Vehicle Impact Barriers for Elevated Structures

Transcription

1 technical notes CAR IMPACT BARRIERS FOR ELEVATED STRUCTURES Wheel stops should not be relied upon to prevent cars falling off elevated structures. Impact barriers must be provided. Impact barriers must satisfy the load requirements of Australian Standard AS Vehicle Impact Barriers for Elevated Structures Result of an Inquest by Peter J. Taylor Date of Issue: July, 2009 Level 6, 1 James Place, North Sydney NSW 2060 p f e. info@tlbengineers.com

2 Contents 1. Introduction 2. Wheel Stops 3. Vehicle Barriers 4. The Coroner s Conclusion 5. Comments References Appendix 1: AS 1170, Part Appendix 2: AS/NZS

3 1. Introduction 1.1 In January 2009, the Deputy State Coroner issued his findings into the death of Mr. Thomas Lee, who in 2006 reversed his car into a car space on the second storey of a multi-storey Sydney car park, and collided with the external steel barrier which failed. Mr Lee s car fell some nine metres to the ground below, and he was killed. (Ref 1). 1.2 Peter Taylor of TLB Engineers was an expert witness in the Coroner s Court. 1.3 During the inquest, evidence was given that the car park was built in 1986 and the vehicle barriers should have been designed and constructed to withstand loads in accordance with the requirements of the Australian Standard AS (Appendix 1 and Reference 3). 1.4 It was found that the structural design of the steel barriers did not meet the 1981 SAA Code loading requirements for impact from cars. There was a suggestion that some reliance for vehicle restraint may have been placed on pre-cast concrete wheel stops, although these were not present at the ends of aisles where the barrier was the only restraint. In any event, it was found by calculation that the steel barriers which may have appeared to have been substantial to a lay person, were not even structurally adequate for pedestrian loading in accordance with AS Wheel Stops 2.1 The 150mm high pre-cast concrete wheel stop did not prevent Mr. Lee s car from impacting the steel barrier. 2.2 The 1971 and 1981 versions of AS stated: Note: Special consideration should be given to the design of kerbings and guard rails to alleviate possible vehicle impact on the structure. (Refer to Appendix 1 and References 2 and 3). 2.3 During the inquest evidence was given that in some buildings there was complete reliance on wheel stops only to restrain vehicles. 2.4 The Coroner s opinion was that this was an erroneous interpretation of the SAA Code (AS ), and consequently, the steel barriers should have been designed to withstand vehicle impact as if the wheel stops were not present. 3. Vehicle Barriers 3.1 Barriers are intended to minimise the risk of injury to persons by containing moving vehicles, especially on elevated floor slabs. 3.2 The SAA Codes recommend minimum lateral loads and heights of load application for the structural design of barriers. 3.3 Both the 1971 and 1981 loading codes required that the horizontal impact load should be 1.5 x 1500 kg for domestic and 1.5 x 2250 kg for other structures, applied at a height above floor level of 0.45m generally and not less than 0.9 m for trucks (References 2 and 3).

4 3.4 Therefore the steel barrier which Mr. Lee encountered should have been designed to withstand a horizontal force of 1.5 x 2250 kg applied at a height of 0.45m above the top of the floor slab. Evidence was given that structural calculations showed the barrier to be quite inadequate to withstand this load. 4. The Coroner s Conclusion 4.1 Because the evidence presented at the Court hearing suggested that in most areas of this building the designers must have relied on wheel stops only to contain vehicles, the Coroner sought to correct this risk of deficiency in existing elevated car parks by recommending that they should all be reviewed to ensure compliance with the current SAA Code AS/NZS which is more specific in relation to the use of kerbs and wheel stops. (Appendix 2 and Reference 4). 4.2 The Coroner s recommendations include: (a)... that the Minister take steps... to ensure that all multi-storey above-ground car parks built before 1989 be required to upgrade vehicle and pedestrian barriers to comply with the current Australian Standard. (b)... that the Ministers take steps... to ensure that Councils have the powers necessary to conduct audits of above-ground car parks built before 1989 to determine whether they are in need of urgent upgrading to ensure safety. The relevant Australian Standard with respect to design loads for both pedestrian and vehicle impact at present is AS (Refer to Appendix 2 and Reference 4). 5. Recommendations 5.1 The Australian Standard AS/NZS Parking Facilities Part 1 Off-Street Car Parking requires that wherever the drop from the edge of a parking area exceeds 600mm, barriers should be provided to withstand the loads required by AS/NZS AS notes that the purpose of wheel stops is as an optional parking guide to limit the travel of a vehicle into a parking space. They are not mandatory. 5.3 The load capacity of existing barriers and/or the provision of new barriers should also be considered during the maintenance and/or refurbishment of existing buildings. References 1. Engineers Australia, March Standards Association of Australia, AS 1170, Part , SAA Loading Code. 3. Standards Association of Australia, AS 1170, Part , SAA Loading Code. 4. Standards Australia, AS/NZS : 2002, Structural design actions, Part 1.

5 Appendix 1 AS 1170, Part S E C T I O N 3, L I V E L O A D S 3.1 FLOOR, CEILING AND LIKE LIVE LOADS General. The live loads assumed for the structural design of floors, ceilings and the like shall be the greatest applied loads likely to arise from the intended use or purpose of the building or structure and these parts shall be designed to sustain, subject to any reduction permitted by Clause 3.9, whichever of the following loads will produce the most adverse effects. (a) The known loads which will be imposed because of the use of purpose of the building or structure. (b) The relevant uniformly distributed load obtained from Appendix B or Clause 3.1.2, as appropriate. (c) The relevant concentrated load obtained from Appendix B or Clause 3.8.3, as appropriate. (d) Loads due to the stacking of building materials or the use of equipment (e.g. cranes and trucks) during construction or loads which may be induced by floor-to-floor propping in multi-storey construction. NOTE: Close supervision of the construction is essential to ensure that overloading due to causes given in (d) above does not occur. Where floor-to-floor propping is employed close control of the propping sequence should be maintained throughout the construction period and the propping system should be approved by an engineer competent in structural design Uniformly Distributed Live Load on Re-stricted Areas. For floors of buildings in which the uniformly distributed live load given in Appendix B is less than 4kPa, and the floor area supported by any member is 3 m2 or less, the uniformly distributed live load on that part of the floor shall be assumed as one of the following, as appropriate: (a) 4 kpa when the part is 1.0 m2 or less in area. (b) The uniformly distributed load per square metre given in Appendix B when the part is 3 m2 in area. (c) A uniformly distributed load varying linearly between that for 1 m2 and that for 3 m2 when the part is between 1 and 3 m2 in area. NOTE: The provisions of this Clause are intended to provide for possible crowd loadings on restricted areas. 3.2 PARTIAL LOADING. Although Appendix B prescribes minimum live loads it shall be assumed also that the prescribed load can be absent from any part or parts of a structure if its absence therefrom will cause more adverse effects on that or any other part. 3.3 IMPACT AND INERTIA LOADS. The live loads given in Appendix B may be assumed to include sufficient allowance for the effects of vertical impact arising through the normal use or purposes of the structure. Forces produced by installed apparatus or processes involving the acceleration of unbalanced masses (as in reciprocating machinery) shall be calculated and treated as additional live loads*. Horizontal impact or braking forces arising from the movement of vehicles shall be calculated and treated as additional live loads. Such loads, considered as acting separately, shall be- (a) for impact..... not less than 1.5 x the gross static force imposed by the vehicle mass; (b) for braking..... not less than 0.5 x the gross static force imposed by the vehicle mass; and, in the absence of more definite information, the assumed mass of the vehicle shall be taken as - (i) On domestic structures kg; (ii) On all other structures kg. The height at which vehicle impact forces are considered to act shall be generally 0.45m above floor level but in the case of motor trucks it shall be not less than 0.9m. NOTE: Special consideration should be given to the design of kerbings and guard rails to alleviate possible vehicular impact on the structure and it should be noted that the impact forces from runaway vehicles on ramps can considerably exceed the loads specified above. 3.4 CRANE, HOIST AND LIFT LOADS. Live loads imposed by the dynamic effect of operation of permanently installed cranes, hoists and the like shall be calculated in accordance with the relevant provisions of AS Live loads imposed by the dynamic effect of operation of permanently installed lifts shall be calculated in accordance with the relevant provisions of AS LONG DURATION LOADING. Where there is likelihood that the live loads from stored materials or installed plant will be of long duration, the appropriate factors, if any, given in the relevant structural design standard for the materials of construction shall be applied. 3.6 THERMAL MOVEMENT LOADS. Loading effects produced on structures through expansion and contraction of materials of construction due to temperature of moisture content changes shall be allowed for where appropriate. In the absence of more exact local information on diurnal and seasonal changes in air temperature the information given in Table 1 may be used. 3.7 OTHER LIVE LOADS Stairways and Landings. Stairways and landings (other than stairways for inspection and maintenance which are covered by AS 1657) shall be designed for a distributed load of not less than that given in Appendix B. Each individual stair tread shall be designed for a load of 1.4 kn concentrated in any position or a distributed load of 2.2 kn/m of tread measured across the width of the stairway, whichever load produces the more adverse effects. Each intermediate landing shall be designed for the uniformly distributed load or for a load of 1.54 kn concentrated in any position whichever load produces the more adverse effects. * See the relevant structural design standards for the provisions to be made for impact loading.

6 Appendix 2 AS/NZS