Building Code Alert – Safety concerns prompt new requirements for curved roofs

The Canadian Commission on Building and Fire Codes has rushed to issue a change to its provisions for arched roof design — the kind of curved roofs that are typically found in curling and hockey arenas. A “Special Change” to Sentence 4.1.7.2.(2) in Part 4 of the 1995 National Building Code of Canada has been authorized as an urgent public safety issue.

The code change was approved October 29. It is advisable that existing arched or curved roof structures with a rise to span ratio greater than 1/10 be analyzed for the unbalanced snow load criteria of Sentence 4.1.7.2.(2) of the NBC 1995 as modified by this Special Change.

The requirements for snow load calculations in the 1995 National Building Code state that only those roofs with a rise to span ratio equal to or less than 1/10 must be designed for both the specified uniform load on the entire roof surface and the partial snow loads set out in Sentence 4.1.7.2.(2). Recent research has indicated that unbalanced snow loading conditions for roofs with a rise to span ratio greater than 1/10 can also occur, increasing the risk of structural failure.
When the Standing Committee learned of the potential design criteria shortcoming in Part 4 of the NBC, the National Research Council engaged the services of a snow and wind load specialist to model arched roofs and make recommendations on changes to the code’s requirements. Based on these recommendations, the Standing Committee requested the removal of the exemption in Sentence 4.1.7.2.(2) for arched roofs with a rise to span ratio greater than 1/10.

The change to Sentence 4.1.7.2.(2) is shown struck out below:

4.1.7.2. Full and Partial Loading
2)In addition to the distribution in Sentence (1), flat roofs and shed roofs, gable roofs of 15 degrees slope or less and arched or curved roofs with rise to span ratios equal to or less than 1/10 shall be designed for the specified uniform load in Sentence 4.1.7.1.(1) computed using Ca = 1.0, distributed on any one portion of the loaded area, and half of this load on the remainder of the loaded area, in such a way as to produce the greatest effects on the member concerned.

Two other changes are also included:
– in Figures H-2(a) and H-2(b) of the Structural Commentaries, the limit on Case II of 2Ss is replaced by a limit of 3 kPa
— a transition formula is introduced in Figures H-2(a) and H-2(b) to account for the rise to span ratio of the roof

The provincial/territorial jurisdictions and the national/provincial/territorial engineering associations are being alerted to this Special Change. Although provincial/territorial adoption may take sometime, there is nothing in the current wording of the NBC to preclude designers from using these procedures immediately.

For more information, you call Cathy Taraschuk, Codes Group,
Institute for Research in Construction, NRC, Tel. 613-993-0049. E-mail cathleen.taraschuk@nrc.ca