Keywords: cold-formed steel purlins, sandwich roof panels, self-drilling screws, rotational restraint, finite element modelling, design method.

This paper deals with the combined action of roof sheeting and cold-formed steel roof purlins in resisting the roof loads; especially when thermal insulation foam is present in the roof panels. It is well known that the influence of the attached roof sheeting on the failure behaviour of cold-formed steel purlins is considerable. The lateral and rotational restraints provided by the roof sheeting to the attached purlins increase the failure load of the purlins. It would be very conservative to neglect the presence of the roof sheeting in the design of the purlins as the influence of the attached roof sheeting plays a beneficial role. Hence, for a given purlin-sheeting system, quantifying the lateral and rotational restraints has importance in design of the purlins. In most cold-formed steel purlin-sheeting systems, the amount of the lateral restraint is very large, whereas, the rotational restraint is always flexible.

In Eurocode 3 a design method is proposed to account for the influence of the lateral and rotational restraints provided by the roof sheeting to the attached purlins. The free flange of the attached purlin is modelled as a beam on an elastic foundation by taking into account the rotational restraint and the load acting on the purlin. The additional stresses induced due to lateral movement of the free flange are calculated and the failure criteria are checked. To use the Eurocode 3 design method, the value of the rotational restraint is required. In this regard, Eurocode 3 gives a calculation procedure to evaluate the rotational restraint based on either experiments or on empirical formulae. However, these empirical formulae have practical limitations and are not applicable for roofs with sandwich panels. In general, the energy consumed for heating and cooling would be more than half of the total energy used in an average home. To maintain uniform temperature, and also to lower noise levels in homes, rigid foams are used for effective insulation in roofs and walls. Rigid foams are made of a strong, yet lightweight, low-density structure, that is both dimensionally stable and moisture-resistant with low vapour transmission. Both polyurethane and polyiso foams are widely used in the fabrication of steel faced building panels for various categories of commercial building construction. By using standard pre-finished metal facings, already commonly used as roof sheets, a factory-finished composite roof panel can be made. This one-piece product combines a lining sheet, insulation and outer sheet, so it can be quickly and simply fixed on site, providing both high quality and reliability. This type of panel was first manufactured in the 1960s and its share of the market has grown steadily all over the world ever since. Today the panels are used for cladding roofs and walls of many industrial and commercial buildings.

In this paper, a design method is presented, based on the developed finite element models, to estimate the rotational restraint in cold-formed steel sheeting-purlin systems with thermal insulation. It is observed from the computational models and experimental tests that the rotational restraint provided by the sandwich panels to the purlin depends on several factors such as (a) shape and thickness of the sheeting, (b) presence of the insulation foam (c) cross-section of the purlin, (d) number of screws per unit length of the purlin, (e) type of screws and their applied position, (f) type of washers etc. The developed design method is validated with experimental tests performed on sandwich purlin-sheeting systems and found to be in good agreement. The design method is useful in calculating the rotational restraint without any experimental tests, and thus supports the proposed design method in Eurocode 3 for designing cold-formed steel purlins.

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