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Design recommendations for multi-storey and underground car parks

The fourth edition of the Institution of Structural Engineers' Design recommendations for multi-storey and underground car parks incorporates recent developments, design feedback and advice from operators, while retaining parts of the previous guidance that remain current and are of value. Whilst the guidance is primarily intended for structural engineers, some material will be appropriate for other construction professionals and car park owners/operators.

NBS Information Specialist Michael Smith looks at some of the changes included in this latest edition.

Background

Concerns about the safety of car parks expressed by the Standing Committee on Structural Safety (SCOSS) and the failure at Pipers Row in 1997, highlighted the need for improved guidance on the inspection and repair of existing car parking structures.

In response to this, a Partners in Innovation (PII) research project entitled Enhancement of whole-life performance of existing and future car parks was awarded to Mott MacDonald in January 2001. The project aims were to address the relatively poor performance and premature deterioration of some multi-storey car parks in the UK. Improvements were also needed to give better whole life cycle costs and sustainability in new construction and maintenance of existing car park stock.

At around the same time the Institution of Structural Engineers (IStructE) relaunched its task group on car parks to revise the 1984 edition of Design recommendations for multi-storey and underground car parks. IStructE published a revised third edition in September 2002.

The Office of the Deputy Prime Minister also initiated several research and development projects to address the safety of multi-storey car parks and their edge protection. As a result, three Partners in Innovation (PII) study reports were produced.

The 4th edition

The fourth edition design recommendations primarily relate to public multi-storey car parks above and below ground, excluding those using mechanical stacking systems. While it is recognised that national variations to design requirements exist, the principles contained in the guidance are intended to be applicable worldwide. A routine five year review by IstructE highlighted that the third edition was being widely used by owners, designers, operators and builders of car parks, as well as engineers. The review also particularly mentioned issues such as:

The incorporation of the requirements of the Eurocodes
An extension of the recommendations to include temporary and demountable car parks
A review of the durability and exposure recommendations
A review of the structural security and safety aspects of edge protection and protective barriers
The inclusion of the requirements of the Safer Parking Scheme
The inclusion of recommendations for setting up a Life-Care Plan
A review of the operational requirements to include developments in electronic monitoring, variable message signing and payment systems.

In light of this there has been a substantial overhaul of the third edition and the new recommendations address key areas of design in increasing detail. However, the revisions follow the same familiar format of the previous documents. Chapter 7 – Fire considerations has had a major revision, and Chapter 8 – Durability of the structure and Chapter 11 – Asset management have been completely rewritten. It is these areas will be the focus of the rest of this article.

Car park owners and designers will find that the new design recommendations reflect current thinking in all aspects of car park construction.

Chapter 7 – Fire considerations

The structure of a modern multi-storey and/or underground car park is considered a low fire risk. However, the cars parked in it pose a fire risk, as do other items such as refuse bins.

This chapter gives recommendations on fire risk, safe means of escape, limiting the impact of a fire and controlling the spread of fire. It considers:

Specific risks - such as petrol spillage and arson
Fire standards - such as BS EN 1363, 1364 and 1365 as well as Approved document B
Fire fighting equipment - sprinklers, fire alarms and hand held fire fighting equipment
Means of escape – considers escape routes and their dimensions.

In multi-storey car parks, fire safety measures are governed by the following factors:

Volume of the building and/or fire compartment
Height of the building
Use of basement construction
Provision for ventilation
Provisions for adequate smoke control
Distance from the boundary or the distance to other buildings.

As well as these, the recommendations suggest that designers should also consider the use category of other parts of the same building and the separation provided, accessibility around the car park to fire-fighting appliances and provision of fire-fighting equipment within the car park, including dry/wet rising mains for the use of the fire brigade, and most importantly, spacing and adequacy of fire-protected pedestrian escape routes.

Car park materials

Reinforced concrete has been the primary material used in multi-storey car parks since they became the accepted solution for volume parking in the mid-1960s. Even where steel frames are utilised, reinforced or prestressed concrete decks are normally used.

Multi-storey car park structural inspection and maintenance generally involves managing reinforced concrete deterioration caused by corrosion of steel reinforcement. Car park decks and ramps are particularly susceptible to chloride ingress, due to de-icing salts brought into the structure on vehicle wheels and bodywork. Chloride ions also attack the edge restraint barriers and fixings and can damage any exposed steel, including structural frames.

A fundamental issue with multi-storey car parks in the UK is that they are constructed to building standards, but they are effectively an extension of the highway network. Being situated in this very aggressive environment determines their consideration as buildings under extreme exposure, and outside of normal building codes.

It is suspected that many car parks do not already receive a structured inspection and maintenance regime. The approach to inspection must relate to many factors other than steel reinforcement corrosion and related maintenance issues. These issues are the focus of the rewritten Chapter 8.

Chapter 8 – Durability of the structure

This completely rewritten chapter considers the exposure environments in which car parks normally operate. In addition to conventional design approaches for durability, car park structures need to be assessed against the specific exposure parameters:

Natural environmental exposure conditions, in particular proximity to the coast
Local operational conditions, especially the use of chloride-based de-icing salts
Design service life, normally taken to be 50 years.

This chapter also considers the form of the construction, as this has been proven to have a significant influence on the severity of conditions that lead to deterioration of vulnerable details. It looks in detail at structural steel, focussing on corrosion protection and composite steel and concrete decking, and their performance in fire conditions. Chapter 8 looks in further detail at:

Chloride ion attack - the biggest perceived threat to the durability of concrete car parks
Durability of design – strength classes for concrete, drainage, ventilation, weather exposure of the top deck and expected service life
Concrete specification – relative to exposure classes in BS 8500
Corrosion prevention – concrete cover and corrosion resistance of reinforcement, other embedded metals and cathodic protection.

It also briefly covers basements and buried structures in relation to exposure and corrosion.

Chapter 8 also investigates concrete finishes, citing that "Puddles, crude irregularities, cracks and stains may contain chloride ions from de-icing salts that will pose a threat to the durability of the structure". Recommendations are offered for finishes for car park deck and ramp surfaces as well as some guidance on abrasion resistance. Chapter 8 then goes on to discuss finishes for parking and pedestrian areas, vehicle ramps and circulation areas, walls, columns and soffits, and finally basements. This section also gives recommendations on membranes and coatings for concrete, focussing mainly on deck waterproofing, but also considering decorative coatings.

Structural problems

The UK car parking industry was given an important lesson when the Pipers Row car park in Wolverhampton partially collapsed in 1997: that it is important to avoid complacency. Pipers Row could easily have collapsed when fully occupied with parked cars and members of the public.

The warning signs for car park structures had already been recognised by the Standing Committee on Structural Safety (SCOSS) in their tenth report, published in 1994(5).

In addition to chloride ion contamination, structural deterioration can be caused by other mechanisms during service, including:

Impact damage from vehicles, reducing the cover
Carbonation of the concrete over time, resulting in reinforcement corrosion
Alkali-silica reaction, producing expansion and cracking
Freeze-thaw action on unprotected or underspecified concrete
Thermal and shrinkage cracking, providing pathways directly to the reinforcement
Surface abrasion and wear
Failure of protective coatings and waterproof membranes
Poor quality or inadequate cover to steel reinforcement.

A car park does not show signs of deterioration everywhere at the same time. The more information that is available when the first signs of deterioration begin, the easier it will be for inspection observations to be understood and remedies effected.

A free-standing multi-storey car park is essentially a functional building basically composed of a series of floors supported on columns, the aim of which is to provide large areas of uninterrupted floor space. However, a major problem is that most car parks are under the operational supervision of general maintenance staff who do not possess the specialist knowledge to detect the signs of structural deterioration.

As multi-storey car parks are structures that are subjected to a severe environment, the inspection and maintenance regime should be programmed in order to minimise major structural deterioration and consequential disruption caused by major repair works. This is the focus of the newly rewritten Chapter 11.

Chapter 11 – Asset management

Car park asset management is the process of safely and predictably maintaining the structure and its component parts in service, over a known design service life, to deliver reliable financial returns.

Good asset management, in engineering terms, is a whole life cost balancing exercise of operational maintenance costs, capital replacement costs of items at the end of their planned life and revenue interruption when maintenance works are required.

Chapter 11 looks at asset management in the form of a Life Care Plan (LCP), which comprises of:

Handover information pack – which should include a health and safety file
Initial LCP - Additional work may be required to fill gaps in the information, which may include a review of records, visual inspection or limited testing
Benchmark inspection and appraisal – in older structures this is required to establish the level of deterioration in the built structure
Development of the LCP – because the plan is a living document, it is never in a 'final' state but is always subject to update and revision.

Because the Life-Care Plan (LCP) is a living document, the LCP data is commonly captured in an asset management database, allowing the information to be stored and updated easily.

Chapter 11 also considers Upgrading of existing structures, looking at using the LCP or the health and safety file as the first point of reference. This section also discusses structural limitations on modifications and change of use, and the particular limitations that apply for modifications to car parks.

Conclusions

Multi-storey car parks contain moving pieces of machinery, their designs must take into account a range of driving abilities and they are exposed to aggressively corrosive environments. Due to this, and an historic lack of maintenance, many car parks are extremely unlikely to see more than 20 years' service without major repair.

If the Life-care Plan process is fully adopted, as the recommendations and guidance suggest, incidents similar to the Pipers Row event should be avoidable.

However, to be fully adopted, the Life-care Plan must become a working management tool to ensure safety and service longevity, and not simply a defence against possible litigation.