Snow load

Snow Loads

Excessive ice and snow loads can overload a buildings structural members and sometimes even cause a roof collapse. Excessive snow loads are usually the result of wind creating large snow drifts. Snow drifts can create concentrated loads on roofs that are well in excess of the loads imposed by uniformly distributed snow. Winter rain storms and ice buildup can further increase roof loads.

The New York State building code did not even consider snow drifting; and therefore, most buildings were not designed for this possibility. Fortunately, few roof failures occur without warning signs. Example, In one school a laminated wood beam broke with such force that security personnel reported hearing what sounded like a gun shot. Warning signs of structural roof problems include roof leaks, cracks in walls and ceilings, and excessive sagging of structural roof elements or ceilings. Hung ceilings often hide the roof structure but will sag if the roof has excessively deflected. Signs of structural roof problems are usually more visible during periods of heavy snow loads.

The roofs of older buildings which have been reinsulated for energy conservation must also be re-evaluated to make sure they can handle additional loads of accumulated snow. Added insulation may increase the probability that more snow stays on the roof longer, thereby increasing snow loads when there are multiple storms. Replacing a non-ballasted roof with a ballasted roof also may reduce the snow load carrying capacity.




Damages to Springfield and Baca County from the large amount of snow fall which causes loss of livestock has been around 10,000.   The figure below shown the roof and the crack in the front of the building caused by snow storm. The front windows were all broken too.

 Snow storm damage the whole structure of building.

 Structure damages by ice dam and snow load

Based on the structural design of a building, snow loads can be a serious concern during the winter. Places where snow loads should be of concern:

• Roof Overhangs
• Multilevel Roofs
• Valleys

Structural integrity is also something to be concerned about. Years of wear and tear of existing structures and their exposure to water and weight can cause rotting, in turn making them weak.

Two means of solving excessive snow load problems are:

• to reinforce roof areas to handle large potential snow accumulation, or
• to remove the snow to maintain snow loads at acceptable limits.

If buildings are showing any signs of structural overloading, an architect or structural engineer should be consulted. Repeated overloading of roofs can significantly weaken the roof structure over time.

DESIGN OR CONSTRUCTION ERROR

A majority of structural failures and damages costs in ordinary building ( some estimates range as high as 80%) occur as a result of errors in planning, design, construction and use rather than stochastic variability in resistance and load. Errors in concept, analysis and execution and other unforeseen circumstances occur even when qualified personnel are involved in design and construction and when accepted methods of qualified assurance and control are employed. Such errors result from human imperfections and are difficult to quantify. Modern code safety checks, such as load and resistance factor design (LRFD), are not developed with such errors in mind. Design and construction errors do not simply change the statistics in load or resistance that are used to derive probability-based design criteria; rather, they change fundamentally the load and resistance models or the relevant design limit states. This sources of ‘abnormal’ load or resistance is better dealt by the engineer recognizing that things can go wrong, though a consideration of hazard scenarios, and through improvements in quality assurance and control. Such performance – oriented thinking is essential to progressive collapse mitigation in general.

Range of design errors

1. Structural systems or concepts, and load paths
• There must be a continuous and sensible load path from the point of application of a load all the way to the foundation. Although this seems evident, load path issues do arise when the design analyses and detailing lose sight of the overall structural concept and become inconsistent with it. They also arise when the structural concept itself is inconsistent with the loads it needs to resolve and resist.
2. Incorrect loads or incorrect assessment of load effects
• Determining the correct design loads is not just a question of extracting values from a building code. It is a critical element of the design. Even building code load values have and continue to evolve as our understanding of natural events, such as winds and earthquakes, and their effects improves and as the statistical foundation for their frequency and magnitude are enhanced.
3. Structural analyses and calculations
• With the prevalence of terms such as computer-aided engineering, it is understandable if people think that computers can do all the work. They can, but they cannot do the thinking. The adage ‘garbage in, garbage out’ remains true, except it occurs faster and faster. Structural analysis, whether computer-aided or not, is fundamentally a mathematical representation of physics. It remains a designer’s responsibility to ensure that the complete structural behavior of a given system is captured, and to ensure that the analysis results are properly interpreted. The problem is actually exacerbated when engineers fail to appreciate the underlying assumptions and limits of applicability of the analysis software they sometimes use as a design aid. 

Wind load and Earthquake load

WIND LOADS

Eiffel Tower

The Eiffel Tower is one example of a building which has a structure that was designed to resist a high wind load.  Winds can apply loads to structures from unexpected directions. Thus, a designer must be well aware of the dangers implied by this lateral load. The magnitude of the pressure that acts upon the surfaces is proportional to the square of the wind speed.


Wind loads vary around the world. Factors that effect the wind load include:
(a) geographic location,
(b) elevation,
(c) degree of exposure,
(d) relationship to nearby structures,
(e) building height and size,
(f) direction of prevailing winds,
(g) velocity of prevailing winds and
(h) positive or negative pressures due to architectural design features
All of these factors are taken into account when the lateral loads on the facades are calculated. It is often necessary to examine more than one wind load case.


Assumption that wind loads, as well as the pressure they develop upon wall and roof elements, are static and uniform. They actually not only pound a structure with a constantly oscillating force, but also increase as a building increases in height. The loading of a tower can be very roughly approximated by an evenly distributed load. It is a vertical cantilever.


The figure below are to investigate the variables which influence the structural behavior of a tall, thin tower. It does not represent actual methods of calculating the total wind force on a tall building. It is intended to demonstrate the interaction between the variables of the equations which govern the structural behavior.

Actual cases

The immediate result of failures of windows and doors was an increase in internal pressure.this occurrence, in combination with overall roof uplift pressures, initiated a chain of events that included removal of roof sheathing, wind and rain entering the building, and the beginning of progressive failure of the building frame.

Building envelope damage may result in total insured loss

The blow-off of roof gravel was instrumental in causing window-glass damage to high-rise buildings in downtown Houston. The windborne debris was a major factor in damage to glass in multistory buildings in south Florida. 

 Roof gravel blow-off caused window glass damage in Houston, Tex

Roof gravel broke fully tempered glass on building in Cutler Ridge, Fla.

The EF Scale are listed below, in order of increasing intensity. 

Scale	Wind Speed		Relative frequency	Potential damage
	mph	km/h
EF0	65-85	105-137	53.5%	Minor or no damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over. Confirmed tornadoes with no reported damage (i.e., those that remain in open fields) are always rated EF0.
EF1	86-110	138-178	31.6%	Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken.
EF2	111-135	179-218	10.7%	Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.
EF3	136-165	219-266	3.4%	Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations are badly damaged.
EF4	166-200	267-322	0.7%	Extreme damage. Well-constructed and whole frame houses completely leveled; cars and other large objects thrown and small missiles generated.
EF5	>200	>322	<0.1%	Total Destruction. Strong framed, well built houses leveled off foundations and swept away; steel-reinforced concrete structures are critically damaged; tall buildings collapse or have severe structural deformations.

EARTHQUAKE LOADS

Earthquake loads are very complex, uncertain, and potentially more damaging than wind loads. The earthquake creates ground movements that can be categorized as:
(a) shake,
(b) rattle and
(c) roll


Every structure in an earthquake zone must be able to withstand all three of these loadings of different intensities. Although the ground under a structure may shift in any direction, only the horizontal components of this movement are usually considered critical in a structural analysis. It is assumed that a load-bearing structure which supports properly calculated design loads for vertical dead and live loads are adequate for the vertical component of the earthquake. The "static equivalent load" method is used to design most small and moderate-sized buildings.


The magnitude earthquake load depends up:
(a) the mass of the structure,
(b) the stiffness of the structural system and
(c) the acceleration of the surface.


This movie is a representation of the movement of a free standing water tower in an earthquake. It can be seen that the as the ground moves, the initial tendency is for the water tower to remain in place. The shifting of the ground is so rapid that the tower cannot "keep up."

After a moment, the tower moves to catch up with the movement of the ground. The movement is actually an acceleratoin. From Newtonian Physics, it is know that an applied force=mass x acceleration. Thus, the force which is applied to the water tower depends upon the mass of the tower and the acceleration of the earth's surface.

The force in this last diagram may be thought of as the "equivalent static load" for which the structure would be designed. This idealized situation demonstrates a concept; it requires modification for actual buildings. These modifications account for building location, importance, soil type, and type of construction. This movement can also be seen in the following movie of lateral earth movement. Note how the mass slowly reacts to the movement of the earth. Eventually, the bending strength of the stem of the tower would be exceeded and it will fail.

Actual cases

The California earthquake of April 18, 1906 ranks as one of the most significant earthquakes of all time. It measured a magnitude of 7.8. Shaking damage was equally severe in many other places along the fault rupture. The frequently quoted value of 700 deaths caused by the earthquake and fire is now believed to underestimate the total loss of life by a factor of 3 or 4. Most of the fatalities occurred in San Francisco, and 189 were reported elsewhere.

Northridge, California Earthquake, January 17, 1994. At the Northridge Fashion Center, near the earthquake epicenter, the second floor of Bullocks Department Store collapsed onto the bottom story. The shear between the waffle slab and the columns caused complete separation of the slab system from the columns and a pancake collapse.

Northridge, California Earthquake, January 17, 1994. A view of the parking structure on the campus of California State University. The bowed columns are of reinforced concrete. The structure has precast moment- resisting-concrete frames on the exterior and a precast concrete interior designed for vertical loads. The inside of the structure failed, and with each aftershock the outside collapsed slowly toward the inside until finally the west side failed totally. The reinforced concrete columns were extremely bent.

Great Hanshin-Awaji (Kobe) Earthquake, January 16, 1995. The picture shows an office building with a partially destroyed first floor. The majority of partial or complete collapses were in the older, reinforced concrete buildings built before 1975. However, significant non-structural damage was also observed for buildings of relatively recent steel or composite construction.

Blast loads (Luccioni et.al, 2003)

Explosive loads become issues that have received considerable attention in recent years which it’s can easily damage the buildings that within the explosion. Hence, new developments in integrated computer hydro codes, such as AUTODYN software were used for this research as a tool to carry out for the numerical analysis.

The analysis of structural collapse of the building was performed in two stages. The first part of the analysis consists on the simulation of the explosion itself from the detonation instant and the second part consists on the analysis of the effect and interaction with the building of the blast wave generated by the explosion. The load that produced by air blast wave was only considered in the analysis and the ground motion generated by the explosion was not taken into account in this research.

An illustration of the role played by the interaction of the blast wave with the building, the propagation of the blast wave in a building without walls is compared with the propagation of the same blast wave in the actual building with walls as shown as below.

Effect of the walls in the blast wave propagation. (a) building without walls, (b) building with walls.

The results obtained for an explosive load of 400kg of TNT located 1m above the ground level, 1m inside the entrance hall and 1m to the right of the axis of the building are described as bellow:

Evolution of damage produced by the explosion. (a) 0.75ms, (b) 254ms, (c) 378ms (d)1.35ms and (e)2.46ms

Comparison with actual damage

The distribution of the remains of the demolition with those registered in the actual building.

The numerical simulation reproduces the fall of the front slabs that resulted hanging from the back part of the building.

Slabs hanging from the highest stages

View of the reinforced concrete frames that remained after the explosion.

 Remaining frames

The comparison between actual damage and that numerically obtained proves that the simplifying assumptions made for the structure and materials are allowable for this type of analysis and nowadays represent the only way to successfully run a complete collapse analysis of an entire building. 

Structural defects

A building needs design based on two methods which are Ultimate Limit State and Serviceability Limit State. When the new building that designed non-compliance with Building Code or called as British Standard (BS), it will lead to the structural defects. Defective construction not only contributes to final cost of product but also to the cost of maintenance, which can be substantial. Hence, defective of construction may lead to the failure of structure or structural defects.


What is structural defect?


'Structural defect' means any defect in a structural element of a building that is attributes to defective design, defective or faulty workmanship or defective materials or any combination of above and that:


(a)results in, or is likely to result in, the building or any part of the building being required by or under any law to be closed or prohibited from being used, or

(b)prevents, or is likely to prevent, the continued practical use of the building or any part of the building, or

(c)results in, or is likely to result in:

(i) the destruction of the building or any part of the building, or

(ii) physical damage to the building or any part of the building, or

(d) results in, or is likely to result in, a threat of imminent collapse that may reasonably be considered to cause destruction of the building or physical damage to the building or any part of the building



Structural defects means structure of building failure in physical state such as:
(a) cracking

(b) corrosion

(c) buckling

(d) carbonation

Carbonation occurs when carbon dioxide from the air penetrates the concrete and reacts with hydroxides, such as calcium hydroxide, to form carbonates. Carbonation of concrete lowers the amount of chloride ions needed to promote corrosion.

(e) sway

(f) settlement and so on 

TYPE OF LOADING ON BUILDING

The loads are broadly classified as:

(a) vertical loads -dead load, live load and impact load

(b) horizontal loads - wind load and earthquake load

(c) longitudinal loads - tractive and braking forces are considered in special case of design of bridges, gantry girders etc.

1. Dead load:


Dead loads are permanent  loads which are transferred to structure throughout the life span. Dead load is primarily due to self weight of structural members, permanent partition walls, fixed permanent equipments and weight of different materials.


2. Imposed loads or live loads:


Live loads are either movable or moving loads with out any acceleration or impact. There are assumed to be produced by the intended use or occupancy of the building including weights of movable partitions or furniture etc. The floor slabs have to be designed to carry either uniformly distributed loads or concentrated loads whichever produce greater stresses in the part under consideration. Since it is unlikely that any one particular time all floors will not be simultaneously carrying maximum loading, the code permits some reduction in imposed loads in designing columns, load bearing walls, piers supports and foundations.


3. Impact loads:

Impact load is caused by vibration or impact or acceleration. Thus, impact load is equal to imposed load incremented by some percentage called impact factor or impact allowance depending upon the intensity of impact.


4. Wind loads:

Wind load is primarily horizontal load caused by the movement of air relative to earth. Wind load is required to be considered in design especially when the heath of the building exceeds two times the dimensions transverse to the exposed wind surface.
For low rise building say up to four to five storeys, the wind load is not critical because the moment of resistance provided by the continuity of floor system to column connection and walls provided between columns are sufficient to accommodate the effect of these forces.  Further in limit state method the factor for design load is reduced to 1.2 (DL+LL+WL) when wind is considered as against the factor of 1.5(DL+LL) when wind is not considered. IS 1893 (part 3) code book is to be used for design purpose.


5. Earthquake load:

Horizontal earthquakes forces ( back-and-forth shaking ) create 'whipping' forces in all parts of a building. These forces must transfer between parts of building to the foundation.