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P R O J E C T

P R O F I L E

P R O J E C T

P R O F I L E

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All advice or information from the Br itish Cement Association is intended for those who will evaluate the significance and limitation of its contents and take responsibilityfor its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted. Readers should note that all BCA

publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

97.324

First published 1992

ISBN 0 7210 1444 5

Price Group C

British Cement Association 1992

Published by the British Cement Association on behalf of

the industry sponsors of the Reinforced Concrete Council.

British Cement Association

Telford Avenue, Crowthorne, Berkshire RG11 6YS

Telephone (0344) 762676

Fax (0344) 761214

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P R O J E C T P R O F I L E

2

Merchant House and Woodchester House are

two office buildings that form a significant part of

the City Harbour development in the heart of

Londons Docklands.The site adjoins the east side of Millwall Docks

and is at the centre of an expanding communications

network - only minutes from City Airport and

immediately adjacent to Crossharbour Station on the

Docklands Light Railway.

The scheme for the five-acre development

includes three major office buildings (totalling some

240 000 ft2), individual office units, a luxury hotel, a

multi-storey car park and other ancillary buildings.

The architecture and planning take full advantage of

the sites extensive dockside frontage, creating a

modern working environment with attractive views.

Merchant House and Woodchester House overlook

the water and border a tree-lined piazza called

Watermans Square.

The development of City Harbour was divided

into five phases, the first of which consisted of

Merchant House and the smaller Woodchester

House. For these two buildings, the development

team chose precast concrete frames. Neither

structural steel nor in-situ concrete frames could

meet the exacting lead-in time and erection

programme requirements; neither could theyprovide the economy and quality of precast concrete.

THE PROJECT

The City Harbour development is on the former Olsen 3 site

of Milwall Docks

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Description

Merchant House and Woodchester House offer

high-quality office accommodation in a central

London location, but with much lower occupancy

costs than those in the City. Their design offers the

flexibility for changing the layout to accommodate a

wide range of future uses.

Each building may be used as a self-contained

office headquarters, with its own entrance, servicing

and car parking, or may be sub-divided into separate

tenancies. The two buildings incorporate varied

floor plans, with balconies and terraces that form a

distinctive stepped profile and provide unobstructed

views across the water.

The buildings have the following principalfeatures:

Eight levels of office accommodation in MerchantHouse and six in Woodchester House.

Ground floor reception lobbies with high-speedautomatic passenger lifts to all floors.

Flexibility for open-plan or cellular office layouton a 1.5m grid.

Raised access floors for electrical and communi-cations installations.

Fully accessible suspended ceilings allowingpartition-head fixings on a 1.5 m module.

Knock-out panels in floors for additional verticalservices distribution.

Fixed sealed double glazing with tinted solar-control glass.

Full air conditioning. Terraces, conservatories and balconies. Dedicated parking with additional visitor spaces in

Watermans Square.

BUILDING SPECIFICATION


The distinctive dockside elevations

Office adjoining one of the terraces

Conservatory office

Terraces on Woodchester House

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4

Space provisions

The net floor area of Merchant House is 55 000 ft2,

with typically 7000 ft2 per floor from ground to fifth

floor levels, and 6000 ft2 and 5000 ft2 respectively at

sixth and seventh floor levels. Woodchester House

has a net floor area of 37 000 ft2, typically 7000 ft2 perfloor; but reducing at the third, fourth and fifth floor

levels. Because of the high water table, the buildings

have no basements.

The internal clear height, from finished floor

level to underside of ceiling, is 2.6 m for all office

areas. A 700 mm deep ceiling void and 150 mm deep

floor void are provided for installing services.

Structure

The superstructures consist of precast reinforced

concrete columns, shear walls, beams and stairs, andprecast prestressed hollow-core floor units. The

precast frames are based on 6.0 x 6.0 and 6.0 x 7.5 m

grids, and are supported on piled foundations.

The floors were designed to carry an imposed

load of 5.0 kN/m2 (including 1.0 kN/m2 allowance

for partitions) plus 0.4 kN/m2 for services.

Cladding

The aluminium cladding system is based on

top-quality products and workmanship, and the

elevations feature dark grey glazing units usedfull-height or within colour-coated panels.

Services

The buildings have full variable air volume (VAV) air

conditioning, with individually controlled terminal

boxes in each structural bay, at all levels, to provide

an efficient and highly flexible system.

Precast concrete frames were specified for both buildings

Details of the curtain wall cladding

Typical office floor section

showing precast concrete frame

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5

Facilities for power, telecommunications and data

wiring are provided within the 150 mm raised floor.

The ceilings have integral light fittings and the lighting

zones can be rearranged to suit future requirements.

Each floor is served by ten-person automatic lifts,

and additional lifts, also serving as firemens lifts,adjoin the goods entrances.

The east

elevations

The dockside setting of the two buildings

Architecture

The site encompasses a landscaped setting on the

former Olsen 3 site, to the east of Millwall Docks on

the Isle of Dogs. The City Harbour masterplan

integrates three separate self-contained headquarters-

style office buildings, seven smaller terraced frontdoor office units, a major hotel, a multi-storey car

park, a health centre, restaurant, and wine bar.

Merchant House and Woodchester House occupy

the east and north sides of Watermans Square - a

paved and tree-planted piazza. The elevations take

full advantage of the sites extensive dockside

frontage to make a strong visual impact.

The exteriors are clad with a high-quality

colour-coated aluminium curtain wall system, the

solid panels having outer panels that act as a

secondary rain screen to the fully weatherproofed

inner system.

In addition to its attractive appearance, the

curtain wall system allows for future refurbishment

or alterations without disturbing the integrity of the

inner system. At ground level the outer cosmetic

panels are a slate-stone-composite material that

provides a more robust and vandal-resistant surface.

Tinted solar-control double glazing complements

and mirrors the surrounding waterside. Plant con-

tainers on terraces and balconies provide visual relief.

DESIGN

Watermans Square

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6

The internal lighting, fittings and finishes in the

offices are all of the highest standard. The interior

design fully anticipated that occupiers would require

flexible internal layouts, and the scheme benefited from

a full space-planning exercise at its conceptual stage.

Structure

A number of different frame designs were considered

and reviewed to determine which would best suit

the project requirements. Least cost and minimum

time were paramount. Flexibility, quality and ease of

erection were also crucial.

Only precast concrete could meet all these

criteria. It was found to be cheaper and quicker to

erect than either steelwork or in-situ concrete

frames, and it beat steelwork on lead-in time.

The use of precast, prestressed concrete floorunits obviated the need for transverse internal floor

beams and kept the floor plate thickness to a

minimum. This in turn generated the maximum

depth of ceiling void. The flat soffits of the floor

units, and the clear longitudinal runs, made it easier

to install services. They also allow for future

alterations to services and office layouts.

The floor units in Merchant House were 150 mm

and 200 mm thick, and 200 mm thick units were

used in Woodchester House. Small service

penetrations were formed in situ through the voids

of the hollow-core floor units. Major penetrationswere preformed within the floor layout, and

knock-out panels are provided to allow for possible

future modification of the services.

The precast reinforced concrete internal spine

beams were restricted to 500 mm deep overall to suit

the distribution of services. The precast spandrel

beams are generally 900 mm deep, with a downstand

providing a fire break in the ceiling void below, so no

time-consuming blockwork was needed. Upstands

for roof balcony and terrace details are provided by

1350 mm deep spandrel beams.The versatility of precast concrete easily enabled

the stepped profile, irregnlar-shaped terraces, and

curved balconies to be produced. A late change in the

location of the lifts was also easily accommodated.

The components are durable and quality assured,

and have immediate structural loading capacity. They

also have in-built fire resistance, both in service and

during construction.

The in-situ concrete slab at ground level is

supported by precast concrete beams resting on the

pile caps.

Lateral stability is provided by a minimum number

of precast concrete shear walls located around the stair

The simplicity of the precast concrete frames

Floor units span onto l ongitudinal beams

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wells and lift cores. Thus the structure behaves as a

braced frame, which permitted economy of

column design. As the building was over five storeys

high, full provision against progressive collapse was

provided by simple horizontal and vertical ties.

The design of the precast columns took fulladvantage of high-strength concrete and the quality

control available under factory conditions. The use

of grade 50 concrete resulted in 450 mm square

internal columns at the ground floor of Merchant

House. The column sections were cast up to

11.4 m long to support three storeys, with cast-in

rectangular hollow section billets to carry the shear

forces from the beams.

The precast concrete stair units, with cantilevered

half-landings, are supported by precast mid-storey

beams and floor landings.

Generally the connections were made, and the

ties installed, by stitching-in with in-situ concrete.

Channel and other fixings, as required for the

following trades, were cast into the precast units to

speed construction on site.

The detailed design, manufacture and erection

of the precast frames were carried out by the

manufacturer. This single-point responsibility was seen

as another benefit of precast concrete construction.

Services

The services were designed for an office occupancy

level of 10 m2 per person with a fresh air supply of

12 litres per person per second and an interual tem-

perature of 21C 2C with 30 to 70% r.h.

The warm air supply of the air conditioning serves

the perimeter wall panels and is controlled from the

rooftop plant room. Each panel is individually isolated

and the system is zoned, with thermostatically

controlled circuits.

The principal feature of the heating system is that

the cold radiation and downdraught effects of the

glazing are offset by the strategically placed heating

panels in the flush under-sill panels. This arrangementensures that the heaters do not encroach upon usable

floor or wall space.

The gas boilers and all other major plant are

contained within plant rooms and screened

compounds at roof level.

The lighting level was set at 500 lux average and

was achieved using continuous twin tube low-glare

recessed luminaires which incorporated return air

slots. On-floor power requirements were assessed at

25 w/m2 with outlets provided in the raised floor.

Other services included hot and cold water, lifts,sanitation, drainage, fire alarms, and facilities for

telecommunications and data wiring.


7

Precast walls for the lift shafts

and stairs act as shear walls

The pland room -

Woodchester House

Precast column units are up to 11.4 m long

Flush under-sill heating

panels in office areas

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8

General

A straightforward approach to construction was

adopted, and met the principal requirements of cost

and speed.

The buildings are founded on a combination of

driven and bored piles. The precast frame proceeded

quickly, with a floor being completed every two

weeks, on average.

No fire protection, either in the form of sprays or

boards, was required. No scaffolding was needed for

the cladding, and access on foot was gained via the

precast stairs. No drying-out period was necessary

which, together with minimal propping requirements,

gave following trades rapid and clear access.

Substructures

Generally, the driven piles have a nominal diameter of

400 mm and a safe working load of 110 tonnes, but

vibration during construction close to the existing dock

wall and sewers was identified as a potential problem.

Therefore, bored continuous-flight-augured piles

(CFA), with a nominal diameter of 600 mm and a

safe working load of 90 tonnes, were used within a

distance of 24 m from the existing dock wall and

within 10 m of the main sewers. Ground vibrations

were monitored for both forms of piling.

Where necessary, the pile caps were restrained

laterally by the precast concrete ground beams.

Holding-down bolts for the first section of the precast

concrete columns were cast direcdy into the pile caps.

Superstructures

Delivery of the frame components was carefully

scheduled so that they could be lifted directly from

the delivery vehicles and placed into position using a

rail-mounted tower crane. This had the capacity to

lift the heaviest component, which weighed 6.2

tonnes, and was supplemented by mobile cranes asrequired. Erection of the frame was unaffected by

weather, and the required storage area was minimal.

The columns were usually erected in three-storey

lifts and temporarily propped. They have bolted steel

base plates and the splice joints were formed with

loose continuity reinforcing bars inserted in

preformed ducts in the ends of the units. When the

units had been lined and levelled, the voids were

filled by pressure grouting.

The main precast concrete beams were securely

seated on projecting RHS steel billets cast into the

columns, and the tops of the beams were fixed in

position by an angle cleat. Generally the beams sitThe first lift of columns

for Merchant House

Column base detail

CFA piling in progressCONSTRUCTION

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9

between columns with a nominal 10 mm gap left at

each end to allow for production and erection

tolerances. The gap was pointed with cement mortar

to enable the remaining void around the billet and

seating cleat to be enveloped in poured grout.

Curved edge-beams were temporarily propped untilthe floor was completed.

The prestressed concrete floor units span from

edge spandrel beams to internal spine beams. They are

seated on wide nibs which are an integral part of the

precast beams. These nibs not only provide ample

bearing for the floor units, but also allow space at the

ends of the units for the longitudinal ties. These consist

of unstressed 12.7 mm prestressing strands anchored

by right angle returns at each end of the building.

Transverse ties were formed by loose

reinforcement placed and concreted into open

hollow cores, and hooked over reinforcement leftprojecting from the top of the precast beams. The

floor units were delivered with the concrete above

every third core already removed at each end. Similar

details were used at the sides of the floor.

The joints between prestressed concrete floor

units, and the stitching between floor units and

beams, were completed using a grade 40 concrete

with 75 mm slump and 10 mm aggregate. This

ensured the diaphragm action of the floor against

lateral loads. No formwork was necessary as the

precast units completely surrounded the voids. The

plant room floor units have a 100 mm thick30 N/mm2 structural concrete topping to carry an

additional imposed load.

The precast concrete shear walls were inter-

connected by vertical lacing bars threaded through

the projecting link reinforcement allowing the joint

to be finished with 40 N/mm2 in-situ concrete.

The stair units were lifted into position on to

simple halvingjoints at floor level, and on to

dowelled bearings at mid-storey height. Flights were

precast to finished floor levels, but landings were

screeded. The cantilever semicircular half landings

were tied with steel channels that also provide lateralsupport for blockwork.

The efficient and disciplined approach to design

resulted in the precast concrete subcontractor being

able to use cross-sections with a high repetition

factor, thus minimizing costs. The resulting

construction is accurate and well within the specified

tolerances. The choice of precast concrete provided

rapid and easy construction to give a robust structure

with economy of design.

The frames contain 1100 precast concrete column,

beam, wall and staifflight components, plus 9400 m2

of precast floor units. The entire structural frame of

Merchant House was erected in 16 weeks at a rate of

Floor/beam/perimeter column

connection detail

Precast stair unit being lifted into position

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10

approximately 95 precast units per week. The surface

finish throughout was ex-factory type A in accordance

with BS 8110. During production, strict quality control

procedures were maintained under a BSI-registered

Quality Assurance Scheme. The units were also

inspected by the design team before despatch to site.

Following trades

The inherently dry construction and total absence of

formwork allowed safe, clear and rapid access for

following trades using the precast stairs. Fixings for the

cladding, temporary handrails, and the conservatory

and plant room steelwork were cast into the factory-

produced units to speed following trades. For

example, the aluminium curtain walling on

Woodchester House was fixed only two floors

behind the precast concrete frame.The curtain walling is an aluminium stick

system and the two-or three-storey high mullions

were fixed before infliling with transomes, and the

solid and glazed panels.

Meanwhile internal concrete block walls were

being constructed just ahead of first-fix services and

joinery. The absence of transverse beams or drops

encouraged the prefabrication and fast installation of

M & E services. Small service holes were formed in

situ through the easily locatable voids in the hollow-

core units. Supports for the services and suspended

ceiling grids were fixed directly into the concretesoffits using drilled and self-tapping devices.

Suspended ceilings and raised floors were installed

before final decoration. Wet trades were confined

mainly to the stairs, lobbies and toilets.

Flat roofs and terraces are of inverted roof con-

struction laid to falls, and were made weathertight

before first fixing began. The waterproof membrane

is covered with 100 mm insulation board protected

by concrete paving slabs.

Merchant House was completed ahead of

programme in 22 months and Woodchester Housewas completed in 20 months.

THE PRECAST CONCRETE SOLUTION

The frames for both buildings were quality assured

Curtain walling

during erection

Paving on the

completed terraces

Merchant House and Woodchester House clearly

demonstrate the advantages of using precast concrete

construction to meet clients requirements for the

following:

Speed and economy - the precast concrete framewas the least-cost solution with the lowest storeyheight, and had a shorter lead-in time than a steelwork

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11

frame. It gave rapid erection with less contractor

preliminaries and quicker returns on investments. It

also gave rapid, clear and safe access for following

trades. High repetition factors and the use of high-

strength concrete produced further cost benefits.

Simplicity - the disciplined approach to design ledto repetitive elements of design, detailing,manufacture and construction. Construction was

uncomplicated, with simple connections, no form-

work, no weather problems, cast-in fixings for

following trades, and minimal storage and propping

requirements. Flat soffits and preformed holes made

services quick and easy to install.

Quality - the robust and rigid frames were builtfrom quality-assured factory-produced components,

to give accurate and durable structures. The quality

of construction was further assured by one-point

responsibility for the detailed structural design,

manufacture and erection.

Flexibility - the precast concrete frames accom-modated balconies, terraces, curves and irregular

shapes, and also permitted a late design change. The

precast floors allow for flexible future office

planning. The flat soffits and large ceiling voids

permit alterations to services, ceilings and office

layouts. Knock-out panels are provided for possible

future large service penetrations. Smaller holes can

be made through the easily locatable hollow cores in

the floor units.

Precast concrete was the only structural frame

material that could meet the cost, speed, quality and

versatility requirements for Merchant House and

Woodchester House. The result is two prestigious

buildings which are a credit to all those involved in

their design and construction.

January 1988: frame of Merchant House

well advanced, cladding in progress

at Woodchester House

May 1987: piling commenced

April 1988:

cladding to

Woodchester House

nearing completion,

internal walls and

cladding in progress

on Merchant House

Woodchester House was handed over in January 1989 and Merchant House in April 1989

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12

Developer Brunel Centre Limited

Architect, structural Building Design Partnership

engineer and quantity

surveyorServices engineer Ralph T. King Associates

Main contractor John Laing (London)

Frame contractor Crendon Structures

Flooring supplier Bison Floors

THE PROJECT TEAMA P P E N D I X

CONSTRUCTIONCOSTS /m2 gross

WoodchesterHouse

Enabling works 15

Piling 15

Substructure 25

Frame 100

M & E 295

Cladding/roofing 335

Finishes 215

External works 35

Preliminaries 125

Sundries 30Total 1190

Merchant WoodchesterHouse House

Gross floor area 6540m2 4500m2

Net lettable area 5100m2 3500m2

Number of storeys 8 6

AREAS

THE PROGRAMME

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13

Woodchester MerchantHouse House

Start May 1987 May 1987

Finish January 1989 April 1989

Duration 20 months 22 months

CONSTRUCTION TIME

TYPICAL FLOOR PLANS

WoodchesterHouse

Floor one

MerchantHouse

Floor one

MerchantHouse

Floor seven

WoodchesterHouse

Floor five

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PROJECT PROFILE: MERCHANT HOUSE AND WOODCHESTER HOUSE

C.H.Goodchild and P.Seilen

BRITISH CEMENT ASSOCIATION PUBLICATION 97.324

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