COP v3.0:structure; understanding-loads

3.5 Understanding Loads 

The performance of profiled metal cladding under wind, snow and point loads depends on its ability to resist the tension (pulling), compression (squashing), and shear (sliding) forces that it is likely to be subjected to during the lifetime of the building.

 

 

3.5A Profiled Steel — Flanges and Webs

A structural steel member typically comprises of sections named webs and flanges. In a roofing profile the sides of the rib act as a web, and the pan and top of the rib acts as a flange

 

Profiled metal cladding acts as a beam, which derives its strength from the ability of its flanges (pan and the crest), separated by the web to resist tensile and compressive forces. This strength can only be maintained while those parts in compression are restrained from buckling.

When a beam is subjected to bending, one flange will be under compression and the other under tension.

3.5.1 Point Load 

 

 

 

When a roof sheet deflects downwards under point load, the pan of the profile is placed under tension and the crest under compression, but in the adjacent continuous span, this condition reverses.
Metal is relatively strong under tension and weak under compression.  Because of this, excessive point load will cause failure by compression buckling of the rib.  Wherever possible, temporary point load such as roof traffic should be placed with the weight in the pan of the profile, so that the load is shared by the two adjacent ribs, across two ribs, or at the purlin line. Permanent point load should be placed across multiple ribs.

 

 

 

3.5.2 Wind Load 

Under wind uplift loading, when all spans of the roof sheet are under upward loads, the crest of the profile is placed under tension and the pan under compression. The deflection and stress patterns are the reverse of those for point load.

 

Failure under wind load for a clip fastened cladding is usually by the clips de-indexing and the cladding sheets blowing off. This is an ultimate failure.

Initial failure under wind uplift for pierce-fastened cladding is usually local buckling of the rib crest adjacent to the fastener. While the cladding can still resist a load, this permanent deformation is liable to cause leakage at that point; therefore, it is a serviceability failure.

 

 

The design load relates to a specific building and is calculated by the engineer. The failure load relates to a specific product or system and is supplied by the manufacturer. Engineers calculate both the Serviceability load and the Ultimate Load. They compare these values with the maximum failure loads of the products and systems they are considering.

  • In design, the serviceability load is no more than 0.72 of the ultimate load.
  • In testing, the serviceability load is about 0.5 of the ultimate load for a pierce fastened product.

Therefore, if a product passes for serviceability it will comfortably exceed ultimate design load requirements. In the above example, the product has failed against serviceability but still exceeds ultimate load requirements.

To determine the performance of corrugated and low rib trapezoidal profiles see 3.14.5 Wind Load Span Graphs

Refer to manufacturer's load/span tables for all other profiles, which should give the maximum recommended span for end and continuous spans when tested as described in 16.7 Wind and Point Load Testing

3.5.3 Deflection 

Deflection is a measured deformation of roof or wall cladding under a load, but there is difference between temporary deflection and permanent deformation.

Temporary deflection of cladding under load is within the elastic limit of the steel; the cladding will regain its original shape and strength properties when the load is removed. Permanent deformation that affects the shape, strength or performance of the cladding is serviceability failure.

With high strength claddings deflection failure is often quite sudden and severe once the point of elasticity is reached, but progressive deflection under repeated loads within the material’s elastic limit is minimal.

Roof design should consider the effect of repeated loads, when expected, because low-strength steel or non-ferrous metals can progressively yield under wind loads or repeated constant foot traffic. Machine roll-curved, crimp curved cladding, and metal roof tiles are usually made from low strength steels.

 

 

 

 

 

 

 

3.5.4 Load Distribution 

The distribution of the load greatly affects the deflection. A load distributed equally along the length of a beam (line load) will cause less deflection than the same load being applied to the centre of the beam.

 

 

 

 

 

 

 

The depth of the profile of metal cladding is another important criterion in the design and use of metal roof and wall cladding. Given the same cross section area, the deflection of the profile will vary with the square of the depth.

 

 

3.5.5 Yield Strength 

Yield strength is the point beyond which material will permanently deform, and Ultimate strength is the point beyond which the material ceases to resist load. Yield strength is affected by the type, grade or temper of the metal measured in Megapascals (MPa). The yield point is more important than the ultimate strength when designing and using metal roof and wall cladding.

The yield point of any metal can vary depending on the temper. If the yield point is higher, it is possible to reduce the thickness of the metal of the same profile and still obtain equal performance.

3.5.6 Section Properties 

Various attributes or effects can be identified by looking at individual shapes or profiles of metal cladding, but their performance cannot be accurately determined by their section properties as their profile shape, and therefore the sectional properties, change during deflection.

Corrugated iron is a common profile traditionally used in roof and wall cladding. It has a symmetrical sinusoidal profile and is equally strong under positive or negative loading. Its ‘neutral axis’’ runs through the centre, or halfway-depth, of the profile. Symmetrical trapezoidal profiles have similar attributes.

 

Symmetrical corrugate or trapezoidal profiles have the advantage of being more easily curved around a radius. Because the ribs are necessarily close together, they have the disadvantage of roof traffic loads having to be spread over two crests or along the purlin line, and they have a lower run-off capacity.

Most profiles fixed on non-residential roofs in New Zealand are asymmetrical trapezoidal or ribbed profiles, with ribs formed at various spacings and different heights. The angle and height of the trapezoid rib determines the profiles performance under compression. A steeper angle generally gives improved performance, but raises cost per square metre as it lowers sheet effective coverage.

The rib spacing determines the position of the neutral axis, which is the point of zero stress. Bigger spacing between the ribs lowers the neutral axis.

Roof cladding should be designed to withstand both positive and negative design loads. Profile designers use computer software to compare the "moment of inertia" (deflection) and "section modulus" (strength) to find a compromise in different profiles.

Strength and deflection are interrelated but not interdependent, and the design strength is determined by the stress (f) at which permanent deformation occurs. Stress is determined by the section property known as the Modulus of Section (Z). Although it is possible to calculate the sheeting performance from the section properties of the profile, only physical testing can prove the actual capabilities of the profile.

Deflection under load depends on the profile section dimensions or section properties, and it is determined by the Moment of Inertia (I) and Young's Modulus (E). Various metals have different E-values, which means the same cladding profile made in aluminium will deflect more than steel. See 16.3 General Methods Of Testing Sheet Roof And Wall Cladding.

The strength or grade of the metal does not affect the deflection. A high-strength steel profile will deflect to the same extent as low-strength steel profile, but using high-strength steel will increase the point at which yielding or permanent damage occurs. See 12.6 Walking On Roofs.

3.5.7 Fatigue 

The performance of metal roof and wall cladding is affected by stress as all metals may be subjected to fatigue under repeated heavy load conditions.

Metal cladding can fail at a much lower point than the yield stress when there is movement under continued fluctuating stress. A cyclic load test is used to determine the performance of cladding under load reversal.

All metal joints will suffer stress because of movement caused by expansion, vibration, traffic or wind. A sealed joint should have enough fasteners to mechanically resist this stress, because sealant or solder alone do not offer enough resistance. Load spreading washers are used in areas subjected to high wind design loads to give greater resistance to any stress-cracking at fixing holes.

High-strength steel is subject to fatigue, which seldom happens in practice. Other metals, such as lead and copper, are restricted in length or the overall panel size to avoid cracking by fatigue. Sharply folded corners should be avoided on these materials and the minimum radii requirements should be followed.

3.5.8 Continuity 

It is important to understand the relationship of strength and deflection when using multiple span profiled metal cladding, because the performance of roof cladding under load depends on the continuity over several spans.

An adjacent continuous span assists the performance of profiled metal cladding, as continuity can reduce deflection up to 50%. In single spans the cladding is free to rotate at each support, but with continuity at a support the cladding is held down by an adjacent span; reducing the rotation and midspan‑deflection.

Profiled metal sheeting deflects less over intermediate spans because it has continuity at two ends, unlike the end span, having continuity at only one end.

 

 

 

 

Tests for point load have established the ratio between the end span length and the intermediate span length, when both spans will fail at approximately the same load.

The end span of profiled metal cladding should be no more than two-thirds of the intermediate span for optimum performance under both point load and wind load. Neglecting to do this can lead to failure of the end spans when are they subjected to foot traffic.

The endspan condition occurs at the eaves, ridge, roof steps and on both ends of penetrations where a full width sheet is cut. Cladding around penetrations may need additional purlin support, spaced at an equal distance to the endspan spacing.