POST TENSIONING OF THE SHARD AND OTHER TALL BUILDINGS

12-5-2015

Graham BowringOperations Director

ConForce Ltd

Contents

Post tensioning - Introduction, Definition Systems used Design Particular considerations re tall buildings Quality Assurance Major Projects Further reading

Post Tensioning - Introduction

It was developed in its present form by a Frenchman, Eugene Freyssinet, soon after WW2 and became a common method for longer span bridges in the 1960’s. There were very few Post Tensioning firms in the UK at that time and knowledge was being built up over time.

In 1969 the unbonded system was invented and use of PT in buildings in the UK grew slowly during the 1970’s and 1980’s.

In late 1990’s O’Rourke built a major shopping centre at Bluewater Park, the use of PT in buildings then became more popular.

Post Tensioning - Definition

Post tensioning is the application of a compressiveforce to a structure, at a certain stage in theconstruction process, which resists part of the deadand live loads applied to the structure later

It is achieved by putting high tensile steel cablesinto the concrete which are stressed at a givenconcrete strength

It is a technique which has a positive, direct effecton the performance of the structure and results inslabs around ¾ the depth of the same RC slab

Post Tensioning - Benefits

Post tensioned slabs have become very commonfor all types of residential commercial andeducational buildings.

It is a method adopted by many ConcreteContractors and Consulting Engineers in the UK

It provides clear benefits over RC in terms ofreduced slab depth, speed, and savings onmaterials with consequent environmental benefits.

Post Tensioning – Benefits especially for tall buildings

In tall buildings the depth savings can beconsiderable; up to around 5 extra floors can beadded depending of course on the height anddetails of the project.

Compared to RC, much less rebar and concreteare used with consequent benefits for storage,lifting, and the environment.

Often, flat soffits are possible compared to otherfloor types.

Systems Available

Unbonded tendons

Bonded tendons

Unbonded tendons

Simple system only comprising plastic coated strands and anchorages i.e. no duct or grouting process

Commonly used in many buildings in the UK Can be provided by ConForce, full system available Very good system for buildings not heavily serviced or where

regime for holes is fixed and known in advance Very efficient design – better lever arm and less friction in the

system during stressing Has major benefits over the bonded system for tall buildings

(next slide)

Unbonded tendons

The anchorages are much smaller (each tendon is a single strand only) and therefore can be better co-ordinated with perimeter fixings

The tendon can be very easily curved in plan – beneficial in complex floor plates

Not subject to temperature restrictions as no grouting process needed

Considerably less material and plant to be stored on site or lifted to the required floor

Unbonded anchorages

Live end anchor with pocket former and strand

Bonded Tendons

A bonded tendon comprises 3, 4, or 5 strands in anoval duct which, after stressing, is injected with grout.

The grout is to bond the strands to the duct and thestructure (basis of design) also for corrosionprotection

The components are: live and dead end anchorages,ducts and strands

In the UK we use as a standard, a prebagged groutdesigned specially for post tensioning

ANCHORAGES

Construction sequence

Edge shutters, bottom mat Anchorages, ducts and strands fixed to required

profiles Complete rebar Concrete 1st stage stressing at cube strength 10 N/mm² Full stressing at 25 N/mm² Measure record and check extensions Trim and seal strands at live ends Grout tendons

Constructional Details –Tendon Placing

Strands and anchors are placed as per therequirements of the drawings and spec

Tolerances are specified by the designer; theheight is important at high and low points and maytypically be +/- 5mm

Lateral position is not so critical to the design; weusually allow + / - 150mm but if fixings have beenco-ordinated could be much less

Strand Coils

Strands are delivered to site in 3 T coils wrapped for corrosion protection

The coil has to be lifted / stored on the site as required for the project

LIVE END ANCHORAGEthis has an angled pocket, in a square

grid they are square to the shutter

DEAD END ANCHORAGE

Ducts fixed to profiles

Constructional Details –Stressing

Stressing relies on good concrete compaction andcorrect transfer strength

Stressing is always done by experienced staff andwith equipment which is calibrated and regularlyserviced

The extension is measured during stressing andcompared with the theoretical as a check; somespecifications require it to be within a certain %age

STRESSING

Always stand to the side of the jack, not directly behind

If there is no access for stressing, top pockets are used

Constructional Details –Grouting

The grout serves 2 purposes – bond to the structure and corrosion protection

Tests are carried out on the grout (fluidity, bleed, volume change, strength) to regularly check its properties

Full grouting of ducts is assured by sealing at the dead end vent and pressurising before locking off at the inlet

Grouting – site set up

Design Codes and methods

Software packages in use are Ram Concept,ADAPT, FLOOR, RAPT, and a simplerspreadsheet for initial schemes from the RCC

We use Ram Concept ( FE software ) and designthe PT slabs to EC2

We also have 3D software available in ConForce Design and detailing document: Concrete Society

Technical Report no 43 ; revised edition publishedin 2005 with updates and guidance on EC2

Design Considerations - 1

The Lead Designer may consider incorporating softzones if slab holes are needed in the future

Penetrations should be decided upon beforetendon placing; holes may be made through PTslabs later but care and special methods areneeded; the building owner should consultConForce

Designers of all p.t. slabs should give guidance ofthe demolition method as required by CDM

Design Considerations – 2Restraint - 1

During stressing of a slab it has to shorten slightly If stiff elements e.g. core walls are at each end the

slab will be prevented from shortening and cracks will form in the slab or at slab / wall junctions

These effects can be overcome but must be dealt with at the design stage. Pour layouts can be changed or RC infill strips put in.

Restraint - 2

Slab with 2 cores cracks will form in slab

Restraint - 3

With 2 pours thus: Cores separated No restraint

Sample PT layout drawing

Architectural requirements

Its common practise in ConForce to co-ordinate anchor positions with the cladding fixings during the design process – just provide us with electronic setting out of the fixings.

If a requirement for service holes near columns detailed analysis will be carried out to check column sizes and punching shear rebar.

If parts of the buildings are of rather complex shapes, even curved edges, as with most types of insitu concrete, this can be done with p.t slabs.

Particular Considerations – 1weather effects

High winds can cause many operations to be delayed therebyputting more pressure on an already very tight programme –the floor to floor cycle otherwise achievable can be around 1to 1½ weeks. The crane times have to be carefully managedand planned in advance so that there is always a small stockof materials available for installation, at the required level;alternatively hoists should be used for lifting materials.

Low temperatures during the winter have often delayedgrouting works to post tensioned tendons; the temperature atlevel 40 for example can be around 3ºC lower than at groundlevel which is sometimes enough for the planned groutingwork to be delayed.

Particular Considerations – 1weather effects

Regardless of temperature, the mixing and pumping of grouton a site requires heavy use of plant materials and services(water has to be taken up to the required level at a goodpressure) all of which are eliminated with the unbondedsystem. The use of this system is covered in Specificationsand Codes such as the CARES Model Spec and ConcreteSociety Technical Report no. 72. Worldwide, the use of anunbonded tendon system is a very common solution to thegrouting problem, it is a system very popular for high rise inthe US and will certainly become more popular in the UK inthe near future.

Particular Considerations – 2axial shortening

Tall buildings always experience the phenomenon of “axialshortening” which is the difference in level, over time,between the slipformed core and the various columns. Thedifferences vary from project to project and can only bemodelled by the Engineer when the detailed programme fromthe Contractor is known but they could sometimes be up ataround 10 – 20mm. These are easily dealt with in PT slabs bysetting up a separate computer model, purely to representthese vertical movements and thereby showing the momentsgenerated and the rebar required.

Particular Considerations – 3co-ordination

During the design process PT designers are required to allowfor holes or fixings in the PT slabs required by: many largeservice openings, fixings for the cladding, beams and fixingsfor the climbing screens, and at some levels crane ties withassociated forces. On a tall building with a high-specarchitectural finish some of these co-ordinated items can becomplex and frequent requiring detailed use of the computermodel for the floor plate – its therefore very important for thisinformation to be finalised and made available to the PTdesigner in accordance with key dates for the designprogramme.

Particular Considerations – 3Dealing with complex floor plate

shapes

Over time, architects and developers are demanding more iconicand perhaps unusual buildings, and this includes the shape of thefloor plate. A simple rectangular and regular grid is now becominga thing of the past; a quick study of recent and current schemesfor tall buildings will give many examples of floor plates which areof a curved shape e.g. circular or elliptical. These shapes demandvery careful attention to detail during the design and drawingprocess; a 2D FE software package such as Ram Concept beingessential to deal with the shape correctly and efficiently.

Particular Considerations – 3Dealing with complex floor plate

shapes

Once the computer model has been finalised in terms of therequired tendon layout, at the detailing stage careful note isneeded of the anchorages at parts of the perimeter wherethey will be at an acute angle to the edge, requiring a largerthan expected pocket for stressing access. All this can bedealt with by an experienced PT designer with the requiredcomputer outputs and tendon and RC drawings produced forthe site installation to be carried out efficiently and correctly.

Quality Assurancesome key aspects

Post Tensioning is a crucial operation for the strength of the structure. The tendons must be placed in the correct positions, stressed to the correct force, and fully grouted.

All site works are carried out as per our QA procedures and our CARES accreditation

QA certificates from materials supplied are kept on file All strand coils are supplied with test certificates All operatives are in direct employ and have a

documented training record and CARES card

CARES CERTIFICATE

Conforce cert: Check scope and date

Major Projects

The structure comprises level 2 which is a majortransfer deck ( also designed to transfer loadsfrom raking steel columns back to the cores ) andlevels 41 – 69 which are flat slabs of a graduallydecreasing size. The total post tensioning slabarea is approximately 20,000 m². The PT floorscomprise the apartments and the Shangri La is afive star luxury hotel occupying 18 floors.Residential space is limited and takes up thetower from 186 metres to 224 metres making it byfar the highest residential property in London.

One of the main challenges was working at thevery high levels on the top floors and specificmethods were developed for the ConForce siteworks for this project. These included careful co-ordination of PT anchors with cladding fixingsand climbing screens; strict programming of ourworks around a 1 week cycle, and forwardplanning of use of the crane for lifting PTmaterials and plant (to avoid losing time in highwinds).

The Shard, London Bridge

Forthcoming Projects

Conforce have just started design for the PT slabs fora 50 storey residential tower in South London.

One of the main challenges will be working at thevery high levels on the top floors and specificmethods are being developed for the ConForce siteworks for this project.

The exterior of the building is a continuously curvingprofile thus making all the floors different; we willprobably set up a computer model for each floor.

The requirements from the Architect and claddingcontractor were such that edge deflections and slabdepths were very tight – this was a key factor inselecting post tensioned slabs .

Axial shortening is being dealt with and requires usto set up a separate computer model for a floor; thevertical displacements are modelled and the resultingrebar detailed as required.

1 BLACKFRIARS RD: 50 storey tower

FURTHER READING

Concrete Centre guide v good for all aspects of PT members can download free

FURTHER READING

Concrete Society Tech Report Sample Spec for PT slabs Covers bonded, unbonded Buildings and car parks

FURTHER READING

There are some papers published re PT tall buildings, many from the US and Middle East, a few examples being:

Al Bidda (Platinum) Tower, Doha; twisted tower, 45 storeys, also varies in width. Comprises external Diagrid, circular concrete core, RC slabs, 3 PT band beams. The PT beams span from core to edge and have movement isolation at core end to accommodate PT effects.

Strata, South London – 43 storey residential tower; central core, PT floors 200mmm thick

ONE MUSEUM PARK WEST, CHICAGO55 storey residential tower

Classic case of a VE process giving good savings A major transfer floor at level 6 was changed to PT with

a big reduction in depth with consequent savings in building height, walls, cladding, lifts, M & E

Above this floor the same depth as the RC as used but about 1/3 of the columns were removed

Very large savings in rebar Total nett saving of $ 4 Million

The Post Tensioning Association “PTA”

The Post Tensioning Association is growing to make a positive contribution to technical developments regarding all aspects of the PT works in the UK

Take a look at www.posttensioning.co.uk ; some Consultants have joined as a designer Member.

Guidance Notes on a number of topics have been produced, the website features project sheets for various contractor members, and PT Specifications are being written

The End

Thank you for your interest !

For further information on ConForce please see

www.conforce.co.uk

Or contact me on 07747 691704

gbowring@conforce.co.uk