Minimum ground snow loads clarified
The impact of heavy snowfalls on buildings at low altitudes in the central and southern South Island led to the Department setting a minimum ground snow load in the Compliance Document for Structure (B1). The following technical guidance on how this minimum ground snow load should be applied is for engineers and designers.
The Department has received questions about the minimum snow load on the ground (sg) specified in the amendment to ComplianceDocument B1/VM1, and whether this applies to both the Ultimate Limit State and the Serviceability Limit State.The snow load on the ground value (sg) is the base value used to determine the design snow load in accordance with AS/NZS 1170. The amendment to the Compliance Document states: ’For Regions N4 and N5, the minimum value of sg must be taken as 0.9kPa‘.
The minimum value specified in the Compliance Document is intended to be for strength, ie, an Ultimate Limit State value only. The Standard’s normal provisions are intended to be used for Serviceability Limit State load cases.
At the next opportunity, the Department will amend the Compliance Document to read:
’For Regions N4 and N5, the minimum value of sg for the Ultimate Limit State only must be taken as 0.9kPa’.
How to determine the ground snow load
For the Ultimate Strength Limit State, the value of s For the Serviceability Limit State, the value of s.g would be calculated from Clause 5.4.3 of AS/NZS 1170.3 using a probability factor of 0.84. This is the kp value corresponding to an APE value of 1/25.1 The value of sg to be used is the calculated value – no minimum would apply.
Background
The minimum strength value of 0.9kPa was introduced after considering snow data in regions N4 and N5. This included data from the snow storm of June 2006 where a snow load considerably greater than the design snow load was measured.
The change to the Compliance Document was made because it appeared that AS/NZS 1170.3 significantly underestimated the snow loads at low altitudes in these regions.
The investigation revealed that little relevant data existed. As a result, the Department commissioned NIWA to gather data from extreme snow events in regions N4 and N5. Once sufficient data has been collected, it is intended that the current Compliance Document provision will be reviewed.
1 Note that Table 3.3 of AS/NZS 1170.0 requires an APE value of 1/25 for the Serviceability Limit State. Table 5.1 of AS/NZS 1170.0 does not have a kp value for an APE of 1/25 so the kp value is therefore determined by interpolation between the values for an APE of 1/20 and 1/50.