Technology 2: Building in Seismic Areas

    Introduction
    Outline of Buildings
    Foundation Material
    Concrete Floor Construction
    Brickwork
    Timber Floor Construction
    Timber Framed Wall Construction
    Roof Structure
    Conclusion
    Bibliography
 


Introduction

Although earthquakes can cause much destruction to buildings and structures, there are some ways in which we can minimise the after effects of the earthquake. The majority of the population would believe that building a rigid structure would stop the force of the earthquake knocking down the structure. The major way in which the effects can be minimised is by creating structures that are flexible and ones that can move around under the immense force.
In this assignment on building in seismic areas I will attempt to outline other possible ways of building structures in earthquake areas so that they can resist the force of it. 

Outline of buildings

The best recommended shape or outline for a building in seismic areas is a square, rectangle or round building. The reasons for this is if one has a building with many extra shapes, the force exerted on them will just break them off from the rest of the structure thus causing more damage.

Secondly buildings should have as much symmetry as possible to balance the stress of the building when in an earthquake. All major openings such as verandahs and garages should be positioned equally around the building to minimise the rotation and thus damage to the building when under the force of an earthquake. 


Foundation Material

The differences when building in a seismic area than a normal site is mainly that in the more severe areas, extra sand or other materials be brought in so that the footing can have a suitable base to transfer its weight to. In clay areas pile and beam or pier and beam may be used so that the slab sits a few centimetres off the ground level thus allowing for some movement during an earthquake. In a timber or steel floored house, steel piers or timber piles would be driven into the foundation to allow extra movement. 


Concrete floor construction

In seismic zones the concrete floor slab is not much different from that used on ordinary sites, but the major difference is that the slab would be reinforced with extra mesh. A mesh with a diameter of 21mm should be used for a slab on ground job. When building a slab on a pier or pile on beam the slab will have little reinforcement as it sits on the beams. When building like this all the structure is tied together using concrete columns and beams with the reinforcing tied together to provide a frame.

The floor slab should be slightly thicker than normal and have more thickening under load - bearing walls so the slab will not crack while moving. 


Brickwork

Before building the brickwork the openings in it have to be designed. A general rule for allowing for openings is not to place them all near each other (eg. Not in the same wall or near the corner or intersection of the building). Also when placing openings, they shall be not less than 500mm apart. When placing openings on external walls, the openings shall be no less than 500mm from the other wall. Where an opening is placed internally the distance from another wall shall be not less than the thickness of the brick times two. (eg. If brick is 110mm thick, then the distance from another wall will be 110mm x 2 = 220mm).

Where openings are placed, lintels will also be needed to reinforce the area above the opening from collapsing. When a brick or other masonry wall is the wall member, a reinforced concrete lintel should be used. The table below shows what reinforcing is needed for a concrete lintel.
 

Span of opening         Depth of lintel     Size of bars         Bearing in wall each end

not over 450 mm         150 mm                     nil                             150 mm
450 mm to 1.25 m       150 mm            12 mm diameter               230 mm
1.25 m to 1.80 m         230 mm            20 mm diameter               300 mm

For Spans over 1.80 m a qualified engineer should be consulted.

The bond of mortar between the bricks is usually where the failure occurs and where cracks start to appear. The best mortar to use in seismic areas is one of a 1:5 or 1:6 (Cement : Sand) ratio but with no lime added as the lime can cause the mortar to become more fragile and soft. This ratio gives a good strong mix and plasticiser can be added to make the mix more workable. It is also recommended that for the mortar to live up to its strength it must be used within 25 minutes.

Fourthly the wall structure should be tied down to the footing so the building is one frame. When the footing is laid by the granolithic contractor, the surface of the footing should be roughened up a bit so that the mortar can get a good grip for the first course of bricks. Also a good idea is to build the corners of the building up 0.50 m then brick in the wall in between the corners. This practice gives the bricks a chance to settle under the weight of the other bricks and form a strong bond.

The reinforcement of the brickwork is a major issue when looking at tying down the structure. When using bricks that have holes in them rods should be placed down these holes then the remaining void be filled with concrete. This process is usually done when using hollow concrete blocks.

When clay bricks are used and the cavity of the wall is at least 50 mm, the cavity is filled at the corners with concrete then reinforcing bars are placed vertically. After this a concrete pillar has been formed. In both cases the reinforcing bars are tied down to the footing and also into the roof frame to connect the whole building as one.

The horizontal reinforcing should be placed no more than 450 mm apart and should be ;
  2 bars each 6 mm diameter, one being set in 50 mm in from the   face of the wall,  OR

  expanded metal extending over the whole wall except for 25 mm   cover at each wall face.

Finally the roof should be fixed to the wall by the use of bolts and threaded rods which come up from the footings up the wall and tie down the roof structure. The tying together of the roof and the wall is important as if there is an earthquake the wall is supported not only at the bottom at the footing, but also at the top by the roof. 


Timber floor construction

The floor system, in this case timber should have at least 150 mm of clear space from ground level to the bearer. If this is not observed the floor will rot and then be more susceptible to failure when the earthquake strikes. Every joist should also have a sufficient bearing of 75 mm long, so that it can be flexible under seismic conditions. The joists should be fixed to the bearers with two nails which are 80 mm or longer and these nails must go into each member at least 35 mm. Alternatively the joists can be fixed to the bearers by an angle iron or joist hanger provided that the joist is not temporarily sitting on the bearer and is not tied. Struts should also be placed at every 2.40 metres in length using 25mm thick timber. 


Timber framed wall construction

 In earthquake areas the bottom plate or Bottom sill must be bolted to the Concrete slab or otherwise fixed to the timber floor frame at 1.50 metre intervals using 100 ´ 50 mm Pine, jarrah or the like. The top plate (Also 100 ´
50 mm) should be bolted to the wall plate at 1.5 m intervals or nailed at 600 mm intervals, but the joint between the two should not be above one another. One in every three studs should also be strapped to both the top plate and the bottom plate to ensure that in the case of an earthquake, the studs do not dislodge and thus cause the roof to fall on the building.

Another rule of thumb to use in timber wall building in earthquake areas is to vertically and horizontally brace the wall structure so the building is secure and will not topple in the case of an earthquake. Where 2 or more walls meet at a corner, vertical bracing should be applied. The braces should be one piece of unjointed timber @ 100 ´ 25 mm and be at a slope of an angle of 30o through to 60o. Where two braces are used on the same wall one should come from the top of the corner and carry through to the bottom sill while the other should start at the top plate and finish at the bottom of the corner. Where the two overlap there will be a recess joint in each to accommodate the members. The bracing will also be recessed into the studs.

To accommodate the horizontal braces, notches must be cut into the wall plate and should be between 1.20 m and 1.80 m from the corner. The brace is fixed to the wall plate by bolts.

When fixing the timber frame to the slab or timber floor structure, Bolts with a recommended diameter of 12 mm be used. Types of bolts that can be used are Tigerbolts/Dynabolts or they can be fixed to the slab by the use of explosive power tools.

Although these recommendations are for timber wall frames, the same principles apply for the newer type of construction in, steel or metal wall frames. As long as both methods use bracing, the general recommendation for walls in earthquake zones has been applied. 


Roof Structure

In Seismic zones, lightweight roofs are recommended as they need lees roofing timber and the roof structure can therefore be more flexible. The lightweight roofs have a covering which would consist of either ;

   Þ Aluminium

   Þ Galvanised Steel or ;

   Þ Plastic Poly - Carbonate

In severe seismic areas, the roof members must be braced longitudinally, if the roof has a span of more than 4.3 m, and the roof is double-pitched. These braces will have a piece of timber that is unjointed, be fixed at an angle of 300 - 600, and be securely fixed by bolts.

Ideally the truss system would be the best for buildings in seismic areas as they are lightweight and their design which is based on bracing and triangulation, is one which is involved in all parts of earthquake construction.

The construction of the roof structure should comply with industry practice as well as the National timber Framing Code. Apart from the above recommendation roofs built in non-seismic areas would be sufficient in earthquake zones. 


Conclusion

In conclusion the general basis of earthquake construction is to firstly tie down the building, and secondly to try and connect the separate structures like the floor, the walls, and the roof as one so different components donít just break away and fail when subjected to the load of the earthquake. Thirdly the shape of the building should be kept as simple as possible (Square or rectangular) and should be placed on a suitable foundation.

In addition to what has been recommended, a precaution should be taken for termites as if they weaken the structure all of the principles and recommendations explained above will be invalid. Also the builder should check materials for any possible defects, so they can be replaced before construction.

Finally building with earthquake designs is a matter of the architect or financier of the project deciding on the risk of an earthquake happening and the amount of finance to be spent. A map of Australia and one of Western Australia are on the next pages and show were the higher risk seismic areas occur in Australia. From these maps the higher risk areas are around Northam (East of Perth) and just on the Eastern Seaboard. 


Bibliography

Þ Building Construction Vol I

Þ AS 1170.4 - Minimum design loads on structures

Þ AS 2121 - SAA Earthquake code

Þ Small buildings in earthquake areas, AF Daldy 



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