TechNote#007 Water Tank Roof Covers

Roy G Harrison,  B.Sc(Eng), MIStructE, MIEAust, CPEng
Steven CONRAD Harrison,  B.Tech(Mfg & Mech.), MIIE, gradTIEAust

Action Effects

In 2002, the loading codes were revised and a philosophy of “Design Actions” introduced specifying criteria and procedures for the structural design of a building or structure in “Limit States” format.

The design of a structure for strength must satisfy the “Ultimate Limit State”, whilst the “in service” load events are to be used to satisfy the “Serviceability Limit State”.  The new code also specifies requirements for “Structural Robustness”, so that a structure is detailed such that all parts are tied together to withstand an event, without experiencing damage to an extent disproportionate to that event.

Typically, when accepting or rejecting a design proposal based on risk, a 5.0% probability of failure is adopted for a given life expectancy.  Lower levels of risk may be adopted, but the costs of achieving them increase exponentially, and 5% is considered the economic basis of decision making, until evidence emerges to suggest otherwise.

Up until the 2002 version of the wind actions code, “normal” structures were designed for an approximate 5% probability of failure in a 50-year period.  In 2002, this was modified to 10% in a 50-year period.  Although this could be interpreted as a 5% probability of failure in a 25-year period.

For failure events randomly dispersed across time, both expected life-span and probability of failure need to be quoted, unless expressed as an equivalent annual probability of failure.

From Holmes (2001), risk “r” of the wind speed being exceeded over the lifetime “L”, can be determined by assuming that all years are statistically independent of each other, and

r = 1 – [1-(1/R)]L

where “R” is the “Return Period”.

ROOF CLADDING DESIGN PHILOSOPHY

The requirements for the design and installation of self-supporting metal roof cladding are set out in AS1562.1 “Design and Installation of sheet roofs – Metal ”.

This code has two components to be satisfied independently, viz

  • Serviceability Limit State
  • Strength Limit State

Serviceability Limit State:

This limit state is driven by the Health and Amenity requirements of Section F of the Building Code of Australia (BCA) which sets down the weatherproofing objectives for roofs and external walls.  These are designed to prevent the penetration of rain or dampness into an “habitable area”.

This requirement is achieved by AS1562.1 by ensuring that “…the cladding system shall provide adequate water resistance when subjected to a 100-year storm (AS2180)…”

The code controls potential leakage caused by de-indexing, unclipping, permanent local deformation, fracture or failure of any part of the cladding, or failure of the fastening, by limiting the residual deformations resulting from the application of a concentrated load.

The principal cause of leakage is from permanent local deformation, of a lower sheet, at a sidelap between supports due to the concentrated load (footstep).

Strength Limit State:

This limit state is assessed by the design action that causes failure of:

  • The sheeting;
  • its fasteners; or
  • its supports.

The manufacturer’s of metal roof cladding present these two requirements in their performance specifications in two separate tables.

  1. Serviceability is controlled by a table of “maximum spans” for a given use.
  2. Strength is controlled by either
  • a table indicating the maximum spans for standardized “wind actions”; or
  • a table indicating “wind pressure capacities” in kPa for various sheeting spans

Examination of these design capacities, when available, reveals that the capacity of roof sheeting for strength at their documented maximum spans is much greater than their serviceability loads.

TANK COVER DESIGN

Having identified the rationale behind the design and installation of roof sheeting in accordance with AS1562.1 the following requirements can be deduced for a cover over a rainwater tank.

  1. A cover over a water tank clearly does not protect a “habitable space”, therefore the serviceability (deflection control) requirements of AS1562.1 as implicit in the span maxima provided by the manufacturers of cladding are not applicable.(It is not imperative that the cover be waterproof.)
  2. Safe access for maintenance and installation to be provided incorporating the levels of safety implicit in the relevant design requirements of the “Structural Design Actions Code” AS1170.1 and the “Fixed Platforms and Walkways Code” AS1657.
  3. The strength of the cover to be designed to present a low degree of hazard to life and other property in the case of failure due to wind actions.

These design parameters are achieved if the cover is designed for

  • maximum spans at which the sheeting system can provide resistance permanent and imposed actions,
  • the imposed actions to be consistent with “non-trafficable roofs”; and
  • wind actions for the structure to have an “Annual Probability of Exceedance” for a 10% risk of failure in a design life of 25 years, for non-cyclonic regions.

Bibliography & Recommended Reading:

Environment

  1.  John D Holmes (2001), “Wind Loading of Structures”, spon press
  2. Standards & Codes for Further Reference & Guidance
  3. ¨AS 1170.0                                    Structural Design Actions Part 0 : General Principles
  4. ¨AS 1170.1                                    Structural Design Actions Part 1: Permanent, Imposed and other Actions
  5. ¨AS 1170.2                                    Structural Design Actions Part 2: Wind Actions
  6. Building Code Of Australia
  7. Annotated SA Development Act

Revisions:

  1. [circa 2005  ] : First Published
  2. [08/10/2016] : First Published as a Blog Post