House

House top and top of other buildings is designed to take static loads and endure the outside environment such as sunlight, rain, wind, falling objects, temperature changes, hurricanes, and/or snow. It must also support loads imposed by people walking on it. This house top structure represents a simple type of a design project in static loading in that the loads are clearly long term and well defined. Creep effects can be easily predicted and the structure can be designed with a sufficiently large safety factor such as 3 to avoid the probability of failure. These type data (static, creep, etc.) are available of the material suppliers (Chapter 7).

A roof design could use a translucent corrugated RP panel structure. This construction material has been used for roofs to admit daylight on a porch, shanty, canopy, and patio, shelter tops such as those used at a bus stop, industrial building, etc. Also used on transportation vehicles such as a boat, truck, airplane, bus, and train. Corrugated materials are available in sheets of different sizes such as from 4 ft x 8 ft to as large as 10 ft x 20 ft. A typical material is 0.1 in. thick with 2 to 4 in. corrugations and a corrugation depth of 1 to 4 in. The usual material of construction is glass fiber-TS polyester RP. Standardized panels are used that have specific physical and mechanical properties. Usual attachment is by using nails or screws to wooden or pultruded plastic supports.

Design approach takes into account the weight of the material that is the major static load applied to the structure itself. Other weight such as snow is available from experience obtained in the area where the structure is to be used. Similarly, weights due to wind load and people load can be determined from experience factors that are generally known.

Main stress caused by the wind is at the anchorage points of the roof to the rest of the structure. They should be designed to take lifting forces as well as bearing forces. By using a lower angle of top structure, the less wind lifting stress. Proper anchorage of the support structure to the ground is also essential.

Localized loads represented by people walking on the roof can be solved by assuming concentrated loads at various locations such as between supports and by doing a short time solution to the bending problem and the extreme fiber stress condition. The local bearing loads and the localized shear should also be examined since it may cause possible local damage to the structure.

Requirements for the design of the structure included factors such as building and fire codes, and those that are reported by the owner of the property. As an example the material selected is to take the required loads without severe sagging (amount to be specified) for a 20 year period with no danger that the structure will collapse due to excessive stress on the material. One typical way in which excessive loading for a single section is handled is to bond two layers of the corrugated panel together with the corrugations crossed. This approach result in a very stiff section capable of substantially greater weight bearing than a single sheet and it will meet the necessary requirements. It also provides thermal insulation because of the trapped air space between the two layers particularly if they are edged sealed.

When using a large surface area of the RP panels expansion with a temperature rise occurs. Recognize that very few of the traditional building materials, such as wood, have significant expansion under normal temperature shifts. The RP materials, when compared to wood, does not have a problem because of its low thermal coefficients of expansion. What also keeps the dimensional changes low is that the corrugated shape can flex and accommodate the temperature changes. When compared to plastic materials such as polyvinyl chloride (PVC) siding, the expansion factor becomes significant and is an important consideration in the PVC method of fastening.

Data on the material (from the material supplier and/or data determined) will have been determined that it meets the static and dynamic loads required based on engineering analysis. Exposure to the environment (includes water and sunlight) can have a significant effect on the properties of the materials. In this example it is assumed that a 50% or more drop in the mechanical properties is expected to occur in 10 years. Based on knowledge of the materials behavior in service this loss of property levels off to a low rate of deterioration so that any potential failure will not occur in 20 years. If necessary increasing the panel strength requirements can compensate loss of properties by a suitable factor of safety that could be 3 in this example. However one knows that with a protective coating on the panel will minimize the effects of weathering so that only 20% loss in properties occurs after 10 years so that initial requirements for the panel performance will be reduced accordingly. The preferred type of coating could be a fluoropolymer that has the best resistance to sunlight and other weathering factors of all of the plastics and will last 20 years. If a coating is applied there usually is a requirement that the panel be rccoat after a specified period in service.

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