Basildon New Town's first multi-storey

UDC 624.03

car park by R. F. Burt, CEng, FIStructE, MInstHE

Synopsis lt is not the intention to add any improved information to that already written on the subject of multi-storey car park design over the last decade, but to record some of the interesting aspects of the design, both structural and otherwise, relating to Basildon's first multi-storey car park which was opened to the public on 6 November 1972.

associated with it are outlined in the hope that those embarking on new designs may find the information of assistance in the preparation of such schemes. Accommodation is provided for 1 112 vehicles and as the structure forms an integral part of the town centre shopping and office complex, its value is enhanced by pedestrian access which links it directly to these amenities. Information relating to the Building Regulations, bearing in mind that this structure is closely integrated with the adjoining shopping development in the town centre, is also given.

The choice of structure and certain detailed information

Introduction The building, which is five storeys high and measures 131 m x 52 m on plan and rises about 15.25 m above ground level, is of split level design, whereby two adjacent floors are staggered at half-storey level and connected by short ramps at a maximum gradient of 1 in 8. With this arrangement eleven parking levels (including the roof) have been achieved. The floor layout is shown in Fig 1.

All cars enter and leave the park from the main road by two entry and four exit lanes. A slip road is provided beyond the exit gates and this permits unimpeded travel beyond the exist barriers and rapid merging with the highway traffic, resulting in less delay at the pay kiosks or coin-operated machines. Because of the large number of cars to be accommodated the car park was divided into an east and west wing, each holding over 550 cars. This was done by installing barriers at all upper floors across the middle of the park in a north/south direction. Identical barriers to those designed as bumper rails were used (Fig 4). To assist traffic circulation these were also painted with black and yellow chevrons and/or bands which have proved very effective. Each barrier was set 0.6 m to 0.9 m apart to permit free pedestrian access.

The main left-hand turn into the entrance is located at the east end of the building and the turning circle provided is 16.75 m in diameter, the inner swept path diameter being 9.75 m. Twin lanes are provided and all control equipment is located on islands 11 5 mm high. A useful reservoir of space is available behind the entrance barriers, allowing cars to stand off the highway while awaiting entry to the car park.

An electrical device in the farm of a ticket issue delay circuit has also been incorporated into the twin gate operating mechanism at the entrance. This ensures that the raising of the barrier is delayed while the other barrier is in operation, allowing single vehicle approach on the entry ramp leading to Floor 1 whcre the motorist may proceed either to the east or west wing as previously mentioned. In this, the motorist is assisted by lane markings, parking signs, directional arrows and illuminated signs indicating when a particular wing is full.

Recommended car park capacities-observations One of the essential requirements in car park design is to ensure uninterrupted movement of cars to and from the car park so that the minimum of delay and inconvenience is experienced by the motorist. This can best be achieved by limiting the capacity of the car park to 1500 cars on four to five floors, depending upon the site available. In addition, the provision of other car parks in strategic positions in and around a town centre serves to absorb the entry of cars and avoids the possibility of cars queuing to enter.

Whether 750 or 1500 are to be accommodated, it is considered prudent to allow for at least two entrance gates and, for every 750 cars leaving the park, two exit gates. This provision has been proved to be adequate. Further sub- division of the car park into smaller units to improve circulation must rest finally with the designer but it has many advantages when catering for capacities in excess of 1000.

Whilst some designers are in favour of providing even larger car parks to accommodate as many as 3000 to 4000 cars under one roof, it is true to say that many motorists are bewildered by the vastness of those already built and experience difficulty in orientating themselves. It is felt that the designer is under some obligation to the motorist who is seeking to park in the shortest possible time. With the adoption of larger parks, much time can be spent in finding a parking bay via a tedious labyrinth formed by cars parked on either side of a network of aisles provided.

It appears, therefore, that by providing a simple layout in conjunction with a smaller car park unit, say 1500 capacity, it will be more readily accepted by the motorist not only from the advantages of speed of entry and parking, but also from the relative ease with which he may locate his car and drive from the park with the minimum of delay.

The foregoing presents a case for sub-dividing car parks when the numbers catered for are in excess of 1000. This reasoning was applied to Basildon's first car park and was found to serve several useful purposes, as outlined:

By dividing the plan area into two (east and west wings) by means of barriers, the distance travelled by any car was found to be half a mile to the most remote parking bay on the car park roof; the pedestrian access to stairs and lifts being so arranged as to be within a short walking distance. The dual set of barriers forming the division between the two sections or wings provided a central aisle for pedestrians, allowing them direct access to the walk- ways located at Floors 3, 4 and 5 on the south side, and hence to the shopping facilities in the town centre. The presence of the barriers served to eliminate possible high speeds which would be encouraged by long uninterrupted aisles. Each wing of the car park has been planned so that it may be closed to traffic, thus enabling maintenance work or cleaning operations to be carried out in safety. Associated with this requirement, separate lighting circuits for each floor of each wing were provided. By sub-dividing the park into two wings, each with its exit ramps, an improvement in traffic circulation was obtained which resulted in a more rapid exit for vehicles.

The Structural Engineer/November 1975/No. 1 l/Volume 53 47 5

, GROUND FLOOR AND F L O O R I ' .

R.(".. rompactor

........

-I In positions shown thb

1

D

Structure The selection of a clear-span layout was in keeping with the basic aim of the design, which was to provide a simple layout combining efficient use of space and column-free parking areas with its obvious car parking advantages.

The structure is of reinforced concrete carried on piled foundations. The parapet walls are concrete, faced with brickwork matching the adjoining buildings on the east and west sides. For column free parking, spans of 16.1 5 m have been adopted, incorporating 90° parking with 4.9 m x 2.45 m parking bays either side of a 6.1 m wide aisle.

The column layout, staircases and ramps have been related to a module of 2.45 m. The deck consists, basically, of long span ribs 630 mm x 300 mm spaced at 2.45 m centres, supporting a 125 mm thick floor slab. At ramps, ribs of similar depth and width are spaced a t 4.9 m centres supporting a 175 mm thick slab and in order to carry the increased load, full advantage was taken of the variable depth of rib available a t the ramp/column junctions, where the columns were en- larged, to provide in the rib an end fixing moment of sufficient magnitude to produce span moments comparable with those

476 The Structural Engineer/November 1975/No. 11 /Volume 53

Fig 7 .

pertaining to the standard rib design, thus enabling all rib soffits to be maintained at the same level.

The cross section through the structure indicated in Fig 2 shows the typical beam and column arrangement. For drainage purposes, all floors and beams were laid to a cross fall of 1 in 100.

The slabs generally are lightly reinforced, the depth being dictated by practical rather than theoretical considerations, permitting both beam and slab reinforcement to be placed without undue conflict.

The clear headroom provided is 2.1 2 m and the storey height 2.75 m. Transverse spine beams which support the ribs are 630 mm x 525 mm wide, their spans varying between 4.9 m and 9.8 m. All columns are 600 mm x 375 mm with the exception of the ramp columns.

Foundations A site investigation showed that the subsoil was composed of London clay to a depth greater than 25 m; the upper eight metres being weathered firm to very stiff brown silty

The Structural Engineer/November 1975/No. 11 /Volume 53 477

P

i I

i -l

( D , 5 l $ 1 S 1 f L m 0.

3

1

Fig 2. Typical section

(D

clay. Below this, very stiff to hard fissured grey silty clay was encountered, representative of the over-consolidation associ- ated with clay of the London Basin.

Careful consideration was given to the question of whether the building should be supported on conventional strip footings and independent bases, or alternatively by straight- shafted bored piles. It was estimated that differential settle- ments of between 12 mm and 20 mm could be expected if conventional strip footings were adopted; these figures being related to a shallow strip foundation constructed 1 a25 to 1.55 m below ground level, exerting a pressure of 162 kN per sq m. The differential settlement figures referred to may well have been acceptable for the garage structure, but because of the pedestrian bridges linking the adjoining properties, such movement would have been expensive to combat. It was decided, therefore, to adopt wholly piled column foundations throughout. These consisted of 450 mm and 600 mm diameter bored straight shafted reinforced concrete piles, individually grouped according to diameter. All piles were spaced generally a t not less than 2.5 times the pile diameter.

The piles were designed as friction/end bearing piles 12.6 m and 15.3 m in length respectively. A factor of safety of 2.5 was allowed in conjunction with a modification factor of 0.8 (as recommended by Professor A. W. Skempton’) to cater for group action when assessing the working load on individual piles. In addition a reduced shaft adhesion value of 0.3 was considered to a depth of eight metres below datum level and thereafter 0.45 down to the toe level when assessing the shaft capacity. Two test piles, 450 mm and 600 mm in diameter, were tested by incremental loading. The load- deflection results obtained from these tests confirmed the design assumptions regarding the differential settlements, which were found to be well within the predicted values.

Wind considerations Calculations were prepared to assess the wind effects on the building and these followed the recommendations laid down in CP3 Chapter V, Part 2. The estimated wind pressure for the particular location was 0.55 kN/sq m and the overall effect of this loading on the structure was negligible because of the broad base of the building in comparison to i ts height.

Although the structure is divided into three sections by vertical movement joints (see Fig l ) , the stability of the building, nevertheless, is unaffected, as the network of columns, together with the various reinforced concrete walls enclosing staircases and lifts, offer adequate shear resistance and stiffness. The end sections (east and west) are additionally stiffened by each of the 300 mm thick reinforced concrete walls located at each end of the building. Each wall serves also to support a ramp on one side and a cantilevered staircase on the other.

Movement joints To allow for temperature variations, two transverse movement joints, nominally 20 mm wide, divide the structure into three sections each approximately 43 m long. These joints are provided at all levels including the foundations and retaining walls and the ground floor slab. With the exception of the latter, all joints in floors, parapet walls and retaining walls contain waterstops.

The separating material between the concrete faces consisted of bituminized fibre board 20 mm in thickness. This was cut back 25 mm from the surface to form a sealing slot. The floor joints were sealed with hot-poured rubber bitumen sealing compound and the vertical wall jointswith polysulphide sealant.

The joints were arranged mid-way between a pair of ribs and the slabs cantilever 1.22 m beyond these. Dowel bars were not provided at these junctions, the horizontal water- stop being the only connection. All joints in the structure were dealt with in this manner, effectively guarding against

water penetration and the possible leaching of harmful salts through the joints. At roof level the 25 mm depth of joint sealant penetrates the full 12 mm thickness of roof surfacing, and into the slot formed in the deck slab.

It is sometimes considered desirable to armour or harden the joint arrises at the movement joints formed in the roofing material (depending on its composition), to prevent their possible collapse under wheel loads. This may be in the form of a mastic strip or epoxy mortar resin laid either side of the joint which, for practical purposes, would need to be about 30 mm in thickness. With this method, which would probably be that most suited to car parks, the slab would be rebated to accommodate the greater depth of mastic to ensure its alignment with the waterproofing of lesser thickness.

It was felt that the additional expenditure involved in forming the joint in this manner was not justified, the material chosen for the general surfacing and described later being considered adequate to withstand the wheel loads at the joints. It is of interest to record that no breakdown of the surfacing or arris has occurred.

Shear connectors at movement joints There are several methods of dealing effectively with the problems arising from the sub-division of a particularly long structure into independent units to cater for movement, as follows :

(a) By providing double columns and beams on a combined footing, with suitable joint filling.

(b) By allowing the sides of adjacent units to cantilever, the ends of which would be separated by a joint.

(c) By forming horizontal scarfed joints within the beam depth and allowing suitable gaps between the two members, including provision for sliding at the bearing.

When considering the design of such joints for the multi- storey car park, it was found that the first method was aesthetically unacceptable and imposed restrictions on vehicle movements. The second method indicated that headroom would have to be reduced. The third method, although considered satisfactory in certain circumstances, relied too much on specialized supervision being available at all stages of construction. It was also found that it would be difficult to provide for the shearing forces occurring within the available cross section. As twenty joints would be required, this method was not pursued.

The method finally devised was a modified version of the scarfed joint principle. This involved the use of purpose-made galvanized mild steel boxes in the form of male and female connectors. The male connector was first cast into a stub cantilevered beam; subsequently the female connector was placed in position and aligned by a bolt, sufficiently tightened to ensure that the bearing surfaces remained in contact. In all, twenty pairs of connectors were incorporated in spans of 4.9 m and 7.35 m comprising 1.22 m stub cantilevered beams which supported 3-66 m and 6.1 m propped cantilevered beams.

In order to standardize the fabrication, all connectors were of the same pattern and designed for the worst condition of loading, namely 335 kN. The contact or bearing area provided was 80 mm x 150 mm.

This design offered an economic and acceptable means of accommodating movement between reinforced concrete members and combined with low fabrication and installation costs made it a practicable proposition. These connectors may also be suitable for adoption in various structures and situations where a simple shear/sliding joint or pin jointed connection is considered necessary. Thus, an existing steel framed building could be extended in reinforced concrete by use of this type of shear connector. The positions of the shear connectors within the structure are indicated in Figs 1 and 2, and details associated with the design in Fig 3.

The Structural Engineer/November 1975/No. 11 /Volume 53 479

Pli-astic 164 hot pourod soolant.

140mm eyeletod hydrofoil

-

-

2 no 32mm dia. ms. borr. 368x178~89 kglm structural tee.cut from m.r. U.C.

SECTIONAL ELEVATION 2 - 2

- Nota: boom roinforccmont omittod for clarity.

Lbhtly tighton 2Omm dia. bolt to onsure machinod surfaces of boaring plat.. or. in c o n t a c t

Mm oithor brock& i s c a s t

into position,

I l- I I I l I I l L

- Full pendration singlo ' v00 butt welds.

Note. al l other wolds to

bo 12mm fill01 welds

b u t o r box roction 20mm m,.. plator. 4 Innor box soction In0 2 O J x l 5 2 x ~ m m m,.. onglos.

SECTION 1-1 Tho wholo of rho a~sombly to bo

hot dippod galvanisod aftor fabrication.

Fig 3. Beam shear connector at expansion joint (max. reaction 335 k N )

Form work Ecomonies were achieved in the formwork by standardizing the beam and column sizes. The slab soffit shutters consisted of purpose made, square and rectangular rebated steel frames inset with 12 mm thick resin bonded plywood panels. This method allowed the plywood panels to be reversed for a greater number of uses. All panels were arranged alternately throughout in pavement pattern, the panels fitting exactly between beam faces. The butt joints between the panels were clipped tightly together to prevent grout loss from the concrete.

Materials and equipment were hoisted by one travelling tower crane 30 m high, operating on 6 m gauge track positioned between grid lines A and B. To cater for the fully fixed end moment conditions applying to the beams in this area, starter bars were left projecting from the column faces on grid line B, enabling the single bay frames and floors at the front of the building to be cast at a later stage. The crane jib was 35.5 m and the lift at this radius was limited to 19.0 kN.

Approximate quantities Concrete pours were limited to about 42 cubic metres daily, which allowed floor bay sizes of 7.35 x 30.5 m to be cast complete with three ribs. Construction joints were formed at mid span in the slabs and between one-quarter and one-third points in the case of the beam spans. To minimize the effects of shrinkage, floor bays were laid alternately wherever possible.

Pumped concrete was used only when the pour was a minimum of 77.0 cubic metres. Over 12 000 cubic metres of concrete was placed in the substructure, superstructure and roads, together with more than 1000 tonnes of reinforcement.

All reinforcement was cut and bent on site and the beam cages fabricated alongside the forms and lifted into position by crane. Sleeves of 38 mm internal diameter to cater for the electrical services were introduced into the beam forms;

480

three per long span and two in each span of the transverse beams.

The approximate quantities of concrete and reinforcement and the minimum crushing strengths at 28 days for each mix used, together with a brief description of its location within the works, are given in Table 1.

Based on these figures and the total number of cars accommodated being 11 12, the approximate quantities of concrete and reinforcement (foundations and superstructure only), provided per car space are 9.7 cubic metres and 0.98 tonne respectively.

Parapet walls All 125 mm thick reinforced concrete parapet walls were faced with Holbrook smooth brown semi-engineering per- forated facing bricks laid in cement mortar (1 :5) with colouring agent added to match the bricks. A splayed concrete feature beam provided at all levels on the periphery of the building supports this brickwork, the first course of which is bedded on a 16 standard wire gauge preformed lipped aluminium underflashing projecting 12 mm beyond the face of the beam to form a drip. It also serves to provide a good visual line and masks any small irregularities in the concrete profile. The flashing is laid in continuous lengths, butt jointed and bedded in bitumen. A 25 mm cavity exists between the concrete wall and brickwork to combat efflorescence and all bricks are tied by means of anchor slots and dovetailed ties. The coping consists of purpose made headers, with two angles chamfered, laid on edge.

In order to prevent the lateral disruption of any section of the coping, 5 mm diameter galvanized mild steel bars were inserted into the perforations to bind the brickwork, each bar being lapped by 75 mm with the preceeding one in adjoining holes to give continuity. Galvanized tying wire was left projecting from the top of the concrete wall at intervals of

The Structural Engineer/November 1975/No. 11 /Volume 53

TABLE l . DESCRIPTION TOTAL AVERAGE LENGTH I VOLUME~OF- ~VOTUME -OF-

NO OF ALLOWING CASTING CONCRETE CONCRETE PILES TOLERANCE OFlSOmm IN STRUCTURE IN ROADS. ETC I LElNFORCEMLh

COVER TO EINFORCEMENI

STRUCTURE. IN

:€INFORCEMEN'

ROADS, ETC. IN

IABOVE CUT OFF LEVEL 1 I l , -

l I SUBSTRUCTURE

biles: 600mm (24')dk 348 15.3M (50.2fi 26N mm 2Omm ReinforcernentF-8-16mm

caps. project 750mrn(2'6'') into at 300mm%.Main bar?

Aggregate ) lOmm dia helical bindi (3750 Ibt/iny%' dia 6.1M (20') long with

LggrLate.

dia 6.1M (203 long with Reinforcement !- S -16mm

lOmm dia helical bindi at 300mm%.Main bo? project 750mrn(2'6'')into caps.

64 - ditto - 12.6M (41.5)

31.25Tonnes

5.0 Tonnes

75 mm

( 3")

75mm

Sulphate resisti

135 cu. metres. ( 1 7 3 ~ ~ . yards.) Sulphate resisti Portland cement 1' 450mm (18")dia

'ile caps,retaining talk , blinding. 70 .O Tonnes - ditto - 1070cu.metres.

(1400cu.yards)

Portland cement. / sulphate resisting /

/

/

27.6N/mm~20mm

cement. Aggregate. / Ordinary portland (40001bf/inz/3;)

(800cu.yards.) Aggregate. 612 cu.metres.

1 : l 5 612 cu. metres (800 cu. yards:) /

31N/mmz/20mm 6900cu.metres.

/ ;round floor slab

10.0 Tonnes /

/ / / eon - mix concrete I d e r ground slab umps and soft rpots.

SUPERSTRUCTURE. olumns,spine beams, ibs.slabs and walls. / / 980.0 Tonne!

/I IERVICE ROADWAY,

lluding bed and Wmm (8') road slab

acking to kerbs and rains.

50mm (6")and 10- I") tean-mix conc re te nder roads and mtways respectively.

cement.

612cu.met res . 1 :l5 (800 cu.prds3 / / Ordinary Rwtbnc

cement.

412 ll87cu.metres 10879cu.metres

/

20 .O Tonnes Total 1096.25Tonnes

450 mm and subsequently tied around the binding bars to anchor the coping.

The joints of the facing brickwork were recessed 6 mm in depth and formed as the work proceeded. Wall joints 20 mm wide were provided in the brickwork a t approximately 14.8 m centres and these were arranged to coincide with the main movement joints in the building. The wall joints and move- ment joints were formed with bituminized fibre board, cut back to a depth of 20 mm and sealed with black polysulphide sealant.

HANDRAIL DETAIL AT RAMPS.

Barriers Until some standardization prevails in the motor industry to ensure that bumper height is maintained a t a uniform level, it would be uneconomical to install protective barriers in car parks to satisfy all conditions. Several types of buffers were considered, but these did not fulfil the design requirements. The design subsequently adopted and shown in Fig 4 will, it is felt, suit the majority of car bumper heights. The cushioning effect of the wooden buffer rai l may also prevent serious damage to vehicles.

In all cases barriers were installed adjacent to walkways and parapet walls to prevent overrun and damage to the walls.

Fig 4. Typical parking barrier

Waterproofing of car park roof There are several systems of waterproofing suitable for use on car park roofs which incorporate a wearing course. Bearing in mind the unusually large areas of roof generally associated with multi-storey car parks which are sometimes provided with this treatment, it becomes obvious that the designer's final choice of material will be influenced not only by cost, but also by other factors which may be related to a particular feature of his design requirements.

The following three methods of waterproofing/wearing course were considered for the car park roof:

a. Mastic asphalt with wearing course This method consists of two coats of roofing grade asphalt laid on a felt isolating membrane to a total thickness of about 20 mm and finished with a wearing course of 30 mm mastic asphalt, the completed treatment being about 50 mm in thickness.

The Structural Engineer/November 1975/No. 11 /Volume 53 481

b. Cold applied polymer resin with traffic surfacing This method consists of applying three coats of polymer resin to the surface. The final coat is usually dressed with a fine layer of graded calcine flint chippings to form a traffic surface. Woven glass fibre is introduced into the application at construction joints and gully surrounds to overcome slight movements in the structure. c. Cold applied bitumen emulsion with wearing course The latter method, which was adopted in the car park design, consists basically of a four-coat application of bitumen emulsion reinforced with a woven glass fibre membrane and finally screeded with a 12 mm wearing course applied direct to the waterproofing. The ingredients of the wearing course consist of Portland cement, mastic flooring emulsion, clean sharp sand and granite chippings. All construction joints and gully outlets are treated quite independently of the four-coat application and a two-coat application of bitumen reinforced with close woven glass fibre is introduced into the surface at these positions.

Advantages (1 ) Less than half the cost of mastic asphalt. (2) Will withstand slight movement of the structure, as in

the case of asphalt. (3) The imposed dead loads from this system of water-

proofing are about one-fifth of the weight of conventional mastic asphalt laid to a thickness of 50 mm. The loading, therefore, is negligible and has little, if any, noticeable effect on the design or on structural costs.

(4) The 12 mm wearing course offers adequate protection to the waterproof membrane against puncturing and subsequent water percolation.

(5) Maintenance costs are minimal and the working life of the waterproofing material is dependent upon making sure that any breakdown occurring in the wearing course, due to abnormal wear or scarring, is dealt with promptly.

(6) Offers a non-slip surface.

Floor finishes and hardening treatment All floors to the car park were first tamped and then trowelled to present an even surface finish. A textured surface was obtained by drawing a stiff broom across the surface in the direction of the fall of the slab to assist drainage to the outlets provided and also to improve tyre grip.

To prevent 'dusting', two coats of a sodium silicate solution were applied to the floors. The chemical reaction between the lime in the concrete and the sodium silicate seals the pores, thus forming an impervious or hard skin, thereby reducing, to some extent, oil, water and petrol penetration, quite apart from eliminating the 'dusting' effect.

Drainage All floors were laid to a cross fall of 1 in 100. The surface water from approximately two-thirds of the total roof area of 6600 sq m at Level 5 was drained into a 150 x 75 mm channel provided with a medium duty cast iron grating and thence by means of fourteen 100 mm diameter and two 150 mm diameter pipes, al l with 100 mm diameter branch connections and cast iron roof outlets to assist the flows. Calculations made in respect of the adequacy of these down pipes assumed a rainfall intensity of 38 mm per hour. The results showed that, for every 645 sq mm of rainwater pipe provided, some 18.6 to 25.6 sq m of roof area could be adequately drained, without the provision of excessively large open channels. These figures are between two and two and a half times greater than those based on the rule-of-thumb method of assessment usually acceptable for domestic purposes.

Drainage of the remaining roof area at Level 5A was pro- vided by eight 100 mm diameter pipes, each catering for a maximum area of 250 sq m with cast iron balcony outlets provided with screw-down gratings. All lower floors are similarly drained and provide for the disposal of water which may arise from a variety of causes.

All pipework above ground level for surface water and foul drainage was prefabricated throughout in mild steel and then galvanized. One-piece stacks to suit the storey heights were provided and the sockets arranged at the underside of the floors to accommodate the section of pipework above this level. Unsightly connections and the abundance of fittings and joints usually associated with cast iron installations were, therefore, avoided. In nearly al l cases down pipes have been located behind columns and are, therefore, screened from vehicles and the likelihood of damage.

Serious consideration was given to the question of incorporating the whole of the pipework installation, both vertical and horizontal, within the beam and column sections, but the design considerations showed that such an arrange- ment was not only impracticable, but also too costly.

The water from the down pipes passes into a surface water drain located in the service road. Here all surface water, including that from the service road, passes through a specially designed three-chamber petrol interceptor before entering the main sewer.

Contrary to the accepted practice of providing trapped gullies at all floor levels, these are not included in the drainage installation because it was argued that the contents of the gully (if dried out) could contain petroleum and therefore provide, rather than prevent, a fire hazard. As the drainage system is well ventilated and any petrol or oil drainings are directed into the three-chamber petrol inter- ceptor, a hazard resulting from the omission of trapped gullies was therefore ruled out.

Toilets Toilet accommodation, with special provision for the disabled, is provided as follows:

Female toilet 5 water closets. 4 stainless steel wash hand basins. 1 hand drier. 1 sanitary disposal unit (not an incinerator).

2 water closets. 6 urinal stalls set flush with floor. 3 stainless steel wash hand basins. 1 hand drier.

Male toilet

The above suite not only caters for the motorist, but because of its location off the main walkway at Floor 3, supplements the existing toilet facilities in the town centre. Limited parking for invalid carriages is also available adjacent to these toilets. The special provisions made for the disabled incorporates the recommendations in BS Code of Practice CP96 Part 1, 1967.

Signing Each floor is clearly indicated by bold 175 mm high numerals painted white on a black square background. These are also provided on the staircases at each access point. In addition, illuminated staircase and parking signs are also provided.

Staircase finishes The staircase walls have a two-coat spray paint multi-colour finish applied directly to the concrete surface, and the ceilings and underside of the stair flights were given two coats of white emulsion paint. The result is pleasing and very effective, no breakdown in the finishes being apparent. The treads, risers and landings have a granolithic finish with carborundum dust surface treatment.

482 The Structural Engineer/November 1975/No. 11 /Volume 53

Fire prevention Building regulations I t is recognized that the Building Regulations, when applied to multi-storey car park designs, are unnecessarily severe, particularly in respect of Part E (Structural Fire Precautions). Although the regulations applicable to car parks fall within Purpose Group Vlll (Storage and General), the fire risk loading is considered to be of a relatively low order when compared with buildings in the warehouse class. This is due to the degree of compartmentation offered by'individual cars in that the fuel contents, together with the upholstery of each vehicle, are reasonably contained, thus effectively reducing the fire hazard; the fuel tanks in themselves serving to check the spread of fire from one tank to the next. A series of experiments carried out at the Fire Research Station, Boreham Wood, where vehicles were set on fire and allowed to burn out, showed the effectiveness of such compartmentation. This was further demonstrated by the fact that the fire did not spread to vehicles in close proximity. Furthermore, the structural steelwork adjoining the ignited car showed no significant rise in temperature.

Adequate natural cross ventilation should be allowed so that exhaust gases, as well as smoke and heat from a possible fire, may disperse freely. The building should, therefore, preferably be constructed with open sides, apart from the protective parapet walls. Failing'this, adequate smoke outlets designed for this purpose should be provided, augmented, if necessary, by mechanical ventilation. The open-sided structure, if adopted, also offers accessibility a t all floor levels for fire fighting equipment to deal effectively with a fire, particularly where hose reels and dry rising mains are not installed as standard items of equipment. As a point of interest, it is only necessary for dry rising mains to be installed in buildings in which the highest floor is more than 12-8 m above ground level.

Basildon's multi-storey car park not only ensures an even distribution of parking within the town centre but, because of its close proximity to existing development, offers shoppers direct access into adjacent shops via bridge links springing from the walkway at Floor 3 (refer to,Fig 1 ). This main walkway which cantilevers 4.8 m provides at the same time a pedestrian route leading directly into the town centre shopping complex. The design allows the tenants of the adjoining shops, if they so wish, to provide access to their premises a t first floor level by bridging the gap anywhere along the southern edge of the walkway (Floor 3). Woolworths' have since altered their premises to include a staircase and escalator leading to this level. The minimum separation distance provided between the shops and the parapet wall of the car park at Floor 3 is 1.525 m.

The upper floors of the car park adjoining these shops span a 21 m wide by 137 m long service road providing a covered approach to the loading bays. The column layout in relation to the road is such that 15.0 m long articulated vehicles manoeuvre with ease into loading bays. A model analysis was carried out at the design stage to establish the space and manoeuvring requirements of such vehicles.

The proposal to locate this building so closely to the existing development was not without its problems, particu- larly as it was first necessary to apply for and obtain relaxation of the Building Regulations in respect of E4 (provision of compartment walls and compartment floors), E5 (fire resistance) and E7 (external walls).

The Department of the Environment, in its circular No. 17/68 (Welsh Office Circular No. 11 /68), offers guidance for designers on the relaxation of Building Regulations Part E (structural fire precautions) relating to above-ground car parks. Application was made to the Department of the Environment seeking approval for a substantial relaxation of these precautions. This was subsequently granted on condition that the covered service area between the car park and the rear of the shops would be fully protected by an automatic,

alternate wet/dry, sprinkler system over its entire area. In addition, drencher protection was advocated above each of the upper level walkways on the south wall face in order to form a curtain of water between the two structures in the event of fire (refer to Fig 2), and extended to positions over doorways in the existing building to protect all loading bays opening onto this roadway.

The service road was further isolated from the lower floors of the car park by a fire resisting wall 125 mm in thickness, in which are provided low level ventilation slots, 2.1 m long by 0.46 m high, the aggregate area of which exceeds the minimum requirement of 2% per cent of the area of the floor level considered. Additionally, all windows facing the car park were reglazed with wired fire resisting glass. Security bars were also fixed to each window.

It was also necessary to re-route all ventilation ducts from the existing buildings which discharged into the car park so that the air intake and foul extract respectively would operate at the roof level of the shops.

Some indications of the important concessions arising from such relaxation are given below:

(a) The total cubic capacity of the building provided without compartmentation was 71 000 cubic metres, thereby exceeding the limit of 21 000 cubic metres specified in Regulation E4.

(b) Because the structure was of non-combustible material, a request was made for the fire resistance rating of four hours, specified under Regulation E5, to be reduced to one hour. In fact, the structure would have qualified for a half hour rating.

(c) The unprotected area (open sided structure) facing the external wall of the adjoining shops required a space separation of l 9 m between building faces in order to conform to Regulation E7, whereas 1 e525 m was provided. The former figure is based on an en- closing rectangle of 12.2 m height and a length of 134 m allowing for sides at least 50 per cent open throughout affording natural cross ventilation. The permitted space separation may be read from Table II of Schedule 9 of the Building Regulations.

Sprinkler and drencher installation The fire protection is designed in accordance with the Ordinary Hazard Group I1 requirements of the 29th edition of the Fire Offices' Committee Rules. At all times during the design stages, close liaison with the Fire Officer and the fire insurers of the plant was maintained to ensure that all aspects of the safety requirements applicable were complied with.

The sprinkler and drencher installation provided operates on the alternate wet/dry system, the flow of water to the sprinkler and drencher pipework being controlled by a set of valves. During the summer months, the system is filled with water and will function as a 'wet' system, whereby the control valve will open automatically, allowing the water to flow, under pressure, to the open sprinkler or drencher heads, at the same time actuating the alarm.

In the winter months precautions are necessary to avoid the risk of freezing and the water in the pipework is drained down, above the alarm valve, and then charged with compressed air-this being known as the 'dry' system. In the event of one or more of the sprinkler or drencher heads operating, air in the system will escape and the reduced air pressure arising from this will cause the valve to trip, allowing water to enter the pipework and flow, under pressure, to the open sprinkler or drencher heads, at the same time actuating the alarm in a similar manner as before.

The heads are designed to operate at 68OC. One vertical air compressor is provided to maintain the air pressure to the installation when it is operating on the 'dry' system. It does not need to be started manually.

The Structural Engineer/November 1975/No. 11 /Volume 53 483

The purpose of the installation is to detect and extinguish a fire in its early stages and to reduce further this hazard by holding a fire in check until it can be completely extinguished by hand appliances, or by the fire brigade.

I t is worth noting that the whole of the plant-hose reel, booster pumps, control panels, suction tank and compressors- are contained within a room 2.2 m x 3.4 m x 3.6 m high. Access is by means of twin doors opening on to the main service road for direct access, if required, by the fire brigade.

Pipes and fittings All pipework is galvanized steel tube. All pipes and fittings are also tapped with a taper thread so as to provide a metal to metal connection without the use of jointing compounds.

Dry rising mains It was necessary to install two dry rising fire mains 100 mm in diameter and these are positioned a t the east and west end of the building. Landing valves are fitted with instantaneous female couplings complete with removable plug a t each landing level. The dry riser is fed from a separate breeching inlet located just above pavement level. To prevent the accumulation of a static charge or misuse by electrical contractors using the pipework for earthing purposes, it is necessary for dry rising mains to be earthed. A resistance to earth, not exceeding that called for by the local electricity supply undertaking, must be provided and the installation must be tested periodically to ensure that this value is maintained.

Hose reels A total of 14 hose reels have been provided. Of these, two are arranged at each floor level to coincide with ramp positions at about one-third points along the length of the building, to cater for split levels 1 /1A to 5/5A inclusive, whilst Floors 3, 4 and 5 straddling the service roadway are served by one hose reel positioned at mid-point along the south side of the building.

In these locations the reels extend to the whole of the floor area to be protected and each reel is installed in corresponding positions at each level. The 20 mm diameter rubber hoses are 37 m in length and are controlled by a manually-operated screw-down valve.

The hose reel pipework, which will contain water a t all times, was insulated with pre-formed 25 mm thick glass fibre rigid sections with a PVC protective wrapping.

Water supply In order to meet the Fire Insurer's requirements to comply with Ordinary Hazard Group II, the water supply must be capable of providing, at the installation control valves, a running pressure of 1.4 bars plus the pressure equivalent of the difference in height between the highest sprinkler head and valves when water is being discharged from the valves at a rate of 725 cubic decimetres/minute and at least 1.0 bar, plus the pressure equivalent of the difference in height between the highest sprinkler head and the valves when the flow rate is increased to l000 cubic decimetres/minute.

The town's water main is located at the rear of the car park in the service road and has a running pressure of 3.1 bars, and a flow rate of 2250 cubic decimetres which is well in excess of the requirements of the above-mentioned rules.

Staircases There are six staircases and these are arranged so that the exits are within a walking distance of 30 m from any position within the car park. By applying this figure, particularly in respect of the larger floor areas associated with car park designs, a sensible layout of staircases and pedestrian routes can be achieved.

With the exception of the open-sided staircases at the east and west ends of the building, al l staircases are enclosed in 175 mm thick reinforced concrete imperforate walls. The shafts, so formed, are ventilated by means of a ventilation unit located on the roof. The open-sided stairs are cantilevered from a 300 mm thick reinforced concrete imperforate wall 11 .O m wide, extending for the full height of the building and carrying an upper level canopy.

The two internal staircases, where points of access are within the parking areas, are each provided with single leaf, inward opening fire doors (2.06 m x 0.84 m). At points of egress double leaf outward opening doors (2.06 m x 0.685 m) are provided. All doors are fitted with an automatic self- closing device consisting of floor springs and top centre pivot. Glazed Georgian wired glass vision panels, 6 mm in thickness and 300 mm in diameter, are included in each door leaf. All doors are of solid construction, 45 mm in thickness and faced with aluminium stucco. All door frames are in hardwood formed from Utile mahogany.

Fire alarm system The fire alarm warning system, which operates on 48 volts DC supply, comprises an indicator panel, battery and cubicle unit, manual contacts, audible alarm and circuit wiring. All contact pushes (smash glass fire alarm contacts) are mounted 1.5 m above main landing levels in all staircases. The main indicator panel includes red dome signals with short caption denoting the area controlled by that contact and separate re-setting buttons for controlling the sirens. This panel is mounted in the Superintendent's office, situated at ground level. The battery cubicle is installed in the main switch room and the audible alarm, consisting of two sirens, is located a t ground level.

Services Lighting The lighting installation was designed to above average standards, to improve visibility and avoid dimly lit areas. As a result, a welcoming rather than dingy image is presented to the public, with the added advantage of combating vandalism and pilfering.

The general level of illumination achieved within the building is, on average, 43 LUX. This was increased to 75 LUX at ramps, staircases and walkways where it was considered essential to provide a higher standard than elsewhere.

Both single and twin 5 ft 65 watt fluorescent tubes in a weatherproof fitting with diffusers manufactured from impact resistant 'Cabulite', chemical composition (cellulose acetate butyrate) were provided. The diffusers are sprung into the body of the fitting and held firmly in position by a spring loaded removable gear tray. The body is constructed from 18- gauge mild steel with welded seams and finished with anti- corrosive epoxy resin powder. A sponge neoprene gasket ensures a complete diffuser seal.

This particular unit, which was new on the market at the time of acceptance, was favoured for its robustness, vandal- proof diffuser, clean neat lines and absence of clips and/or 'Allen' screws (which have a tendency to be lost during maintenance), to hold the diffuser in place. The high standard of workmanship, combined with lower installation costs when compared with other makes, produced an overall minimum saving on fittings in the region of 30 per cent.

All light fittings are ceiling mounted and arranged in two rows, 6.1 0 m apart, over the one-way aisles at each floor level. They are positioned at right angles to the beams and staggered, on plan, at 7.30 m centres and are thus masked by the main down-stand beams or ribs projecting 450 mm below the slab soffits, serving to eliminate glare on approach. Twin 2 ft, 20 watt, weatherproof fittings of similar con- struction have been installed in all staircases and the same fitting has been adapted for use as emergency lighting in these and at various strategic positions along the walkways.

484 The Structural Engineer/&ovember 1975/No. 11 /Volume 53

All circuit wiring is enclosed in galvanized heavy gauge conduit or trunking which is installed on the surface. Separate lighting circuits and switching are provided for each clear span and this arrangement takes into account the vertical divisions previously referred to. In this respect, therefore, the lower floors have two circuits whilst the upper floors have six, viz. three serving either half, the controls for which are located in the Car Park Superintendent’s office on the ground floor with overriding switches in the main electrical intake room located on Floor 1 (room size 4.5 m x 2.45 m x 2.6 m high).

The lighting on the service road and walkways on the south, east and west sides have separate circuits which are controlled by solar dial time switches with over-riding facilities in the Superintendent’s office to cater for overcast conditions which may suddenly prevail.

The staircase lighting operates twenty-four hours daily, seven days a week. The switches are provided in the main electrical intake room, rather than in the Superintendent’s office, to ensure against the likelihood of the lighting being switched off in error.

Floodlighting The roof area of 6600 sq m is lit by twelve 500 watt Tungsten Halogen floodlights mounted on four folding type lamp columns, 7-75 m high. These columns, which were modified with a 400 mm diameter base plate complete with stiffeners, were bolted down to the upper floor, Level 5A, and spaced at approximately one-fifth points along the length of the building and over column heads along grid line B. Three floodlights attached to the three-way brackets are mounted on each lamp column and angled to suit the coverage required, the essential requirements being to ensure that overspill of light beyond the boundaries of the car park did not occur. For maintenance purposes, one trolley mounted spring counterbalance unit is provided for lowering the columns. The whole of the column shaft and attachments were shot- blasted and zinc sprayed.

Emergency lighting An emergency lighting system has been provided which serves not only the staircases, but also the walkways. Sub-circuit wiring from the battery plant located in the electrical intake room is taken to the staircase and walkway emergency light fittings by means of a separate conduit system. The emergency light forms an integral part of the principal fluorescent lighting unit and is wired from a separate terminal block within the fitting. The emergency supply, which is fully automatic, will operate immediately upon mains failure, ensuring not only continuity of essential lighting, but also the service to the fire alarm installation. The emergency lighting equipment is capable of maintaining a supply in a normal 240 volt circuit for one hour after failure of the mains supply,

Ramp heating To ensure freedom from accumulations of snow and ice, a heating system was installed in each of the eight ramps serving Levels 4A to 5 and 5 to 5A. The heating cables were laid in a 50 mm thickness of sand/cement screed and protected by a 12 mm thickness of waterproofing, as adopted for the main roof. The embedded cables are rated at approximately 120 watts per square metre and are controlled by means of an ‘Icelite‘ controller.

Lifts Where a car park is designed on the split level principle, it is difficult to serve every floor directly from a lift, particularly where economics govern the number to be provided. It was decided, therefore, that four eight-passenger lifts of the high rise type would be installed, each with a deep platform to accommodate push chairs. These lifts are located at either end of the building, with direct connections to Floors 1 to 5 inclusive, but not at the mezzanine (half) floors. The cars and doors of the lifts are faced with satin finished stainless steel; this material was chosen to combat vandalism and has proved successful.

Audible alarms for summoning assistance are provided in each lift and these operate in the usual way by pressing an emergency alarm button ; this also activates a signalling device which is connected to the Superintendent’s office.

Conclusion The car park has been in use for approximately two years, during which time it has functioned satisfactorily and has fulfilled earlier expectations. Users are generally of the opinion that the layout is straightforward and easy to follow, with shoppers well catered for by the various access points which lead directly to the town square and also into the various stores.

Vandalism has occurred and fire alarms and hose reels have been damaged. Arising from this, fire alarm contact glasses have had to be replaced frequently to maintain the system, and the need for enclosure of hose reels and associated valves in robust cabinets is under consideration.

It is hoped that this article will make a contribution towards overcoming some of the problems likely to be encountered in the design of multi-storey car parks.

Acknowledgements The author wishes to thank the Chief Engineer of Basildon Development Corporation, Mr. T. R. Button, BSc (London), CEng, MICE, MlnstHE, for his permission to present this paper.

Reference l . Skempton, A. W. ‘Cast in situ bored piles in London Clay,’

Geotechnigue, London, December 1959.

The Structural Engineer/November 1975/No. 11 /Volume 53 485