Roof Uplift Loads

Asce sei 7 16 contains a number of revisions in the wind load chapters of the stan dard.

Roof uplift loads.

When specifying net uplift pressure on steel joist do any of you indicate design approach say asd or the exact load combination say 0 6d w used to arrive at that value. Uplift load pressures from wind flow that cause lifting effects. Let s assume that the calculations for our building require a roof that can withstand 23 pounds per square foot of uplift force in the field of the roof. At the lower edge of the roof for example the rafters can experience 240 pounds of uplift minus the dead load of the roof framing and cladding which is no more than 10 lb.

Because fm approvals requires a 2 1 safety factor roofs with field of roof uplift forces of 30 pounds per square foot are the maximum for 1 60 ratings and 37 5 pounds per square foot are the maximum for 1 75 ratings etc. This can be exacerbated during high wind as air infiltration into the building can increase pressure below the roof whilst the speed of wind over the roof surface can reduce air pressure above it in much the same way it does over an aircraft wing. With sji promoting both lrfd and asd for the design of steel joists isn t it misleading for the joist designer to not have this info. Wind uplift occurs when the air pressure under the roof is greater than the air pressure above it.

Lateral load a pulling and pushing horizontal pressure that can cause a building to move off its foundation. The uplift loads acting on a roof are influenced by a number of factors geographic location the local building code dimensions exposure use and occupancy aka risk factor and type. It would appear that the factors for wind design are all fixed and not debatable but there are presumptions made during the wind design of every roof. This presentation examines these revisions and how they impact low slope roof assem bly design in resisting wind uplift.

As we head up the exterior walls though there is less building material to resist the wind. It also describes wind uplift design of roof assemblies in accordance with asce 7 16 with several illustrative examples. In a study of wind load on roof tiles the vulnerability analysis indicated that the net wind uplift loading on tiles caused an increase in tile destruction rather than the external surface pressures li et al. A typical uplift load limit assumes a maximum wind speed of about 90 miles per.

6the 0 6 reduction factor on d is intended to apply to the calculation of net overturning stresses and forces. Shear load wind pressure that is horizontal and could make a building tilt. Thus for the structure s design along with other loads wind load must also be considered. U is wind uplift load from negative i e suction pressures on the roof.