Construction of Cyclone Resistant Buildings (25-06-2020) by PROFESSOR APPLIED MECHANICS DEPARTMENT L D COLLEGE OF ENGINEERING AHMEDABAD – 380 015.
• Earthquake • Cyclone • Flood • Fire • Landslides • Liquefaction • Tsunami
Effect of Disasters
• Fundamental of Designing Cyclone Resistant Buildings Engineered and Non-Engineered • Planning aspects of a building in cyclone prone areas
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Design Loads • Dead Load • Live Load • Wind Load • Earthquake Load • Many other types of loads Lateral forces create discomfort to structures – Wind & Earthquake
Dead Load + Live Load +Wind or EQ Load DL + LL + WL or EQ Earthquake Force F = mass x acceleration = ma Wind Force F = Intensity of wind x Area of Obstruction Torsional Rigidity
Cyclone Resistant Design • No damage condition Earthquake Resistant Design • Damage allowed but no collapse
During an earthquake, lighter the building and the roof, the better is the performance of the house. Lighter roof would not induce as much load on the walls, and the walls would be able to transfer the loads easily during an earthquake. On the other hand, during a cyclone, heavier the roof, the better is the performance of the house. It would resist strong loads due to the wind pressure, hold itself and the house in place.
Cyclones are among the most destructive natural phenomena. The impact from cyclones extends over a wide area, with strong winds and heavy rains. However, the greatest damage to life and property is not from the wind, but from secondary events such as storm surges, flooding, landslides and tornadoes.
• Design wind speed V z = V b k 1 k 2 k 3 – V z = design wind speed at any height z in m/s – V b = basic wind speed in m/s – k 1 = probability factor (risk coefficient) Table 4 – k 2 = terrain, height and structure size factor – k 3 = topography factor p z = 0.6 V 2 Design wind pressure z
Wind Speed & Damage relation Category m/s mph Damage HC1 33 - 42 74 - 95 Minimal HC2 43 - 49 96 - 110 Moderate HC3 50 - 58 111 - 130 Extensive HC4 59 - 69 131 - 155 Extreme HC5 > 69 > 155 Catastrophic
Non-Engineered Building – Vulnerable Engineered Building • Well designed – Perform Satisfactory • Poorly designed - Vulnerable
HOW HIGH WINDS DAMAGE BUILDINGS ? • The walls of the building are pulled apart by winds moving swiftly around and over the building • This creates suction on the walls and roof, effectively causing the equivalent of an explosion
Structural configuration is the single most important factor for better performance of buildings subjected to cyclones. Recommendations to improve performance of non-engineered building with minimum construction cost : 1. Symmetrical building for balanced distribution of forces in the structure. 2. Restrict height of buildings to one/two storeys 3. Ensure that lightweight floors and roofs are securely fastened to the walls.
4. Lightweight roofs should have a hipped shape (sloping in four directions) rather than a gable shape (sloping in two directions). Lightweight roofs should be not less steep than 20 degrees (generally, the steeper the better up to about 30 degrees) to improve wind resistance. 5. Lightweight roofs should have minimum overhangs at the eaves. In fact is would be better to have no overhangs and to introduce a parapet. The need to shade windows and doors from sun and rain may be met by separate canopies.
• Timber Houses • Steel Frames • Masonry Houses • Reinforced Concrete Frames
• Light weight structure
Steel Portal Frame Building with Light Roofing System
• These are usually safe in cyclones. There are countless examples where the loss of roofs has triggered the total destruction of un-reinforced masonry walls.
• The design of low rise reinforced concrete frames is usually controlled by the seismic hazard. But care still needs to be taken to ensure that the concrete frames can accommodate the wind forces. There have been a few isolated examples where, ignoring this, has led to disaster.
Components failure • Roof Sheeting • Roof Tiles • Rafters • Connections • Walls • Foundation • Windows & Doors
Roofs that aren’t anchored properly can get uplifted during a cyclone/windstorm. Light weight temporary shelters, which are not held properly, may blow away. CGI sheets/tiles could blow away if not anchored well.
The pressure of the wind builds up on the ceiling inside the house to such an extent that the clay tiles are blown away.
THE ROOF CORRUGATED GALVANIZE SHEETS ARE GAUGED BY NUMBERS. THE HIGHER THE NUMBER THE THINNER THE MATERIAL.EXAMPLE 24 GAUGE GALVANIZE IS SUPERIOR TO 28 GAUGE. HOW DOES ROOF SHEETING FAIL IN CYCLONES?
THE ROOF ROOFING MATERIALS GALVANIZED SHEETS IF GALVANIZED SHEETS ARE USED 24 GAUGE IS RECOMMENDED IF YOU MUST USE 26 GAUGE WHICH IS THINNER, THIS IS HOW TO HOLD YOUR SHEETING TO THE ROOF STRUCTURE. At ridges, eaves and overhangs - fixings every two (2) corrugation. All other locations, fixings every three (3) corrugation. Maximum spacing.
THE WALLS THE WALLS MUST BE SECURELY TIED TO THE FOUNDATION TO PREVENT THE WIND FORCES LIFTING UP THE ENTIRE BUILDING OR BLOWING IT OVER.
Foundation (too small for light weight building) pulled completely out of ground
FOUNDA TIONS Box action give very high integrity
FOUNDA TIONS THE REINFORCEMENT IN THE CONCRETE BLOCK WALL TIED TO THAT OF THE FOUNDATION.
Apart from roofs, the elements requiring the most attention are windows and doors. Sadly, these are often neglected even when buildings are formally designed by professionals. Glass windows and doors are, of course, very vulnerable to flying objects. There are only two solutions: • use impact-resistant glass (very expensive) • cover the glass with storm shutters Attention must also be paid to secure doors with strong bolts or braces and to fix door and window frames firmly to the walls.
WINDOWS
Location The location of the building is important. We often have little choice in the matter, perhaps because of financial constraints. It is as well, therefore, to recognize when a building is being located in a more vulnerable area. The rational response would be to build a stronger-than- normal house. Such vulnerable areas include open-ended valleys, which act as funnels for the wind, and exposed hill crests. Both conditions lead to acceleration of wind speeds with the corresponding increase in damage potential.
BASIC DESIGN CONSIDERATIONS • When choosing a site for your home…
BASIC DESIGN CONSIDERATIONS • When choosing a site for your home…
If your lot is on a slope don’t place the house The wind and water can dislodge the house. like this unless it is properly anchored. A house is best built on a flat firm site Cut and fill is a common means of leveling a house site. provided it is well drained . Avoid building on the fill. Foundation should be on solid ground. This house is safer , cut into the side of the hill.
Shape is the most important single factor in determining the performance of buildings in cyclones. Simple, compact, symmetrical shapes are best. The square plan is better than the rectangle. The rectangle is better than the L-shaped plan. This is not to say that all buildings must be square. Even more important than plan shape is roof geometry. For lightweight roofs it is best that they be of hipped shape (sloping in all four directions, usually), steeply pitched (25 to 30 degrees), with little or no overhangs at the eaves (with parapets if possible) and with ridge ventilators where these are practicable.
DESIGN OF THE HOUSE
RELATIVE PLANNING
References • A manual on Safe Construction Techniques prepared as part of the OAS/USAID Caribbean Disaster Mitigation Project (CDMP) • CYCLONE RESISTANT BUILDING ARCHITECTURE by GoI – UNDP, Disaster Risk management Programme • Wind Storms, Damage and Guidelines for Mitigative Measures, Document No. :: IITK-GSDMA-Wind03-V3.0 Final Report- .
Thank you
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