Lesson prepared by the Swiss Agency for Development and Cooperation for the trainings at the Housing Reconstruction Centres Ballakot and Battagram 13 August 2006 1 Definition Confined 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. 13 August 2006 2
Concept The walls are tied down to the foundation. The ties work like a string around a parcel. 13 August 2006 3 Difference with RC frames Confined Masonry Walls first Concrete later Reinforced Concrete Frame Concrete first Walls later 13 August 2006 4
Difference with RC frames Confined Masonry Walls ensure rigidity Reinforced Concrete Frame Column beam connections ensure rigidity Straightforward transmission of efforts Complicated transmission of efforts 13 August 2006 5 Difference with RC frames Confined Masonry Walls are anchored to the frames through indentation. Reinforced Concrete Frame Walls are usually not anchored to the frames and will fall out. (Additional re-bars will improve anchoring) Later anchoring of walls can be difficult 13 August 2006 6
Difference with RC frames Confined 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 13 August 2006 7 Summing up Confined Masonry It s the walls that do the work. Reinforced Concrete Frame It s the columns and beams that do the work. That s why they are called load bearing walls. Walls are only infills. From here on we ll talk about load bearing walls made with the confined masonry system. 13 August 2006 8
Walls (the walls shown here are not confined) 13 August 2006 9 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) + 13 August 2006 10
Load bearing walls They can be made out of: Bricks Stone Cement blocks + cement or cement/lime mortar + reinforced concrete confinements 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) 13 August 2006 11 Load bearing walls All load bearing walls must be confined. or All confined walls are load bearing. 13 August 2006 12
Earthquake resistance When we talk earthquake resistance, we are particularly interested in the capacity of a wall to withstand horizontal loads. Horizontal load Horizontal load Confined walls work also for horizontal loads. 13 August 2006 13 Horizontal loads 20.5 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. 13 August 2006 14
Distribution of walls The 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 13 August 2006 15 Distribution of walls The shear walls must be placed symmetrically and as far away from each other as possible to avoid rotation. 13 August 2006 16
Shear walls Shear 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. 13 August 2006 17 Shear walls Shear walls should not be higher than 8 feet. If the wall is higher, an intermediate bond beam must be introduced at 8 feet. 13 August 2006 18
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: Type of soil Description Min. wall surface on ground 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) Blondet 2005 Hard Intermediate Soft of loose Rock Gravel Hard clayish sand Loose sand soft clay 1.0% 1.2% 1.4% 13 August 2006 19 Shear walls Example: Total surface of shear walls on the ground = 1.0 to 1.4% of roof surface in this direction. Blondet 2005 The same calculation must be repeated for the other direction. 13 August 2006 20
Construction details Roof slab Bond beam Columns Walls Plinth beam Foundation 13 August 2006 21 Construction details Step by step 1. Column reinforcements 2. Foundations 3. Plinth beam 4. Walls 5. Doors and windows 6. Bond beam 7. Roof slab 8. Extensions 13 August 2006 22
Column reinforcements General rules Bend hooks at 45 degrees Alternate position of hooks 3 Always use steel bars with ridges (smooth rebars may only be used for stirrups) All rebars must be covered by at least 1 inch of concrete 1 13 August 2006 23 Column reinforcements Prepare columns reinforcements: Through plinth and bond beams place stirrups at 4 intervals. Keep this interval for 2 ½ feet in the lower and upper part of the column. 13 August 2006 24
Foundations Place column reinforcement on a 2 bed of concrete on the bottom of the foundation 2 13 August 2006 25 Foundations Don 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. 13 August 2006 26
Plinth beam Prepare the reinforcement of the plinth beam Don t forget: stirrup ends bent at 45 degrees! 13 August 2006 27 Plinth beam Place 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. 1 ½ 1 ½ 1 ½ Distance blocks (height 1 ½ ) 1 ½ 13 August 2006 28
Plinth beam Connect beams to columns using straight rebars and pins Overlapping 50 cm 13 August 2006 29 Brick walls Use only bricks with frogs or vertical holes Water the bricks before use 13 August 2006 30
Brick walls Use the Flamish rather than the English bond. It has an equal number of joints in each layer, thus ensures more regular mortar joints (i.e. all of the same thickness). 6 joints 3 joints 6 joints 3 joints Flamish Bond English Bond 13 August 2006 31 Brick walls End walls in zig-zag towards columns Avoid continuous joints 13 August 2006 32
Brick walls All joints (horizontal and vertical) must be between 3/8 and ½. Join walls carefully by alternating brick connection. 1 st layer 2 nd layer 13 August 2006 33 Brick walls Don t build higher than 4 feet per day! Protect fresh walls in hot climates against drying out. 13 August 2006 34
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 13 August 2006 35 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 sill level Seismic band rebars Ø 3/8, h = brick course Seismic band at lintel level Stitches 13 August 2006 36
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 13 August 2006 37 Doors and windows Reinforcement around windows and doors 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 13 August 2006 38
Bond beam Prepare the reinforcement of the bond beam Don t forget: stirrup ends bent at 45 degrees! 13 August 2006 39 Bond beam Connect bond beams to columns with straight rebars and pins. 13 August 2006 40
Roof slab For spans up to 11 feet Ø 3/8 @ 8 Ø 3/8 @ 8 2 6 ؽ @ 10 Distance blocks 1 ½ 13 August 2006 41 Roof slab Example of reinforcement bar distribution for a span of 11 feet. 13 August 2006 42
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. 13 August 2006 43 Roof slab Water the slab and keep it wet for one week. 13 August 2006 44
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! 13 August 2006 45 Roof terrace If you want to make a roof terrace, the terrace walls must be anchored like all other walls. 13 August 2006 46
Roof terrace Avoid putting your terrace wall on a cantilevered cornice, as it will fall on your head during an earthquake. 13 August 2006 47 Slope roof You can use the column reinforcement bars sticking out of the roof slab to anchor the beams of a slope roof. 13 August 2006 48
Retaining walls The walls of your house must not be in direct contact with the ground! 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. 13 August 2006 49 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. 13 August 2006 50
Gaps between buildings To avoid damage among two buildings, or an extension to the original building, a gap of 1/100 th 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! 13 August 2006 51 Gaps between buildings Close gaps with bricks placed in front of them. They will brake away during an earthquake, that s all. 13 August 2006 52
Find the error No distance blocks or spacers in the roof slab. These rebars exposed on the surface will rust quickly and become ineffective. 13 August 2006 53 Find the error Stirrups are not bent at a 45 degree angle. 13 August 2006 54
Find the error The overlapping length is insufficient 13 August 2006 55 Find the error Bond beam must be cast together with the roof slab. 13 August 2006 56
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. 13 August 2006 57 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. 13 August 2006 58
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: 13 August 2006 59 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. 13 August 2006 60
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. 13 August 2006 61 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 sismo resistente en viviendas e mamposteria, Asociation Colombiana de Ingenieria Sismica, 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 13 August 2006 62