Wind and snow load

Wind and snow loads on roofs

Althought Malaysia is not undergoing any seasonal climate. But the effect of snow effect is significant in contributing to the building defect. For us to be in malaysia, the following information can be refered for additional information.

When snowfall comes, the wind will make the snow loading to be different.

Depend on wind velocity, the snow will move in different orientation. For example:

Light wind velocity (0.3 – 1.5m/s) – drifting movement

High wind velocity (1.6 – 3.3m/s) – horizontal movement

The wind velocity also affect the deposition of snow. Like erroded sand and soil, the snow will be deposited at the low wind velocity area.

Higher winds often blow the snow away leaving the roofs (depends on types of roof) almost bare and deposit large amount of snow on the ground or against structures and may result in the formation of snow ramps.

Snowdrift formations are highly dependent on the wind velocity patterns across the roof, which in turn are functions of wind direction and duration, types of roof, and the environment near the building.

In areas where the wind is being bloe harder and accelerating, the existing now will be blow and causing removal of snow. Figure below give simple explation on how snow moves arround the building.

* shaded area is the area which exposed to increasing wind load

When snowfall happens, the wind flow field is disturbed for buildings in the wake of other buildings. In the near wake the mean flow and turbulence intensity are affected by separating shear layers and vortices shed from the upstream building edges.

When snow falls in the presence of wind, the snow may accumulate on buildings in areas such as valleys, the lee side of peaked or arched roofs, lower roofs sheltered by higher roofs or behind obstructions on roofs.

Redistribution of snow may also occur in periods without snowfall. Snow may be

transferred onto the roof from its surroundings, snow may be redistributed across the

roof or snow may blow off the roof.

Snow transport can be divided into suspended transport and unsuspended transport.

Unsuspended transport takes place in a layer 1–25 cm above the surface. Saltation and creep are the two kinds of unsuspended transport. Saltation refers to the way in which the drifting snow particles appear to jump along the surface, and is the dominant mode of transport for particles larger than 0.1 mm. Creep describes particles that roll along the surface.

Suspended transport or suspension is defined as snow transported by turbulent wind at a higher level (approximately 1 – 100 m) than unsuspended transport.

Normally, a single snow event will not does any harm to the roof critically. Repeated snow events that do not have time to melt can accumulate and surpass the roof design’s live load.

Roof Failure

A partial or complete roof collapse can occur for several reasons. Most of the causes are

preventable if measures are taken beforehand to address roof design, snow removal and

ensure that roof drains and gutters are clear and flowing freely. A few of the reasons that roof failures occur are:

1. Incorrect roof live load design. This can either be from allowing from inaccuracy in building design, from unpridicted future climate changes, and reduction in the live load during the design phase.

The reason the reduction is allowed is based on the belief that the wind typically blows during a snow event and a reduced amount of snow will accumulate on the roof. Collapses have occurred when the actual snow load was above the reduced snow load.

2. Problems with the installation of the roof steel. Most bar joist roof frames are welded, while some of the primary steel beams have bolted flanges. If the welds are not done correctly or the bolts are not torque to specification, failure of the roof structure can occur.

3. Roof drains and/or downspouts become blocked or frozen and melting snow or rain can not adequately drain from the roof.

4. Over time, additional dead load (weight) is added to the roof, which will reduce the available live load or roof design. The increased dead load can come in the form of adding HVAC equipment, new roof covering or hanging conveyors from the roof steel.

5. Imbalance of snow load on roof. Imbalance of snow load can only occurs naturally through the redistribution of snow which entirely causes by the blowing of wind around the building.

From the picture, the steel bar is deforms causes by the snow loading.

Hurricane Damage: Katrina Storm Case Study

Malaysia is free from the natural disaster like hurricane. But for future study, the history can become a guide for the future building design. The application of wind tunnel contributre sighnificant contribution to the design. Katrina is one of the major storm event that sacirfies so much life and does more damage than any other hurricane events occurs in United States. Below is some of the information that relates to building defect caused by the Katrina storm event.

A structural engineer assessing a distinction of damages caused by high winds before the storm surge swept through the site. Pictures below shows some of the typical views along the areas with the most severe damage.

Typical views of devastation along the Mississippi coast caused by

Hurricane Katrina. Determining the extent of wind damages prior to the storm

surge sweeping through these sites

From the Katrina storm event, slow-rising flood waters from the storm surge reached a height of 30-inches above the first floor, causing extensive water damage to interior floor and wall coverings. 

High winds damaged the roof covering, caused wind-blown debris to penetrate the roof sheathing, and removed roof and wall sheathing panels from the windward gable end.

Wind-driven rain entered breaches in the roof covering and gable wall openings causing water damages to the ceilings and partial second story.

The forces from wind caused the majority of structural damages, while the surge caused extensive water damages to the interior.

The storm surge reached a height of about 10-feet for certain areas, and caused the destruction of the exterior wall which facing the beach.

Series of photographs that documented damages to the house around the victimised area. Note that minimal damage was present to the roof covering, while severe damages were on the lower part of the structure.

Earthquake Effect to Building Defect

Earthquake effect on building

For reference, malaysia is located outside from the volcanic area and earthquake zone which ease our structure engineer in design process. The earthquake effect today cannot be neglected as some part around malaysia is feeling the seismic activity. As reference, some of the well known country that often exposed to the seismic activity is Taiwan and Japan. But Indonesia, our neighbour is also exposed to the seismic activity and malaysia can feel the aftershock even the earthquake happens thousands of kilometers away.

During an earthquake the foundation of the building moves with the ground and the superstructure shakes and vibrates in an irregular manner due to inertia of their masses (weights).

As the ground moves, the building will tends to move in the opposite direction as if being pushed by an imaginary force.

The building structure attempts to resist this force and in doing so absorbs the energy released. Weaker construction will provide less resistance and energy absorption and thus result in damage to the structure and in certain cases failure.

Main causes of failure of a building include, but not limited to, heavy roof, excessive opening in walls, deficient foundations, poor interlocking of walls and poor site conditions.

Below is some of the cases of damage due to earthquake activity towards building defect around the world

Damage due to soil liquefaction in apartment buildings in Niigata, Japan, Niigata. (Earthquake magnitude 7.5), June 16, 1964. Liquefaction is a phenomenon that happens when the strength of saturated soil is reduced by the earthquake’s sudden movements and as a result  the soil behaves like a heavy liquid

Soft story damage in a building during Loma Prieta Earthquake (magnitude 6.9), October 17, 1989

The Agassiz statue, Stanford University,San Francisco Earthquake (magnitude 7.8), April 18, 1906

Damage due to ground deformation in a school in Anchorage, Alaska Earthquake (magnitude 9.2), March 28, 1964.

San Francisco Bay Bridge 2nd level collapse,Loma Prieta Earthquake (magnitude 6.9), October 17, 1989

Severe damage in a concrete parking structure, California State University, Northridge, Northridge Earthquake (magnitude 6.7), January 17, 1994

Massive earthquake and fire damage in San Francisco, San Francisco Earthquake (magnitude 7.8), April 18, 1906. The fire resulting from the earthquake was a major contributor to the losses

Soft story damage in a building during Northridge Earthquake (magnitude 6.7), January 17, 1994. Soft story is typically the result of one the stories being laterally softer than the others due to large window and door openings or parking garages

The information above shows all the building defects and even failure. For additional knowledge, the information belows shows some of the precaution steps in design to prevent the seismic activity for causing critical damage to the building.

Measures for Achieving Seismic Safety

SITE SELECTION

 Site Investigations will assist in identifying potential danger of sliding, erosion, land subsidence or liquefaction during an earthquake. The local practice of managing any such hazard should be given due considerations. A safer site is the one having

- No danger of landslides

- Sufficient plantation on slope

- Trees not too close to the house

- Mild slope

- Far from river banks

Potential hazardous sites

Steep and unstable slopes

Building should not be constructed near steep and unstable slopes. Cliffs made of soft or crumbly, clay loam; deposits materials, etc. should be avoided.

Areas susceptible to landslides and rock fall

Landslides or rock fall areas should be avoided while selecting a site for building

construction. Some slopes may look stable, but failure could be started by

an earthquake. The building could be completely damage by the landslide.

However, building in these areas can be constructed after providing proper retaining

walls and green barriers. 

Fill Areas

Building should not be constructed on loose fill. In a filled ground, the bearing capacity of foundation subsoil is low and settlement of foundation may occur.

Geological fault and Ruptured areas

Geological fault and ruptured areas that are usually visible, permanent, deep and active should be avoided for construction. Buildings should be constructed at least 250 m away from these lines

Forest and trees

The forests are really useful to stop landslides. But for safety consideration, buildings should not be constructed close to any big tree, as there might be a possibility of falling of the trees during earthquake. 

Too Close building

Building should not be constructed close to another building: there might be a possibility of falling of building during earthquake. Pictures below clearly indicated that harms may comes from building which built too close together.

APPROPRIATE PLANNING

The shape, size and layout of the building is important for its seismic safety. Constructions with asymmetrical plan and elevation are more vulnerable to earthquake than those having symmetrical plans elevations

Regular shape

Regular shaped buildings like square, rectangular, or circular resist the earthquakes more effectively as compared to irregular buildings and are therefore preferable in earthquake prone areas. During the earthquake movements the corners of non-uniform buildings are stressed more and may be damaged. Complex shaped buildings, shall therefore be made simple by providing gaps at appropriate locations. Some complex shapes and their simplified solutions are shown

Short walls

In long and narrow buildings, longer wall is weak against earthquake forces and can easily fall down during an earthquake. Therefore, if long and narrow buildings are constructed, they should be divided into two or more blocks with sufficient gap between them. The foundation of these blocks may be connected to each other and separation can be made only in the superstructure. The other alternatives include provision of cross walls and buttresses as shown below.

Box Effect

One of the essential principles of earthquake-resistant construction is to use a compact,

box-type layout. Furthermore, all the components of the building such as walls, floor

and roof structure, should be well tied up with each other, so the building could act as a

box during earthquake vibration. The maximum length of wall between cross walls shall

preferably be limited to 15 ft for an effective box action

Material and its defects due to environment

Timber 

Timber has long been used by man especially in building construction. It is the most useful material available for wall, floor, roof and other structural framing. All commercial timbers can be classified into softwoods (such as Pine, Fir and Damar Minyak) and hardwoods (such as Chengal, Meranti and Kapur), depending on the characteristics of their grains, weight and moisture content.

In general, timbers either of softwoods or hardwoods have a moisture content of between 12 to 15 per cent. Normally, a well dried timber has a moisture content of 12%. If the moisture content of the wood exceeds 20%, fungal rots, insect infestation and termite attack will eventually take place. This will further lead to structural failure.

Before timber is being used for building construction, it is important for the material to be seasoned and preserved. The primary aim in seasoning timber (either of air or kiln seasoning) is to render timber as stable as possible, for the timber increases its strength properties as it dries. On the other hand, the preservation of timber, usually by chemical processes either before manufacture or after, concentrates on fungicidal preservation, flame-proof protection and water-repellence application.

Stone

Stone has been used in building construction for thousands of years. Stone comes in different types and properties ranging from the hard impervious such as granite, slate, marble to the softer and pervious sandstone and limestone. Although stones will last for many hundred years, its tendency of decay in any kind of weather is possible.

Weathering occurs in three situations. First, the attacks from soluble salts especially when it comes up from the ground where there is no damp course, in locations near seas or from a heavily polluted atmosphere. Second, trouble arising from the slow build-up of soot deposits and dust, leading to possible onset of decay due to small vegetation organisms. Third, the straight forward erosion by wind and rain. Stone will become saturated when it is exposed excessively to driving rain. As a result, its surfaces becomes marked and rough. Besides weathering, stone may also decay through faulty materials and workmanship.

stone can be deteriorate by the wetting or drying cycles either causing frost damage or introducing soluble salts that crystalise, expand and "blow" the stone. These salts may come from a polluted atmosphere, from the mortars, may be inherent within the stone or from the ground through rising damp or road salt splashes.

Sulfation is an especially common form of decay in urban areas. The calcium carbonate of limestone converts to calcium sulfate by the action of acid rains. Initially a hard protective skin of sulfate is formed but this is the beginnings of a blister that expands and blows off, exposing soft friable decayed stone behind. Cleaning is excellent maintenance, as calcium sulfate is soluble in water and the skin can be washed away, before it causes damage. The skin is always unsightly because it binds black tarry and sooty pollution by products.

Organic growth with its aerial roots and rapid growth is a menace as it quickly penetrate the joints and can threaten the whole stability of a building. Organic acids secreted by suckers and tendrils can damage the surface and the mortar. A heavy growth keeps the wall moist whilst inhibiting healthy rain washing, and can accelerate frost damage. If climbing plants are desirable, they need to be maintained and their growth controlled, perhaps by covering the building with stainless steel wire to keep the plant from clinging to the building itself.

Limestone is also affected by salt attack. road salt can caused crystallisation on the stone face - not destructive but causing unsightly efflorescence. Crystallisation behind the face can damage the stone.

Below is some of the picture of stone deterioration

Brickwork or burnt clay block

Brickwork or burnt clay block has been used in many historic buildings in Malaysia, particularly the ones built during the British occupation. Some of the colonial buildings have exposed brick walls and others are plastered and painted. Old bricks are slightly different than modern bricks. The texture of modern bricks looks closer and smoother, and the edges are straighter and sharper compared to the old materials. Colour and size are also different. Brick may decay through weathering process including sulphurous smoke caused by polluted atmosphere, water penetration through small holes and openings of the brick as well as mortar joints; and dampness in wall caused by no damp course in locations near sea or river. Brick may deteriorate due to harmful vegetation and also mould or fungal growth that accumulate in the brick surface. Brick can also decay due to cracks caused by structural movements. Such structural movements may come from building foundations when subsoil is compressed through the decades or centuries followed by wall deflections due to the foundation weakness or an uneven loading distribution from above wall structure.

Plaster

Plaster tend to deteriorate over a period of time. Plaster normally contains lime, sand and water; and sometimes chopped animal hairs to give tensile strength. Plaster are used widely in decorative panels, ceiling renderings, cornices and internal walls. Causes of deterioration include direct exposure to driving rain, condensation, evaporation, air pollution, aerosols, capillaries, thermal stresses, vegetal causes, insect attacks, animals and human activities. Plaster may become cracked due to either shrinkage or movement in the substrate. Shrinkage usually occurs early in the life of the building but substrata movement is often the reason for failure in historic situations.

Deterioration can be caused by direct or induced mechanical stresses. Much of these depend on changes linked to the humidity present in the masonry, both due to external causes (rainwater) and internal (diffusion of vapour from inside to outside). These phenomena are linked with consequent micro-variations in volume (freezing-thawing), chemical or electrochemical phenomena (efflorescence, oxidation, incompatibility of a chemical type) and biophysical pathologies (moulds, algae).

The presence of a considerable amount of damp behind the facing can be caused by particular geometry of the facade (stringcourses, drip moulding, balconies, etc.) that facilitate water penetration and lead to water stagnation, capillary rising and saline migration towards the rendered surface.  

In many historic buildings, defective plastered rendering occurs mostly on external walls, columns and ceiling. In a humid tropical climate, the defects of rendering are normally caused by biological attacks arising out of penetrating rain, evaporation, condensation, air pollution, dehydration and thermal stress. Other causes may come from mould or harmful growth, insects, animals and traffic vibration. Prior to being decomposed and broken apart, plastered rendering may become cracked due to either shrinkage or movement in the substrate itself.

Steel

Deterioration of steel at the material level stems primarily from incompatibility

factors, such as chemical attack, that lead to corrosion. There are a large

number of ways that steel can corrode. A partial list of corrosion types includes

chloride-accelerated, concentration cell, crevice, deposit, electrochemical, electrolytic,

galvanic, pinpoint, and thermogalvanic. Section loss may occur due to corrosion,

reducing the strength or stability of a steel member or system. Proper surface treatment

of steel, such as corrosion resistant paint, often can reduce or prevent the occurrence

of corrosion.