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.