Design codes - buildings / structures

This Technical Measures Document has not yet been reviewed by HSE Reviewers.

The relevant Level 2 Criteria are:

• 5.2.1.3
• 5.2.1.6 (38) c, f
• 5.2.1.7
• 5.2.1.8

Related Technical Measures documents are:

• Control room design
• Explosion Relief
• Inspection/Non-Destructive Testing (NDT)
• Secondary containment
• Active/passive fire protection
• Design codes - pipework
• Roadways / site traffic control / immobilisation of vehicles

Introduction

Buildings and structures are provided on major hazard plant for a number of purposes. Buildings may serve to simply protect the plant or control systems from weather conditions or may be provided as accommodation or shelter. More importantly they may be part of the overall containment strategy ie to prevent, control or mitigate major accident events. Other structures are provided as support for plant either within buildings or externally. Failures of buildings and structures closely associated with major hazards plant may directly impinge upon the plant itself thus initiating a hazardous event. It is therefore clearly important that buildings and structures are designed to withstand all foreseeable loadings and operational extremes throughout the life of the plant.

General principles

Buildings and structures should be designed to sound engineering principles in accordance with appropriate design codes and fit for purpose. Considerations should be:

• Extreme weather e.g. wind loadings, snow loadings, flooding;
• Seismic activity;
• Fire, especially where flammable substances are held;
• Explosions overpressures from plant excursions;
• Corrosion effects from local atmospheric conditions;
• Ventilation to contain and deal with releases;
• Special requirements e.g. warehouses, control rooms;
• Emergency ingress and egress;
• Impact from vehicles and dropped loads from lifting equipment;
• Thermal expansion;
• Positioning of non-process buildings.

Structural design of buildings to withstand natural events

Buildings should be designed in accordance with BS 6399 when considering extreme weather. Part 2 provides advice on design for Wind Loadings and Part 3 for Imposed Roof Loads. There is a strong probabilistic element in the design methods in that it is not possible to define the precise loads to which the building or plant structure may be subjected.

BS 6399 is only appropriate if the response of the structure can be considered to be static; structures with a dynamic response are not covered within the scope.

Historical meteorological data for the plants location which is required to form the basis of the criteria for design should be obtained from the UK Meteorological Office. Other relevant codes are BS 8100 for Lattice Towers and Masts and Shore Protection Manual.

Design of non-standard structures and buildings will require special consideration, including structural analysis calculations as appropriate.

Plants should be provided with adequate stormwater drains to deal with potential flooding. As with other extreme weather situations, the starting point is to consider historical meteorological data. Some regions will be far more vulnerable to flooding and particular attention should be paid to this aspect in positioning safety critical plant, equipment and control systems to allow safe shutdown. Design methods for dealing with stormwater are described in 'Contain liquid spills and improve safety with a flooded stormwater sewer, Mason and Arnold, Chemical Engineering 91, 105'. The design should include catchment basins correctly sized to ensure that contaminated water is not released to the environment. There are two possible systems ie a gravity flow system and a fully flooded system. In gravity flow systems the lines are designed to run about three-quarters full at a slope of about 0.6 to 0.8% to a catch-basin with a sand trap and liquid seal. In the fully flooded system a dam is placed at the entrance to the collection sump which causes the sewer to become fully flooded. The advantage of this system is that it prevents the passage of flammable vapours and burning liquids along the sewer.

A further most important aspect to consider when designing buildings to cope with flooding is the possibility that tanks may float when subject to buoyancy forces. This will be particularly important where tanks are within wells or deep bunds and may bring about catastrophic failure of the tank and associated pipework with subsequent releases of hazardous substances to the environment and possible domino effects.

In general, the UK is a low risk area for earthquakes and it is almost always possible to dismiss the risk from such events on frequency grounds. Where it is considered that the consequences of catastrophic plant failure is of such magnitude that it may not be tolerated at the estimated frequency then the approach taken for the Nuclear Industry as outlined in the HSE document ' Nuclear Safety. Safety Assessment Principles for Nuclear Reactors, 1979' may be appropriate. Earthquake-resistant design involves the consideration of the complete design including ground conditions. Design methods are given in 'Uniform Building Code, International Conference of Building Officials, USA 1991' of detailed dynamic analysis based upon the design basis earthquake or 'DD ENV 1998 Eurocode: Design provisions for Earthquake Resistant Structures (Draft)' may be used.

Structural design of buildings to withstand plant excursions

Buildings and structures should be designed to withstand fires and explosions, if their failure causes additional hazards or domino effects. Methods of Fire Protection are discussed in the Technical Measures Document on Active / Passive Fire Protection. In general, where structures are required to provide fire resistance for a period of time in the event of fire, water spray or insulating coatings can be applied.

Buildings and structures are vulnerable to overpressures, shock or blast waves and missiles generated by explosions. These may be:

• Flash fires/deflagrations;
• Vapour Cloud Explosions (VCEs);
• Boiling Liquid Expanding Vapour Explosions (BLEVEs);
• Pressure bursts;
• Exothermic reactions.

Experimentally, normally designed plants have withstood overpressures of about 0.3 bar (5 psig).

Where the design basis for safety relies upon the building or structure remaining intact following significant explosions/overpressure then appropriate design methods should be used. Design for such events is often carried out by considering the equivalent static pressure exerted by the blast. However it is preferable to use dynamic structural analysis. It is important to use a blast profile that accurately reflects the event being considered. Condensed phase explosives (TNT) blast profiles that are readily available are not representative of profiles for vapour cloud explosions etc. Design methods often include allowing for some measures of explosion relief via fragile roofs or walls which allow venting to a safe place so as not to injure people or damage neighbouring property. This is particularly relevant to warehouses storing drums/cylinders of flammable substances (see HS(G)51 Storage of flammable liquids in containers).

Relevant design codes include 'The design of structures to resist explosions TM5 1300' , 'Protective Construction Design Manual , ESL-TR-87-57' and 'Fundamentals of Protective Design (conventional weapons), TM5-855'.

Missiles may be classified as primary or secondary. Primary missiles are generated from explosions or overpressures within vessels or pipes causing their fragmentation whereas secondary missiles are generated as objects pick up energy from a blast wave. Consideration should be given to eliminating possible secondary missiles such as loose equipment, light fittings etc in the design of buildings vulnerable to blasts.

There are various methods available to determine the effects of missiles upon buildings and structures and design barricades to protect more vulnerable plant. Some simple empirical methods such as that provided in 'The Design of Barricades, for Hazardous Pressure Systems, CV Moore, Nuclear Eng. Des. 5, 81 1967' and 'High Pressure Safety Code, Cox BG, Saville G, High Press Technology Assoc. 1975' and more complex models such as 'Explosions Hazards and Evaluation, WE Baker et al , Elsevier, Amsterdam 1983'. Factors that need to be determined are the size, initial velocity, angle of departure, flight trajectory and the target vulnerability.

For occupied buildings, a methodology is presented in the recent CIA/CISHEC guidance CIA Guidance for the location and design of occupied building on chemical manufacturing sites. Further details are given in the Technical Measures Document on Control Room Design.

Piping containing hazardous fluids should be protected from damage by external mechanical impacts such as those imposed by explosions and missiles. Pipe supports and bridges should be designed with sufficient mechanical strength for the loads exerted on them. This is considered further in the Technical Measures Document on Design Codes - Pipework.

Maintenance of buildings (damage to plant)

Falling masonry and steelwork can initiate major accidents by damaging plant, it is therefore important that buildings and structures are maintained to a high degree of integrity. Regular inspections should be carried out by a competent person and systems should be in place to ensure that any remedial work required is undertaken promptly. See also Technical Measures Document on Inspection / Non-Destructive Testing (NDT).

Structural design of Bunds

Bunds are an essential secondary containment for hazardous liquids, particularly where there are large quantities stored or in process. (see Technical Measures Document on Secondary Containment). Materials of construction should be capable of withstanding the mechanical and thermal shock that occurs on catastrophic failure of the primary containment. Bunds are generally fabricated from brick/mortar or concrete but where liquids are being stored above their boiling point additional insulation, e.g. vermiculite mortar, may be added as cladding to reduce the evaporation rate. Such materials provide adequate chemical resistance to most liquids. However, where surfaces may be exposed to strong acids for longer periods, acid resistant coatings such as phenolic resins should be used.

Care must be taken in the design of the bund wall to withstand the dynamic loads upon bund walls when a large liquid release occurs. Previous practices have been to design bunds to withstand only the hydrostatic load within the tank from which it is released. It has been estimated that the dynamic load at the base of the bund wall may be six times this hydrostatic pressure.

Where bunds are particularly deep, consideration will need to be given to buoyancy forces when filling with liquid. which may cause catastrophic failure of the tank and associated pipework.

Further consideration of the design of bunds is included in the Technical Measures Document on Secondary Containment.

Drainage/spillages

Buildings and structures should be designed to deal with flammable and toxic liquid spillages. It is normal to provide 3 separate effluent systems ie open drainage channels/sewers for clean stormwater run-off, a closed domestic sewer and a closed sewer for aqueous effluent. The requirement for stormwater drains is covered above.

Aqueous effluent systems should be designed to prevent spread of hazardous liquids and vapours around the site. This is particularly important for volatile and/or non-water miscible flammable substances which may find sources of ignition some distance from the origin of the spill. Run-off from plant areas should be directed to interceptors or sumps which may provide separation of non-water miscible substances and sampling prior to discharge. Consideration should be given to:

• Neutralisation prior to discharge;
• Discharge to drums or standby tanks for disposal or re-use;
• Level measurement/alarms to detect spillages;
• Cleaning of sumps to prevent build up of solids;
• Protection against freezing;
• The use of appropriate materials of construction for sumps, floors and drainage channels.

In most instances standard materials of construction ie concrete, brickwork will be adequate. However where strong acids are likely to be present for prolonged periods, consideration should be given to the use of acid resistant coatings. This should be extended to protection of structural steelwork that may be exposed to corrosive liquids and vapours.

Further consideration of design of plant drainage systems is included in the Technical Measures Document on Secondary Containment.

Status of guidance

There are a large number of design codes covering buildings but most are outside the scope of this document. Relevant codes are listed below together with general references and HSE guidance notes on particular relevant aspects.

Codes of Practice relating to design of buildings/structures

• BS 6399 Loading for buildings, British Standards Institution.
  Part 2 : 1997 Code of practice for wind loads
  Part 3 : 1988 Code of practice for imposed roof loads
• BS 476 Fire tests on building materials and structures, British Standards Institution.
  This standard specifies the time / temperature profile for the testing of fire resistant materials under fire engulfment conditions.
• BS 5493 : 1977 Protective coating of iron and steel structures, British Standards Institution.
• BS 3416 : 1991 Bitumen base coatings for cold applications, suitable for use in contact with potable water, British Standards Institution.
• BS 470 : 1984 Inspection, access and entry openings for pressure vessels, British Standards Institution.
• ASME B31.3 Process piping : 1999, American Society of Mechanical Engineers.
• Uniform Building Code, International Conference of Building Officials, USA, 1991.
• Eurocode DD ENV 1998 Design provisions for Earthquake Resistant Structures (Draft).
• TM5 1300 The design of structures to resist explosions.
• ESL-TR-87-57 Protective Construction Design Manual.
• TM5-855 Fundamentals of Protective Design (conventional weapons).
• The Design of Barricades, for Hazardous Pressure Systems, CV Moore, Nuclear Eng. Des. 5, 81 1967.
• High Pressure Safety Code, Cox BG, Saville G, High Pressure Technology Association, 1975.
• Explosions Hazards and Evaluation, WE Baker et al , Elsevier, Amsterdam 1983.
• CIA Guidance for the location and design of occupied building on chemical manufacturing sites, CIA/CISHEC, 1998.
• HS(G)176 The storage of flammable liquids in tanks, HSE, 1998.
  Supersedes both HS(G)50 The storage of flammable liquids in fixed tanks (up to 10000 cu. m in total capacity) and HS(G)52 The storage of flammable liquids in fixed tanks (exceeding 10000 cu. m in total capacity).
  Paragraphs 141 to 152 give guidance on the provision of bunds. Individual bunds are recommended for large tanks, bund walls should be 0.5 m or greater and bunds should hold 75% of the volume of the largest tank. Recommendations for spacing and construction are also given.
  Table 2 provides minimum separation distances of tanks from buildings.
• HS(G)71 Chemical warehousing: the storage of packaged dangerous substances, HSE, 1998.
  Gives general advice on the design of warehouses. Paragraph 108 recommends bunds are used to ensure segregation of incompatible materials should leaks occur.
• PM3 Safety at autoclaves, HSE, 1998.
  Paragraph 20 requires that supporting structures should be designed of good construction and sound materials, BS 470 should be used as the minimum requirement.
• HS(G)64 Assessment of fire hazards from solid materials and the precautions required for their safe storage and use, HSE, 1991.
  Annex C provides information on fire resisting building materials and structures (based upon BS 476).
• HS(G)58 Evaluation and inspection of buildings and structures, HSE, 1990.
  Paragraph 7 details possible causes of problems e.g. poor quality construction, damage due to overloading, hot or corrosive atmospheres.
• GS28/2 Safe erection of structures Part 2 Site management and procedures, HSE, 1998.
  Table 2 gives types of support providing stability and their applications.
• HS(G)139 The safe use of compressed gases in welding, flame cutting and allied processes, HSE, 1997.
  Paragraph 61 recommends that storage buildings containing fuel gases should have wall sections or a lightweight roof to act as explosion relief, venting to a safe place.
• EH70 The control of fire-water run-off from CIMAH sites to prevent environmental damage, HSE, 1995.
  Paragraphs 31 to 41 give guidance on possible methods of containment of fire water, including bunds, lagoons, tanks, sacrificial areas, flexi-tanks, catch pits, interceptors, drain seals (to prevent material reaching sewers), booms and sand bags. It recommends that permanent provision for the containment of large quantities of fire-water run-off, typically several thousand cubic metres and above, may be achieved by lagoons and tanks. These can be remote from, and may serve, several storage areas, receiving the run-off via a gravity flow or pumped drainage system. For bunds, it recommends bund capacity as 110% of the largest tank, allowing 10% for foam addition. Recognition is given that bunds cannot be sized to allow for fire-water. Bund walls should be strong enough to resist hydrostatic pressure which arises when a loss of containment occurs.
• HS(G)140 Safe use and handling of flammable liquids, HSE, 1996.
  Appendix 2 gives information on fire resisting structures for LPG and Highly Flammable Liquids.
• CS21 Storage and handling of organic peroxides, HSE, 1991.
  Paragraph 15 provides recommendations on materials of construction of floors and walls for buildings.
  Paragraph 16 recommends the use of lightweight roofs as explosion relief.
  Paragraphs 17 and 18 provide advice on separation distances, spill control, escape routes and fire protection.

Further reading material

Lees, F.P., 'Loss Prevention in the Process Industries: Hazard Identification, Assessment and Control', Second Edition, 1996.

Mecklenburgh, J.C., 'Process plant layout', Godwin, London, 1985.

HSE, Nuclear Safety. Safety Assessment Principles for Nuclear Reactors, 1979.

Case Studies Illustrating the Importance of Design Codes - Buildings / Structures

Warehouse Fire