by Ar. Rey S. Gabitan
Head, UAP Emergency Architects
Typhoon-Resilient Design Strategies:
The main strategy in protecting buildings from strong winds is to maintain the integrity of the building envelope, including roofs and windows, and to design the building to withstand the expected lateral and uplift forces. The following are some strategies that can be employed to make the construction system more wind-resilient and which could possibly withstand more than 250 kph winds:
Building Shape
o The most important single factor in determining the performance of buildings in typhoons is the building shape. Simple, compact, symmetrical shapes are best.
o The best shape to resist high winds is a square. The square plan is better than the rectangle since it allows high winds to go around them. The rectangle is better than the L-shaped plan. For rectangular shapes, the best layout is when the length is not more than three (3) times the width.
o If other shapes are desired, efforts should be made to strengthen the corners.
o If longer shapes are used, they must be designed to withstand the forces of the wind.
o For groups of buildings, a cluster arrangement can be followed in preference to row type.
Roof Form/Shape
o Use a hip roof. This is the strongest type with all sides of the roof sloped. Hip roofs offer much less wind resistance than gable roofs.
o For gable roofs, use a high pitched roof.
o Avoid a low-pitched roof. Roof pitch angle at least 25°. Experience and experiment have shown that the hip roof with the pitch in 25° to 40° range has best record of wind resistance.
Roof Overhangs
o Avoid large overhangs as high wind force build up under them.
o Roof eaves can be limited to 18 - 20 inches.
o If overhangs or canopies are desired, they should be braced by ties held to the main structure.
o Roof overhangs for verandah, patio, terraces and balconies should be designed as separate construction rather than extensions of the main roof of the building. They can break-away from the main roof structure without damaging the rest of the house.
Roofing Sheets
o if possible, use long-span roof sheets.
o If the sheeting is too thin or there are too few fittings, the nails or screws may tear through the sheet. If galvanized sheets are used, 24 gauge is recommended.
o Roofing sheets usually fail at ridges where capping comes off, at gable ends where sheetings rolls up sideways and at eaves where sheets lift up. At ridges, eaves and overhangs, provide fixings at every two (2) corrugations. At all other locations, provide fixings at every three (3) corrugation at maximum spacing.
o Use fittings with a broad washer or dome head (zinc nail). To use more fixings for each sheet, put in the laths at closer centres and nail closer together.
o Screws hold better than nails so fewer screws can be used. But the sheeting must be thick or they will tear through.
o Nails do not hold as well as screws. Use nails with wide heads and long enough to bend over below the lath. Galvanized coated nails are better than ordinary wire nails.
o Spacing for laths and number of fixings will vary with the gauge of sheeting used. Laths should be placed closer together for thin sheets to provide space for extra fixings. A guide to the number of fixings and spacing of laths is shown below.
26 450 mm – 600 mm
25 600 mm – 750 mm
24 600 for nails, 900 mm for screws
o The connections of cladding/sheeting to the truss need to be designed for the increased forces, especially at the corners and the roof edges considered as zones of higher local wind suctions. Failure at any one of these locations could lead progressively to complete roof failure. The following are recommended:
• Sheeted roofs:- A reduced spacing of bolts, ¾ of that admissible as per IS:800, recommended. For normal connections, J bolts may be used but for cyclone resistant connections U – bolts are recommended. Alternatively a strap may be used at least along edges to fix cladding with the purlins to avoid punching through the sheet. Properly connected M.S. flat can be used as reinforcing band in high suction zones. The corrugated sheeting should be properly overlapped (at least 2 1/2 corrugation) to prevent water from blowing under the seam. Spaces between the sheeting and the wall plate should be closed up to prevent the wind from getting under the sheeting and lifting it. This can be done by nailing a fascia board to the wall plate and rafters.
Roof Frame Construction
o Roof trusses and gables must be braced.
o Provide a more rigid fastening system for the roof frame like metal tie-down straps (typhoon straps) that tie the roof structure all the way down to the foundation
o If the rafters are not secure, the ridge can fall apart when strong wind passes over the roof. The ridge can be secured by using:
• Collar Ties - Timbers connecting the rafters. Nail them to the side of the rafters.
• Metal Straps over the top of the rafters.
o The connection of roof framing to the vertical load resisting elements i.e. wall or post, by providing properly designed anchor bolts and base plates is equally important for overall stability of the roof. The anchoring of roof framing to masonry wall should be accomplished through anchor bolts embedded in concrete cores. The weight of participating masonry at an angle of half horizontal to 1 vertical should be more that the total uplift at the support. In case of large forces, the anchoring bars can be taken down to the foundation level with a structural layout that could ensure the participation of filler and cross walls in resisting the
uplift.
o Adequate diagonal or knee bracing should be provided both at the rafter level and the eaves level in a pitched roof. The purlins should be properly anchored at the gable end. It is desirable that at least two bays, one at each end, be braced both in horizontal and vertical plane to provide adequate wind resistance. Where number of bays is more than 5, use additional bracing in every fourth bay.
o In order to reduce wind induced flutter/vibration of the roof in cyclonic regions, it is recommended that all members of the truss and the bracings be connected at the ends by at least two rivets/bolts or welds. Further the cross bracing members by welded/connected at the crossings to reduce vibrations.
Doors and Windows
o Avoid openings which cannot be securely closed during a typhoon.
o Doors and windows must be protected by covering and/or bracing. Hurricane shutters can protect windows from most wind-blown debris.
o Enhance the protection of openings by considering the addition of impact-resistant windows, doors, louvers, etc. An alternative is for glass panes be strengthened by pasting thin film or paper strips. This can introduce some damping in the glass panels and reduce their vibrations.
o Recourse may be taken to reduce the panel size to smaller dimensions.
o Since the failure of any door or window on the wind-ward side may lead to adverse uplift pressures under roof, the openings should have strong holdfasts as well as closing/locking arrangement.
Masonry Walls
o It is not uncommon for un-reinforced masonry to fail in severe cyclones. Walls braced by ring beams and columns have remained safe.
o All external walls or wall panels must be designed to resist the out of plane wind pressure adequately. The lateral load due to wind is finally resisted either by walls lying parallel to the lateral force direction (by shear wall action) or by RC frames to which the panel walls must be fixed using appropriate reinforcement such as seismic bands at window lintel level.
Flood-Resilient Design Strategies:
Location
o Avoid as much as possible any high-density development in low-lying areas (prone to flooding).
o Observe required easements along sides of waterways.
o House located on a river bed, close to running water, is very vulnerable to flooding. Not only the house, but also its contents are vulnerable to destruction due to heavy rains. Houses should not be built in such obviously vulnerable locations, or if they are, they should be designed to resist the hazards of their location.
o Houses must be located away from places subject to landslides where soil may move down a steep slope, debris flows where soil gravel and rocks may be washed rapidly down by heavy rainfall, and flashfloods.
o The house floor must be elevated above the surrounding area, with special consideration for possible area flooding, either by ground water, sea storm, or by tsunami.
Foundation
o Invariably a typhoon is accompanied by torrential rain and tidal surge (in coastal areas) resulting into flooding of the low lying areas. The tidal surge effect diminishes as it travels on shore, which can extend even upto 10 to 15 km. Flooding causes saturation of soil and thus significantly affects the safe bearing capacity of the soil. In flood prone areas, the safe bearing capacity should be taken as half of that for the dry ground. Also the likelihood of any scour due to receding tidal surge needs to be taken into account while deciding on the depth of foundation and the protection works around a raised ground used for locating cyclone shelters or other buildings.
Building on-stilts
o Where a building is constructed on stilts it is necessary that stilts are properly braced in both the principal directions. This will provide stability to the complete building under lateral loads. Knee bracings will be preferable to full diagonal bracing so as not to obstruct the passage of floating debris during storm surge.
Wall Openings/Window Openings
o Openings just below roof level be avoided for storm resiliency except that two small vents without shutter should be provided in opposite walls to prevent suffocation in case room gets filled with water and people may try to climb up on lofts or pegs.
Earthquake-Resilient Design Strategies:
Foundation
o Soil for a good foundation that can carry the weight of a house must be well drained so that it is dry and not waterlogged. Waterlogged soil can become liquefied in an earthquake—turn to a semi-liquid—so that structures sink into the ground.
Plan/Building Configuration
o Maintain the symmetry of a structure by distributing the seismic force resisting component evenly in all directionsis crucial in reducing the earthquake impact.
o Avoid soft storeys and asymmetrical floor plans, which can induce torsion.
Structural Framing
o The building needs a coherent structure. If the structure is coherent and strong, it protects the whole house.
- An integrated structural ring beam around top of walls connected to columns
- Triangular gable end walls must be structurally supported
o A wood frame structure mounted on a concrete frame/stub wall must be fully anchored together. A bracket made of noncorrodible metal, must be cast into the bearing structure to provide a structural connection for the whole wall through to the foundation. The wood column must be bolted to the bracket, and the bracket must be fully secured into the concrete.
o The joints of wooden roof trusses need to be bolted together and tied with metal straps to provide flexibility but not collapse under the forces of nature. Metal roof trusses must be welded together, welded to purlins, and welded to wall reinforcement for strength.
o All wall openings are to be tied to wall material.
o For reinforced concrete frame buildings with CHB walls, ties are to be cast into columns at 40cm spacing, and are to be a minimum of 8mm diameter.
o All houses are to have completely framed pitched triangular roof trusses. Roof trusses are to be placed over columns and tied to them.
o Most roof truss joints, and particularly central ones, are to be bolted, not nailed.
o Roof truss joints of 3 or more elements are to have a metal strap joining each roof component.
o Wood blocks are to be used for fixing purlins to roof trusses.
o Joints in roof structural wood are to be made with step joints, not with 45º cuts.
o Joints in roof structure are not to be made near the middle of a span.
o All wooden parts of a house structure are to be cross braced, stumps, walls, and roof.
o Wood roof structures are to be cross braced in both directions.
Wall Openings/Window Openings
o Openings in load bearing walls should not be within a distance of h/6 from inner corner for the purpose of providing lateral support to cross walls, where ‘h’ is the storey height up to eaves level.
References:
1. Handbook on Good Building Design and Construction in the Philippines prepared by the GTZ Office Manila, UNDP Regional Center in Bangkok and the Secretariat of the International strategy for Disaster Reduction, 2008
2. Cyclone Resistant Building Architecture prepared by Ankush Agarwal, Technical Officer (Hazard Vulnerability Reduction), GoI – UNDP, Disaster Risk Management Programme, March, 2007
3. Building Infrastructure Resilient to Disasters prepared by Asiri Karunawardena, Chaya Samarakkody and RavindraBalasooriya
4. Guidance Notes on Safer School Construction prepared by the Global Facility for Disaster Reduction and Recovery (ISDR, INEE, World Bank).
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