Uplift pressure

Factory Mutual provides loss prevention data sheets that explain how to protect buildings from wind damage. Pressure coefficients that define increased uplift at corners and edges adjust the calculated uplift pressures. A laboratory uplift pressure test rates roofing assemblies. An uplift pressure of 2.9 kPa (0.42 psi) must be withstood under FM conditions to meet the Class 1-60 requirements. The FM approval guide is revised aimuaHy (37). 

Landfills in wet areas. Because of the problems associated with contamination of local groundwaters, the development of odors, and structural stabihty, landfills must be avoided in wetlands. If wet areas such as ponds, pits, or quarries must be used as landfill sites, special provisions must be made to contain or ehminate the movement of leachate and gases from completed cells. Usually this is accomplished by first draining the site and then lining the bottom with a clay liner or other appropriate sealants. If a clay uner is used, it is important to continue operation of the drainage facility until the site is filled to avoid the creation of uplift pressures that could cause the liner to rupture from heaving. 

Tray supports must prevent trays from being lifted by upflowing vapor. Trays are usually designed to withstand an uplift pressure of about three times the tray pressure drop at maximum vapor and liquid loads (49) or 0.25 psi (107) (normally, whichever is larger). 

Groundwater frequently represents one of the most difficult problems during excavation, and its removal can prove costly. Not only does water make working conditions difficult, but piping, uplift pressures and flow of water into an excavation can lead to erosion and failure of the sides. 

Uplift pressure acts against the base of a dam and is caused by water seeping beneath it that is under hydrostatic head from the reservoir. Upiift pressure shouid be distinguished from the pore water pressure in the material beneath a dam. The uplift pressure on the heel of a dam is equal to the depth of the foundation below water level multiplied by the unit weight of the water. In the simplest case, it is assumed that the difference in hydraulic heads between the heel and the toe of the dam is dissipated uniformly between them. The uplift pressure can be reduced by allowing water to be conducted downstream by drains incorporated into the foundation and base of the dam. 

The Goda formula clarifies the concept of uplift pressure on the caisson bottom, since the buoyancy of the upright section in still water and its uplift pressure due to the wave action are defined separately. The distribution of the uplift pressure has a triangular shape. 

In the extended Goda formula (named here the Goda-Takahashi formula), the wave pressure acting along a vertical wall is assumed to have a trapezoidal distribution both above and below the still water level, while the uplift pressure acting... 

For the ordinary vertical breakwater, Ai,A2, and A3 are taken as unity since the Goda formula was originally proposed to describe this type of breakwater. The modification factor Ai represents the reduction or increase of the wave s slowly varying pressure component, A2 represents changes in the breaking pressure component (dynamic pressure component or impulsive pressure component), while A3 represents changes in the uplift pressure. These modification factors are introduced to express the pressures on other t es of caisson breakwaters. 

The usual requirement that the permeability of the cover layer should be larger than that of the underlayers cannot be met in the case of a closed block revetment and other systems with low permeable cover layer. The low permeable cover layer introduces uplift pressures during wave attack. In this case, the permeability ratio of the cover layer and the filter, represented in the leakage length, is found to be the most important structural parameter, determining the uplift pressure. This is also the base of analytical model. 

Apart from the cracks due to wave impacts, the mattress should also withstand the uplift pressures due to wave attack. These uplift pressures are calculated in the same way as for block revetments. For this damage mechanism the leakage length is important. In most cases, the damage mechanism by uplift pressures is more important than the damage mechanism by impact. 

It is not expected that instability will occur at once if the uplift pressure exceeds the gravity forces. On the other hand, the above result turns out to be in good agreement with the experimental results. The experimental verification of stability of gabions is rather limited. Small scale model tests have been performed by Ashe and Brown see Fig. 19.15. 

Mchenry, D. The effect of uplift pressure on the shearing strength of concrete. 3rd Congress on Large Dams, Stockholm, 1948. 

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