Prefabricated buildings have gained widespread acceptance as a result of their reduced fabrication costs. These prefabricated buildings commonly have a building framework with spaced apart upstanding side beams and roof beams between which are connected a plurality of generally horizontally extending support members. These horizontally extending support members are commonly known as girts when they are located in the side walls of the building and purlins when they are located in the roof of the building. Fasteners are then used to attach exterior cover panels to the girts and purlins to provide an exterior covering for the building. In order to compete with custom fabricated buildings, the cost of component fabrication and the cost of erection of prefabricated buildings must be minimized. At the same time, corrosion, especially at the fastener joints, must be minimized in order for the useful life of the prefabricated building to be acceptable.
Two different systems have evolved for fabricating and erecting prefabricated buildings. Both systems fabricate and erect the side beams and roof beams in about the same way with their differences lying primarily in the fabrication and erection of the support members and the cover panels. One system (the inside system) requires the workmen erecting the building to install the cover panels from the inside of the building while the other system (the outside system) requires the workmen erecting the building to install the cover panels from the outside of the building. Each of these systems has certain advantages and disadvantages.
The inside system uses cover panels with inwardly projecting spacer legs along opposite sides of the cover panel that project inboard of the exterior section of the cover panel. Mounting flanges are provided on the inboard ends of the spacer legs and are arranged so that the mounting flanges on adjacent cover panels can be overlapped. These overlapped mounting flanges are used to attach the cover panels to the outside of the support members with the fasteners. It will thus be seen that the fasteners are located inside the building and thus not exposed to the outside environment. This is the primary advantage of the inside system since corrosion is significantly decreased by the fasteners not being exposed to the outside environment. In this system, the support members used to support the cover panels are prepunched with holes spaced along the length of the support members during the fabrication operation and prior to building erection. Likewise, the mounting flanges on the cover panels are also prepunched with holes during the fabrication operation and prior to building erection. These prepunched holes are located so that the holes in the mounting flanges on the cover panels align with each other when the mounting flanges of adjacent cover panels are overlapped during building erection and also so that the prepunched holes in the mounting flanges align with the prepunched holes in the support members when the cover panels are positioned against the support members during building erection.
During building erection, the workmen position the cover panels so that the mounting flanges on adjacent cover panels are overlapped with the prepunched holes aligned and so that the overlapped mounting flanges on the cover panels lie against the support member with the prepunched holes through the cover panel mounting flanges and the support member aligned. While holding the cover panels in this position, the workmen insert conventional threaded fasteners from the inside of the support member through the aligned prepunched holes in the support member and the cover panel mounting flanges. The workmen start an internally threaded nut on that end of the fastener projecting through the holes behind the cover panel mounting flanges and then tighten the fastener and nut to attach the cover panels to the support members. The limited space available between the exterior section of the cover panels and overlapped mounting flanges requires the workmen to install the fastener from the inside of the support member so that the fastener extends first through the support member and then through the cover panel mounting flanges. The cover panel mounting flanges are too thin to engage the threads on the fastener sufficiently to maintain the strength of the connection between the cover panels and support members thus requiring the use of the nut on the fastener behind the cover panel mounting flanges to keep the cover panels in place. The limited space between the exterior section on the cover panels and the overlapped mounting has also required that the nut be tightened by turning the fastener while holding the nut against rotation. Because of this, the use of self-drilling and self-tapping fasteners which could eliminate the prepunching of holes has effectively been prevented in connection with this system. As a result, the prepunching operations used with this system have kept the fabrication costs of the cover panels and support members relatively high. This system has also required that the building framework be located with more accuracy than normally required so that the holes in the support member were properly aligned with the holes in the cover panels.
In the outside system, cover panels are used whose exterior sections could be overlapped and placed against the support members on the building framework. The workmen install fasteners holding the cover panels onto the support members from the outside of the building through the overlapped edges of the cover panels and into the support members. This allows the fastener to be screwed into the support member so that the head of the fastener clamps the overlapped edges of the cover panels against the support members. Because the support members are sufficiently thick to engage the threads on the fastener and maintain the strength of the joint, the need for a nut can be eliminated, thus permitting the use of self-drilling and tapping fasteners. On the other hand, it is difficult to determine the location of the support members from the outside of the building once the cover panels have been positioned for installation. As a result, the workmen typically predrill holes through the cover panels at those locations where the cover panels will lie against the support members. The cover panels are then overlapped so that these predrilled holes are aligned with each other and with the support members. The workmen, using the predrilled holes through the cover panels as a guide, then install self-drilling and self-tapping fasteners through these holes and into the support members so that the fastener threadedly engages the support member to keep it in place while the fastener head clamps the overlapped cover panels onto the support member. While this system helps minimize the fabrication costs of the cover panels and support members, the erection costs are maintained relatively high due to the predrilling of the holes through the cover panels prior to being positioned on the building framework. This system also leaves the heads of the fasteners exposed to the outside environment and thus requires special fasteners or special techniques to reduce the corrosion of the exposed head of the fastener.