are only infills.

From here on we’ll talk about load bearing walls made with the confined masonry system.

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Confined masonry

(the walls shown here are not confined)

Walls

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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 also

3. Carry the lateral pushes (horizontal loads)

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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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Earthquake resistance

• When we talk earthquake resistance, we are particularly interested in the capacity of a wall to withstand horizontal loads.

• Confined walls work also for horizontal loads.

Horizontal load

Horizontal load

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Horizontal loads

In an earthquake the ground moves sideward. That’s like pushing the roof. The walls must help to resist this horizontal load.

A serious earthquake (0.35 g) has the same effect on a house as tilting it at an angle of 20.5 degrees. That’s why walls must be strong.

20.5°

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Distribution of wallsThe earthquake resistance of a house is much better, if least two strong walls without openings (called shear walls) are placed in each direction.

These shear walls must be placed as far away from each other as possible.

Special case with walls in

the centre

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Distribution of walls

The shear walls must be placed symmetrically and as far away from each other as possible to avoid rotation.

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Shear wallsShear walls cannot have any openings and no slits for

pipes!

Shear walls should ideally be as long as they are high. Minimum length should be 2/3 of their height.

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Shear wallsShear walls should not be higher than 8 feet.

If the wall is higher, an intermediate bond beam must be introduced at 8 feet.

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Shear walls

The total surface on the ground of all shear walls (L x B) in the same direction must be between 1.0% and 1.4% of the roof surface (with 2 floors, add up each floor), according to the soil.

Example:

If you roof surface is 400 sqft, the total shear wall surface touching the 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 clayish sand

Intermediate

1.0%RockGravel

Hard

Min. wall surface on ground

DescriptionType of soil

© Blondet 2005

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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 other direction.

© Blondet 2005

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Confined masonry

Construction details

Foundation

Plinth beam

Walls

Columns

Bond beam

Roof slab

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Construction details Step by step 1. Column reinforcements

2. Foundations3. Plinth beam4. Walls5. Doors and windows6. Bond beam7. Roof slab8. Extensions

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Column reinforcements

General rules

1“

3“

Alternate position of hooks

Bend hooks at 45 degrees

All rebars must be covered by at least 1 inch of concrete

Always use steel bars with ridges (smooth rebars may only be used for stirrups)

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Column reinforcementsPrepare columns reinforcements:

Through plinth and bond beams place stirrups at 4”intervals.

Keep this interval for 2 ½feet in the lower and upper part of the column.

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FoundationsPlace column reinforcement on a 2” bed of concrete on the bottom of the foundation

2”

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FoundationsDon’t put big stones near to the columns in the foundations. The concrete would not be able to get correctly into the reinforcements.

Place bigger pipes into the foundations to prepare future passages for sewage and water pipes.

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Plinth beamPrepare the reinforcement of the plinth beam

Don’t forget: stirrup ends bent at 45 degrees!

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1 ½” 1 ½”

1 ½”

1 ½”Distance blocks (height 1 ½”)

Plinth beamPlace plinth beam reinforcement on foundation using ‘distance blocks’ or ‘spacers’.

The distance blocks must also be put on both sides to keep the distance to the formwork.

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Plinth beamConnect beams to columns using straight rebars and ‘pins’

Overlapping 50 cm

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Brick wallsUse only bricks with ‘frogs’ or vertical holes

Water the bricks before use

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Brick wallsUse the Flamish rather than the English bond. It has an equal number of joints in each layer, thus ensures more regular mortarjoints (i.e. all of the same thickness).

Flamish Bond English Bond

6 joints3 joints6 joints3 joints

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Brick wallsEnd walls in zig-zag towards columns

Avoid continuous joints

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Brick wallsAll joints (horizontal and vertical) must be between 3/8” and ½”.

Join walls carefully by alternating brick connection.

1st layer

2nd layer

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Brick wallsDon’t build higher than 4 feet per day!

Protect fresh walls in hot climates against drying out.

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Improved wall anchoring•To improve connection between columns and walls, use “stitches”made of rebar anchors (Ø 1/3” or 8mm).

•Place anchors every 1 ½ foot.

•Place the anchor bars in a bed of concrete the height of a brick layer.

•Anchors must end with a hook.

•Link anchors with small stirrups.Stitches

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Improved wall anchoring• Add horizontal through bands at sill and lintel level, even if you don’t have a window. These bands will secure the wall against being knocked out during an earthquake.

Seismic band at lintel level

Seismic band at sill level

Seismic band rebars Ø 3/8”,

h = brick course

Stitches

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Installations in walls•Don’t break the wall to place electrical or water pipes.

•Try to keep these pipes outside the wall.

•If they have to be in the wall, place them while mounting the wall, by interrupting the wall in zig-zag and filling in with concrete like you do with the columns.

© Blondet 2005© Blondet 2005

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Doors and windowsReinforcement around windows and doors

Add 2 rebars on both sides of any window or door

Add 2 rebars on both sides of any window or door

Rebars are bent on both ends and must be well connected to seismic bands. 8”

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Bond beamPrepare the reinforcement of the bond beam

Don’t forget: stirrup ends bent at 45 degrees!

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Bond beamConnect bond beams to columns with straight rebars and

pins.

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Roof slabFor spans up to 11 feet

Ø ½” @ 10”

Ø 3/8” @ 8”Ø 3/8” @ 8”

6”2”

Distance blocks 1 ½”

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Roof slabExample of reinforcement bar distribution for a span of 11 feet.

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Roof slab

• A roof pulled over the wall edge is another good method (protects the walls against rain).

• But you still have to place a bond beam over the walls as shown on the previous picture.

• Make sure that the slab rebars go correctly through the bond beam.

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Roof slabWater the slab and keep it wet for one week.

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Column – roof slab connection• If you want the column rebars

to stick out of your roof for future extensions, they must be:

• Long enough (at least 2 feet).

• Cast in concrete (to ensure correct anchoring).

• Otherwise it’s better to bend them into the bond beam before casting the slab!

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Roof terrace

If you want to make a roof terrace, the terrace walls must be anchored like all other walls.

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Roof terrace

Avoid putting your terrace wall on a cantilevered cornice, as it will fall on your head during an earthquake.

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Slope roof

You can use the column reinforcement bars sticking out of the roof slab to anchor the beams of a slope roof.

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Retaining walls

• If you build against a slope, you must make retaining walls at least 2.5 feet away from the building.

• Retaining walls keep away the pressure of the slope and of the water.

• If you make retaining walls in dry stone masonry or ‘situ’ they must have an inclination of 1 to 5.

The walls of your house must not be in direct contact with the ground!

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Retaining walls

• To lead away rain water (including all water that has rained on the slope above your house), make a drainage canal around the house.

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Gaps between buildingsTo avoid damage among two buildings, or an extension to the original building, a gap of 1/100th

of the height is compulsory.

This gap must be absolutely empty and clean (no mortar rests).

Example: Height of the building 12 feet (3.6m), the gap must be 1 ½” (36mm).

Take into account any future second floor!

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They will brake away during an earthquake, that’s all.

Gaps between buildingsClose gaps with bricks placed in front of them.

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Find the error

• No distance blocks or spacers in the roof slab.

• These rebarsexposed on the surface will rust quickly and become ineffective.

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Find the error

• Stirrups are not bent at a 45 degree angle.

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Find the error

• The overlapping length is insufficient

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Find the error

• Bond beam must be cast together with the roof slab.

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Find the error

• Bond beam must be cast together with the roof slab.

• Depending on the direction (i.e. in the direction of the shorter span), the horizontal reinforcement bars must end in a hook.

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Find the error

• Slab is cast against another house.

• There is no movement joint to protect your house against the hammering of your neighbour’s house during a quake.

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Find the error• These distance blocks or

spacers don’t have a wire to attach them to the rebars. They can move while pouring the concrete.

• You should prepare them as shown below, each block with a wire:

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Find the error• Concrete has not been vibrated

correctly to fill every gap.• You should use a vibrator when

casting concrete, or at least hammer against formwork to make sure that the concrete fills every hole.

• Don’t add more water to the concrete to make it more fluent. It will become weaker.

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Find the error• Wall is to high

without intermediate seismic band.

• An opportunity has been missed: for little more money a much stronger wall could have been made.

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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-Engineered Construction, 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) and Building Materials and Technology Promotion Council (BMTPC), www.nicee.org/EQTips/IITK_BMTPC.htm