T 20.001 structural erection
Rev 03 [Jan 09]

Designing to make management of hazards associated with erecting structures easier

INTRODUCTION

1. Designers can play a major part in making it easier to manage the hazards associated with erecting structures.

2. Some designers have been found to have a narrow view of design, viz, that it requires structural analysis followed by detailed design. This is not adequate for the purposes of their duties under the CDM regulations, which require them to take into account how something will be erected followed by how it will be maintained and, in due course, demolished.

This guidance note makes designers aware of some of these issues.

HAZARDS ASSOCIATED WITH STRUCTURAL ERECTION

3. The erection of structures requires people to work on partially complete structures, where they are exposed to the hazards associated with instability and working at height. In addition:

• a) They often have to work close to machines, eg, cranes, which are used during the erection of structures; 
• b) Sometimes, unplanned work exposes them to health hazards, which arise out of applying a remedial process, eg, cleaning and then painting corroded steel; and
• c) Delays in the programme could mean that many tradesmen are working in close proximity to others, eg, under or above.

4. Therefore, the hazards associated with erecting structures and associated temporary works may be summarised as including:

• a) Temporary instability;
• b) Falls from height;
• c) Lifting: Overturning of cranes;
• d) Working on or near fragile materials;
• e) Handling heavy unwieldy loads;
• f) Collapse of temporary works equipment;
• g) Danger to adjacent properties.

WHAT DESIGNERS SHOULD DO

Therefore, in order to prevent, as far as possible, workers exposure to the hazards in 4, designers should consider:

Designing to minimise temporary instability

5. Structures in their temporary state could become unstable for many reasons for this, which include:

• a) Omission of temporary works, which usually occurs when the design is out of the ordinary and usual erection practice is insufficient, eg:
  • i) Slender rafters, which require additional bracing until the roofing is fixed;
  • ii) Portal frames where the cladding contributes to sway stability;
  • iii) Long-span members, which require bracing until another one is connected, eg: some bridge beams
• b) The part-erected structure is inherently unstable, which could occur when:
  • (i) It is stabilised by other [remote] parts of the permanent works, eg:
    • Shear walls,
    • Shear cores,
    • Adjacent structures;
  • (ii) Provision for lateral stability is either by unsymmetrical bracing or contributed to by other members to be added later, eg: cladding; 
  • (iii) It is subjected to significant unanticipated [by the designer] construction loads, eg:
    • Complete roofs being lifted;
    • Timber or pc floors supported on “green” masonry walls are loaded out too soon;
  • (iv) A member has inadequate seating on another member in the temporary state, because erection tolerances add up unfavourably, eg:
    • pre-cast slabs on narrow flange beams,
    • purlins on main rafters,
    • beams on corbels.
  • (v) Members in isolation cannot sustain normal erection loads, eg:
    • Pinned base columns under lateral loads from ladders,
    • Long-span beams and trusses.
• c) Temporary supports are removed prematurely, which usually occurs when a structure is apparently complete, eg:
  • (i) Composite beams and panels supporting “green” concrete,
  • (ii) Portal frames, which rely on ties into the floors to carry significant horizontal thrust at their base;
  • (iii) Guying systems supporting columns.

6. While it is preferable for designers to eliminate these hazards by design, the minimum requirements are for them to:

• a) Advise the Contractor that these hazards exist;
• b) Inform the contractor about design assumptions and design forces, eg:
  • i) Construction loads allowed for,
  • ii) Portal base horizontal thrusts, etc.

Designing to minimise falls from height

7. When erecting structures, workers often find themselves in precarious positions, eg, straddling unattached beams, working towards an open edge, etc. This is against the law and, although it is a matter to be controlled by contractors on site, designers should give consideration to details, which could help to limit the workers exposure to these hazards or mitigate the consequences of them, for example by:

• a) Contacting suppliers of temporary edge protection and discussing how their products could be integrated into the design of the permanent works;
• b) Designing elements, which allow the attachment of anchorage lines or nets, where appropriate;
• c) Accounting for erection and manufacturing tolerances to minimise the need for vigorous manhandling while slung at height, eg:
  • (i) pc slabs on steelwork,
  • (ii) Steel beams between columns;
• d) Specifying a good quality sub-base for ground slabs, which would carry the loads from the necessary cranes and mobile platforms required for the erection of the building envelope, eg: for MEWPs erecting steel envelopes;
• e) Removing the need for some work at height, eg, getting rid of sag bars for purlins;

8. Further information can be found in Technical Series T 20.008  Working at Height.

Designing to minimise hazards while lifting

9. This usually means being aware of the conditions under which a crane could overturn or collapse.

10. While it is not always possible to limit the weight of components: members or frames, or the radius over which they have to be lifted into position, designers should give consideration to the following:

• a) Cranes need working room therefore avoid heavy lifts or large lifting radii on congested sites;
• b) The radius of a lift limits the weight that a mobile crane can lift; even moderate weights lifted over a large radius could create a lifting hazard;
• c) It is always helpful to know the weight of components.  Therefore, inform the contractor about maximum loads of components;
• d) It is essential to know where the centre of gravity is, especially if it is not the middle of the load;
• e) Lifting points are always helpful;
• f) Cranes need good foundations therefore do not specify designs, which require heavy lifts on sites where the ground is poor;
• g) On exposed sites, wind on assemblies with large effective areas could create a lifting hazard;
• h) Long span large section beams have a significant momentum when they start to swing.

11. Where you have designed heavy items to be lifted or have contemplated lifting moderately heavy loads over a large radius, discuss the options with a competent crane supplier.

12. Further information can be found in General Information Series I 002 Safe Working with Cranes.

Working on or near fragile materials

13. This is a problem largely, but not solely, associated with roofing.  Designers should only specify non-fragile components and assemblies.

14. A non-fragile roof assembly is defined by ACR[M]001:2000 and the principles of this test could be extended to other assemblies.

Handling heavy or unwieldy loads

15. This is a problem of specification.  If lighter alternatives exist, specify them instead.  For example:

• a) Use light concrete blocks or bricks instead of heavy ones. Where heavy blocks are unavoidable, eg, for acoustics, specify half size blocks to reduce the weight. Further information is given in Technical Series T 20.015  Concrete blocks;
• b) Where steel sections have to be lifted manually, eg: some lintels, consider composite members like back-to-back channels or angles instead of I-beams. Further advice is given in Technical Series T 20.005  Refurbishment;
• c) Where standard details govern the specification, discuss the possibility of removing the heavy objects with the specifier, eg: concrete kerbs;

16. Where it is not possible to specify lighter alternatives, inform the contractor about their weight. Or investigate whether mechanical installers exist and design these components to be compatible with their use. For example, machines are available to install heavy glazing units and concrete kerbs.

17. Even “light” components can be unwieldy and difficult to manhandle if their shape is unusual or the centre of gravity is away from the geometric centre. Therefore designers should consider:

• d) Providing seating cleats for members;
• e) Providing lifting points, which allow vertical and horizontal members to be dropped into position vertically and horizontally;

Collapse of temporary works equipment

18. While it is not the duty of the permanent works designer to design the temporary works, it would help designers of temporary works if the design of the permanent works incorporated components, which could be used to stabilise the temporary works, eg: by providing for the tying of scaffolds.

19. Further information can be found in Technical Series T 20.006 Temporary Works Equipment.

Designing to minimise danger to existing adjacent structures

20. Sometimes the location of existing buildings or structures could limit the erection processes that are possible. For example:

• a) Noise and ground vibrations may not be acceptable, eg, close to hospitals;
• b) Excavation [without agreed precautions] may not be possible close to canals or other watercourses;
• c) Crane operations are restricted by a number of constraints – see I 002.

USEFUL REFERENCES

BS 5531:1998 – Safety in erecting structural frames

Erectors Manual – A guide to Health and safety in Steel Erection [BCSA Publication No 16/93]

National Structural Steelwork specification for Building Construction – BCSA Publication No 1/89