5. Design Requirements: Guidelines for Multi-Point Suspended Scaffolds (MPSS)

• ISBN: 0-7794-9927-1
• Issued: April 2006
• Content last reviewed: June 2009

Section 142.2(1) requires that a professional engineer design the MPSS. The professional engineer must be licensed in Ontario and must have the necessary knowledge and experience to design MPSSs and their components.

5.1 Loading Considerations:

Section 142.2 (2) indicates both the live loads and dead loads that must be used in the design of the MPSS.

5.1.1 Dead loads:

Dead loads are the weight of the MPSS and its components including the loads imposed by the guardrail system. Fixed scaffold and ancillary structures that are constructed as part of the MPSS may be considered as part of the dead load.

5.1.2 Live Loads:

The live loads to be taken into consideration when designing an MPSS are, but are not limited to:

• construction material
• equipment
• debris from demolition, including grit from abrasive (sand) blasting
• workers
• loads from traversing or otherwise moving the MPSS.

The minimum uniformly distributed live loading that the MPSS is to be designed to is dependent on its intended use.

The MPSS must be designed for a minimum live load of 1.2 kN/m² uniformly distributed over the platform surface, except:

• if the MPSS is intended for masonry work, it shall be designed for a live load of at least 2.4 kN/m² excluding storage of masonry units;
• if the MPSS is used for heavy-duty work such as demolition or storage of masonry units or other related material or equipment, it shall be designed for a live load of at least 3.6 kN/m² uniformly distributed over the platform surface.

5.1.2.1 Additional Live Load Considerations

Subsection 142.2(3) requires in addition to the loads specified in subsection (2), a multi-point suspended scaffold shall be able to support or resist the following concentrated loads and wind loads.

5.1.2.1.1 Concentrated Loads

Subsection 142.2(3) requires that in addition to the uniformly distributed live and dead loads, the MPSS must be able to support or resist a concentrated load of 1.1 kN on an area measuring 0.3 metres by 0.3 metres located on the platform at a position having the most adverse effect on the component under consideration. This concentrated load will be factored with a live load factor of 3.0 for MPSS and structural members and 4.0 for suspension systems. 5.1.2.1.2 Loads due to Abrasive (Sand) Blasting operations

If the MPSS is to be used for abrasive blasting operations, then an additional live load allowance for 25 mm grit accumulation across the entire platform area must be included. It is recommended that the unit weight of the grit be specified and verified during field inspection.

5.1.3 Wind Loads

Subsection 142.2(3)(b) indicates that the MPSS shall be designed for wind loads determined in accordance with Table 2.5.1.1 (Design Data for Selected Locations in Ontario) as per section 4.1.8 of the Building Code Act, 1992, assuming a probability factor of at least one in ten.

The effect of wind loading on any environmental enclosures on the MPSS must be taken into account.

5.1.3.1 Special consideration for the wind loading

The wind load derived from Table 2.5.1.1 of the Building Code Act, 1992, may be reduced by up to 30% if the professional engineer who designs the MPSS determines it appropriate to do so. If the professional engineer does reduce the wind load, he/she must make such a determination in writing.

5.1.4 Any Other Applied Loads

Subsection 142.2(3)(c) indicates that in addition to the above loads, the MPSS shall be designed for any other loads likely to be applied to it.

Examples include:

• Stock piling or storage of material
• Dynamic effects of moving equipment
• Material receiving areas

5.2 Methods for Designing the MPSS

The regulation offers alternative design methods which may be used for the design of the MPSS:

• Limit States Design
• Working Stress Design
• Ultimate Strength Design (Testing to failure for components)

5.2.1 Limit States Design

Subsections 142.2(2),(3),(4),(5),(6),(7) and (8) of the Regulation set out the design and load criteria of an MPSS, based on the Limit States Design method in section 4.1.3 of the Building Code Act, 1992.

The values of the load factors stated in subsections 142.2(7) and (8) of the Regulation replace the values of the load factors described in section 4.1.3 of the Building Code and must be used instead. The live load factors for the MPSS are different from those of the Building Code as the MPSS is a temporary structure while the Building Code addresses permanent structures. The MPSS must be designed to have sufficient strength and stability so that the factored resistance is greater than or equal to the factored loads as presented in section 4.1.3.2 of the Building Code. The equation below (see section 4.1.3.2 of the Building Code) was modified for the design of the MPSS by not considering the factored loads due to the effect of temperature change (aT)T.

ΦR ≥ effect of: a_DD + γ Ψ[(a_L)L + (a_W)W]

Where

Φ = Resistance factor applied to the specified material property or to the resistance of a member, connection, structure or foundation, which for the limit state under consideration takes into account the variability of dimensions and material properties, workmanship, type of failure and uncertainty in the prediction of resistance. The value of Φ shall not be more than 0.9 and in some cases it may be less.

R = Resistance of a member, connection, structure or foundation, based on the dimensions and on the specified properties of the structural materials.

(a_D) = Dead load factor (refer to Table 1 below).

D = Anticipated dead load of MPSS and its components.

γ= Importance factor applied to factored loads other than dead load which takes into account the consequences of collapse or failure as they relate to the use of the structure (refer to section 4.1.3.1 of the Building Code). The value of γ shall not be less than 1.0 and may be increased greater than 1.0 in some cases for critical components.

Ψ = Load combination factor applied to factored loads other than dead load to take into account the reduced probability of a number of loads from different sources acting simultaneously (refer to section 4.1.3.1 of the Building Code).

(a_L) = Live load factor (refer to Table 1 below)

L = Specified live load for platform surface.

(a_W)= Wind load factor (refer to Table 1 below).

W = Wind load in accordance with section 4.1.8 of the Building Code using the probability factor of at least one in ten for temporary structures..

Note: The above definitions have been modified to reflect the terms related to Multi-point Suspended Scaffolds .

Table 1 provides a comparison of the load factors for the design of an MPSS.

5.2.2 Working Stress Design

Subsection 142.2(9) of the Regulation allows the designer the option to design the MPSS and its components using Working Stress Design. However, the safety factors for the scaffold and its structural members are required to be at least equal to what would otherwise be provided under the Limit States Design.

5.2.3 Ultimate Strength Design (Determination Of Failure Load By Testing)

Subsection 142.2(10) of the Regulation indicates that if a component is tested to failure, then a suitable safety factor may be applied to the failure load to determine the capacity of the component. Where the failure load of a component has been determined by testing and verified in writing by a professional engineer, the minimum safety factors listed in Table 2 shall be used.

5.3 Redundancy of the System Design

Subsection 142.2(12) of the Regulation requires that an MPSS be designed, constructed and maintained in such a way that the failure of one means of support or suspension will not cause any part of the scaffold to collapse or fail under the most adverse loading condition as determined by the professional engineer who designs the scaffold in compliance with section 142.2.

This requirement means that the suspension system and its components must be designed such that if a suspender or a suspension component fails under any loading condition, the remaining suspension system shall continue to support the platform with the load factors and safety factors specified in subsections 142.2 (7)(8)(9)(10). A failure mode analysis is normally required to be carried out to determine this design requirement.

The above suspension system redundant design feature is required where the MPSS is both stationary or when it is being moved or traversed to another anchorage position.

5.4 Other Design Considerations

5.4.1 Fall Arrest System

Section 142.7 requires a worker on an MPSS to use a fall arrest system when the MPSS is being erected, dismantled, traversed or otherwise moved. The fall arrest system is independent of the MPSS and must be designed in accordance with subsections 26.1, 26.6, 26.7 and 26.9 of the Regulation such that the worker is protected at all times. The design and layout of the fall arrest system must take into consideration the design of the MPSS.

It is recommended that the design engineer of the MPSS evaluate the loads induced by the fall arrest system on the permanent structure.

5.4.2 Movement limiting devices

The design engineer of the MPSS shall consider and specify movement-limiting devices to prevent the unintended movement of the scaffold when traversing or otherwise moving (refer to section 142.2(13)).

5.4.3 Wind speed

It is recommended that the design engineer of the MPSS specify on the design drawing the maximum wind speed at which operations such as traversing or moving of the platform should cease or when further precautions would be required. A wind speed indicator may be provided and located on the platform to allow the workers to monitor wind conditions.

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