Technical Specification for Aarsleff’s Centrum Ground Beam

System General - Design The Centrum Ground Beam System comprises square section precast concrete beams which are generally set out on an orthogonal grid pattern to align with the superstructure. Movement joints are not reflected in the ground beams or substructure. The installation of the ground beam system is carried out to a level precision accuracy of +0/-5mm tolerance by a setting out engineer involved throughout the whole process. Centrum ground beams sit directly onto the piles, tied by a high tensile steel dowel grouted into a hole drilled into the pile, this is then grouted into a preformed hole in the ground beam. In this case the dowel’s function is to transfer horizontal shear force from the ground beam into the piles once the ground beam has been laid. It should be noted that no moment is transferred across this joint. Ground beams can also be supported by other ground beams via halving joints, which are also dowelled and grouted. Here the dowels play an important role in locking the joints together so that the torsion is transferred from one beam into bending in the restraint beam. Ordinarily the tops of the beam are at a constant level, but stepped foundations can be created by adjusting the half joint configuration. The Centrum Ground Beam System does not require pile caps and pile positions are generally restrained by orthogonal beam arrangements at corners and by making use of the stiffness of the beams to restrain intermediate piles (see appended calculations), however, should circumstances dictate, the ground beams can easily accommodate precast or insitu pile caps.

Figure 1 – An Illustration of a Potential Ground Beam System

A typical layout is shown in Figure 1.

Stability is maintained by designing the beams to accommodate the torsional moments arising from pile installation tolerance (positional inaccuracy and out-of-plumb). All piling will be surveyed to ensure tolerances are achieved prior to the beams being installed. Where piles fall outside of tolerance, a number of remedial measures have been developed to ensure that loading on the pile is kept within design limits.

Figure 2: Typical layout of ground beams on square section piles with typical connections.

The typical details identified in Figure 2 are as follows:

C 1 - Corner Junction. The primary beams are installed directly onto the pile and secured with a pin

connection (hereafter referred to as the dowel). The dowel bases through a steel, spiral grooved grout tube cast into the ground beam. The secondary beam sits into the recesses of the primary beam by use of a halving joint and is secured with a dowel connection.

C 2 - Beam to pile. The primary beam is installed on top of the pile and is secured by a dowel bar already

fixed into the pile. This then passes through a steel spiral grooved grout tube in the ground beam. C 3 - T-Junction. The primary beam is supported off the pile and located with the projecting dowel bar from

the pile. The two secondary beams sit into the recess in the primary beam by use of halving joints and are secured with dowel connections through the steel spiral grooved grout tube.

C 4 - Intermediate support. The secondary beam is supported off the primary beam by use of a halving joint

and dowel connection that sits into the recess of the primary beam.

When the ground beams are all interconnected lined and level all the dowel connections are grouted. The grouting takes place in 2 stages.

Figure 3: Typical beam to pile connection.

Figure 3: Typical beam to pile connection

Design Process

A full project specific design is carried out for each Centrum Ground Beam System project and includes 4 stages. The first stage is to prepare the general arrangement drawings that will show the site layout, indicating the position of the building(s) on site. These drawings will contain such information as follows:

• The building(s) on plan.

• Easting and Northing coordinates of the building(s) for setting out purposes.

• Finished floor levels and beam top of concrete levels.

• Cross sections noting construction details and datum levels.

• Pile layout – indicating the pile positions on plan, plus a pile table noting information such as pile setting out coordinates, pile loads and pile cut off levels.

• Site setting out station triangulation survey. In order to prepare a design, the client and/or the client’s representative will supply sufficient information to allow design works to commence. This information will include, but not be limited to:

• Easting and Northing coordinates to all external masonry corners of the building(s)

• Survey drawing noting all site stations complete with setting out and AOD levels

• Full dimensioned working drawings

• External works details noting all proposed external ground levels

• Location on plan of service penetrations (if required)

• Line loads to be applied to the ground beams

• Site investigation report for assessment of the ground conditions

Once the General Arrangement drawings have been produced, the drawings are issued to the client and their design team for comment and approval.

The second stage of the design project is the structural analysis of the piles and ground beams together. This stage determines the bending moments, shear forces and torsional forces on the ground beams from the given set of line loads. The pile loads can then be determined and used to calculate the pile design. The bending moments and shear forces on the ground beams are then used to design the ground beams reinforcement. The third stage of the ground beam design is the fabrication details for the ground beams’ manufacture. Calculations are carried out for each individual ground beam in order to determine the rebar requirements in the beam using the results obtained in the structural analysis. From there, drawings are produced for each ground beam that indicate all the dimensional information and rebar layout to allow the manufacture. Each ground beam also has its own unique mark number for identification purposes both in the factory and on site. The fourth stage is site installation procedures, an installation drawing is produced to show all beams in their respective positions in the layout, complete with mark numbers which in numerical order denote the order in which the beams should be laid so that the installation team can easily identify the position of each ground beam and its corner co-ordinates for positioning purposes. A separate spreadsheet is also issued to the setting out engineer with these values so that they can be automatically uploaded into the total station to eliminate inputting errors. The designs are produced electronically using 2D or 3D modelling techniques. A design risk assessment which covers lifting and handling is also carried out for each project in line with all current legislation. Beam Design As part of the stage 3 design, a structural analysis is carried out using software which uses linear elastic analysis to determine bending moments and shear forces within the beams. Simply supported beams may also be analysed by hand calculation which complies with British Standards. The analysis results are used to determine the reinforcement requirements in accordance with BS8110:1:1997. The beams are positioned in order to utilise the pile capacity as much as possible. The end reactions on the beams are limited to allow for torsional moments during construction caused by misplaced and out of position piles.

The design checks include:

▪ Ultimate vertical bending capacity of beam ▪ Ultimate shear capacity of beam ▪ Ultimate bending capacity of nib ▪ Ultimate combined shear/torsion capacity of nib ▪ Horizontal force across beam to beam dowels ▪ Torsion of beam ▪ Bearing stress of halving joint

Reinforcement cages at the beam ends in order to resist torsional forces are standardised and incorporate closed torsion links in accordance with BS8110:2. Should the torsional forces fall within the capacity limits of the end cages; no further torsion checks are required. If the torsional forces exceed the capacity of the end cages, the calculations are re-assessed and adjustments made to the reinforcement to suit. If this isn’t possible, the layout is amended and tie beams introduced or beams spans are reduced so as to reduce the torsion to an acceptable level. Eccentric loading such as that caused by cavity wall construction, together with construction tolerances, will produce a torsional moment in the ground beams. To deal with torsion and its effects, beams are placed either on the centreline of the pile or offset towards the centre of loading and allowance of construction tolerances and any load imbalances are then taken into account. The eccentricity of the load is calculated and the applied torsional moment is calculated based on this value, from this torsional moment the maximum torsion at the beam ends, based on the beam span, can then be calculated. The resultant moment is summarised in figure 4 below:

Figure 4: Torsion caused by out of balance loads

fk/yme

P

Mt (kNm/m)

Inne

r Lea

f

Bew

am S

. Wt.

Out

er L

eaf

The beams are designed to carry the torsion to the transverse beams (see Figure 4). These ‘restraint’ beams are consequently designed for an additional end moment, which is generated through uneven bearing stresses on the half-joint. It is important to note that the connection between the ground beam and the pile is a pinned joint and therefore capable of transferring only nominal moment into the pile.

Figure 5 Torsion is carried back to the restraint locations.

Mt max.

Additional bending in

restraint beam due to

transfer of torsion.

All torsion carried

back to restraint

beam locations.

Centrum ground beams are designed to accommodate standard industry type precast flooring such

as beam and block or hollow core units, the detail of which is usually considered during the design

stage, however, the precast floor units can either bear directly onto the ground beams or bear onto

masonry under build to suit the sites finished ground levels.

Ground conditions will be noted in the site investigation which forms part of the contract specification.

The beams are typically designed using a concrete cube strength of 50N/mm2 with 30-50mm cover to

reinforcement (links), subject to the individual design requirements of the project. This is suitable for

a variety of ground conditions and satisfies design chemical class DC-4 to BS8500-1:2006. Special mixes

may be ordered and used for particularly aggressive ground.

Design competence All designs are prepared in house with the option for outsourcing designs to an Aarsleff approved Structural Engineers practice if required, all sign off and checking functions are controlled in every case by a senior Aarsleff design Engineer. Design work is undertaken by experienced engineers. Construction Tolerances With a precast foundation system, the ground beams and piles can be manufactured to very high tolerances, a unique aspect of controlled, offsite production. However, allowances must be made in the design for construction tolerances. The design is based on the ground beams bearing directly onto the piles, the centreline of the ground beam generally but not necessarily coinciding with the centreline of the pile, and ideally, the load from the ground beam should be transmitted through both the centreline of the ground beam and the centreline of the pile. The piles are designed for the specified axial loading they are to transmit, with the pile designer allowing a tolerance in the design for out of position piles on plan up to 75mm, and an out of plumb allowance of 1:75. If the pile is placed out of position, but is within the specified tolerance of +/-75mm on plan, the beams may be subject to torsion due to the eccentricity of the applied loads. This torsional moment is catered for in the design of the ground beam end cages and the adjacent beams, the philosophy being that the ground beam resists the torsional moments with only a nominal moment being applied to the piles. Should a pile be further out of tolerance a design check on the beam with the known eccentricity from the survey, the beam is re-evaluated with respect to bending and axial forces to determine if it’s integrity has been compromised. Where additional forces are transmitted to the pile this will be checked by the pile designer. The result of the design check can result in the pile being passed at its received position, abandoned and/ or additional relief piles being specified. The implications on the ground beam are discussed below in Remedial Actions.

Positional Accuracy of Pile

As discussed, the positional accuracy of the pile can be affected during installation. During the piling

operation, the setting out engineer will carry out an ‘as built’ survey of the piles installed, with the

Total Station. The data gathered will be input into the original co-ordinate table used to initially set

the pile out. This will produce a figure, measured in mm of how far the pile has been installed to the

original position. The ground beam designer will then re-consider any piles outside the 75mm

tolerance, he will then carry out a design check in relation to the ground beam and the out of position

pile and specify a remedial method, if required. For any piles that are within the specified tolerances,

no further action is required.

i. Temporary Redial Works Piles might be positioned outside the centre line of the ground beams. Whilst they may work in design, ‘in use’ the piles cannot hold a ground beam during the construction phase without running the risk of the ground beam overturning and slipping off the pile until it has been connected to the balance of the piles and ground beams. 2no toe jacks are used to temporarily support a ground beam before the lifting chains are removed. Minimum SWL of a jack to be 5tonne. Toe jacks are removed once the beams are locked in place.

ii. Permanent Remedial – Precast Concrete Pile Caps Precast concrete pile caps may be required when a pile has hit an obstruction and is driven so far out of position that it isn’t possible to achieve stability during installation with a temporary remedial measure, or that the resulting loads on the piles or beams require the introduction of a pile cap to unevenly distribute the load as directed by the design engineer. Pile caps are generally up to 600mm square, precast concrete and installed on the top of the pile and the underside of the ground beam. To accommodate the siting of the pile cap the cut off level of the pile is reduced by the appropriate amount to suit the cap. The installation of the pile cap is then the same as a ground beam and the ground beam is installed. A pile cap can also be specified for use as a stanchion based and not as a remedial. iii. Permanent Remedial - Precast Bridging Beam Should a pile be broken or out of position by such an amount that it’s load bearing properties are no longer within its capacity or acceptable FOS limits, then a bridging beam with extra piles can be utilised. The piles that support the bridging beam are cut down an extra beam depth below the specified cut off level, and the bridging beam is then lowered onto these piles and the connection fully grouted as a standard ground beam. The abandoned pile is cut down to below the soffit of the bridging beam so as to ensure there is no load transfer into the abandoned pile. See figure 6.

Figure 6: Remedial bridge beam.

i. In-situ Bridging Beam Insitu bridging beams can also be used on site where the timescales involved in procuring a precast bridging beam would cause unacceptable delays. The reinforcement design for the bridging beam would be specified by the ground beam designer and suitable QA procedures for site works adopted. ii. Repositioning of Dowels. If a pile has changed position, along the length of a ground beam, (as opposed to the beams width), this can be accommodated by core drilling a new hole in the ground beam in order to allow the pile to beam dowel connection to be established. Prior to core drilling the new hole in the beam, the calculations for the beam are first checked to establish that the change in bending moments and shear forces on the beam, due to the altered spans are within the beam’s capacity. Once the beam has been checked, the beam designer will inform site and core drilling can commence. Should the recalculated bending moments and shear forces on the beam exceed the beams shear and/or bending capacities, the ground beam is abandoned and a new beam manufactured and delivered to site. In cases where the affected beam has not as yet been manufactured, the beam designer will still carry out a design check, and the position of the grout tube can be amended in the mould, plus amendments to the reinforcement (if required), before casting. See figure 7.

Figure 7: Re-positioned dowel

Misplaced pile

Intended pile

position

Core drilled hole to

accommodate misplaced pile

Figure 7: Re-positioned dowel

i. Insitu Build Up In some cases, the eccentric loading from an out of position pile may cause a bending moment and axial load that is still within the capacity of the pile, but the location of the dowel pin may be too close to the edge of the pile (and hence, outside of the pile’s reinforcement cage) to be able to drill a hole in order to place the pin. In cases like this, an insitu build up that is cast around the head of the pile can be utilised. The insitu build up is to generate a sufficient volume of concrete so as to allow safe drilling for the dowel bar. The build-up is not designed to pull the load back to the centre of the pile although this can be beneficial. The torsion on the ground beam generated by the out of position pile is checked by the beam designer. The build-up cage is composed of pairs of U Bars lapped in both directions, the dowel bar is located within this steel cage, therefore, it is assured that the insitu build up is adequately tied to the pile.

H10 U-bars

in pairs

Shutter ring

H10 U-bars

in pairs

Projecting

H12 dowel

Insitu

build upPile to be cut down by a

minimumm of 300mm and

cage exposed

Figure 8: In-situ build-up

Concrete used will be either ready mixed from a certified batching plant or a proprietary pre-mixed

product where water only needs adding on site. Concrete strength to be minimum 50N/mm2 at 28

days, backed by offsite test results at Centrum Factory/ready mix plant and additional test cubes taken

on site. For each and every occasion where this solution is used, the beam designer will carry out a

specific design.

Manufacture

General Centrum Ground Beams are manufactured at our approved facilities in the UK:

• Centrum Pile Ltd, Hawton Lane, Balderton, Newark, Nottinghamshire, NG24 2BU

• Centrum Pile Ltd, Units 3 & 4, Walker Industrial Estate, Tuxford, Nottinghamshire, NG22 0PQ The facility is under the direct control & ownership of Aarsleff Ground engineering, a recognised manufacturer, installer and supplier of precast concrete piling products. The manufacture of the Centrum Ground Beam System benefits from the quality systems adopted by these facilities, which are certified under ISO 9001. The moulds in use are ex pile moulds 450x450mm. The moulds are steel which can be mechanically locked into position to avoid dimensional inaccuracies and movement. An overview of the factory procedures is presented below. Materials Materials are procured on a job by job basis as dictated by individual project specifications. The concrete mix designs are produced by Aarsleff technical team, in accordance with the current British Standards and strength proven by regular cube tests. Where materials or their suppliers are changed, the impact on the mix is determined and the necessary steps taken. Table 1 below lists the materials used in the concrete production for the Centrum Ground Beam System. Material certificates are kept on record at the facility and are available for inspection upon request. Certificates are provided by suppliers for:

▪ Sands ▪ Coarse aggregate ▪ Cements ▪ Reinforcement steel. (CARES).

Item

Information Available

Aggregates Certificates for source of material Grading certificates

Cement Certificates for source of material Laboratory test certificates

Water Local water authority potable water and recycled water from plant facility

Cement Replacement

PFA. Certificates for source of material

Additives Cemex Isoflex 781 Superplasticiser.

Reinforcement Supplied by ROE group or other CARES approved supplier

Shutter Release Agent

None used when beams are manufactured in steel moulds. Renosol CW1 used when special beams are manufactured in timber moulds

Curing Agents Casting undertaken in enclosed buildings. None used at this time

Concrete Mixing Centrum’s on-site batch plant facilities hold current accredited third party certification meeting requirements for BS EN206-1:2000 Annex C. The plant is completely computerised and regularly calibrated by the relevant manufacturers. A unique printed record is produced for each concrete batch, detailing weight of each constituent material, water/cement ratio and admixture dosage. The system calculates ambient moisture content in the sand and aggregate and makes adjustment to the water/cement ratio. Concrete sampling, cube manufacture and testing is carried out at the facility in accordance with British Standards. Daily sampling is carried out for each mix used and cubes are crushed at 9 and 28 days on cube crushing machines which are calibrated and maintained in accordance with BS EN12390. The concrete is designed to correspond to class DC-4 in accordance with BRE Special Digest 1. Manufacture of Ground Beams The production process is as follows:

▪ Final design confirmed with the Client.

▪ Casting details are forwarded electronically to the factory where they are initially checked for any special requirements.

▪ Aarsleff place an order with the factory (Centrum) prior to any beams being cast.

▪ Delivery dates are agreed with the factory.

▪ The Contracts Department will issue the factory with a production sequence based on site

requirement including a schedule taking account of the beam reference numbers shown on the approved drawings.

▪ Drawings are issued within the factory to the reinforcement, casting and mould supervisors.

• Any non-standard moulds or end formers are ordered and produced from an external specialist (steel or timber).

▪ Reinforcement cages are assembled away from the mould on trestles and spacer blocks (cover subject

to the project design) are fixed.

▪ The moulds are set up using the appropriate end forms to achieve the desired cast.

▪ The end forms are magnetic and clamp to the mould assisting in accuracy of the ground beam.

▪ Once set up the moulds and end forms shall be oiled to assist de-moulding without damaging the beams.

▪ The cages are then lifted by overhead crane or forklift truck to the relevant mould and inserted.

▪ The moulds are then mechanically tightened and spiralux void formers, projecting dowel bars and lifter anchors are installed.

▪ Prior to casting production, a QA Foreman will inspect the moulds and check for correct

reinforcement, cover, dimensional accuracy and general compliance with the casting details.

▪ Once set up has been approved by the QA Foreman the beams will be released for casting.

▪ Self-compacting concrete is ordered as per the mix design for the project. Any non-standard concrete mix will be added to the reinforcement drawing.

▪ The mould is filled to the correct depth. Ensuring that the concrete flows around the reinforcement, void formers, dowel bars and lifting anchors.

▪ When the casting supervisor is satisfied that the mould is fully filled only then will they proceed to the next beam to be cast to prevent concrete curing and a cold joint forming within any beam.

▪ The beams will be given a lightly trowelled finish to the unformed face.

▪ Casting of high volume beams should be carried out only when continuity of concrete is guaranteed.

▪ No water is allowed to be added to the mix once is has been dispatched.

▪ When the casting has been completed the moulds say be covered to protect from cold weather and enhance the curing process.

▪ Records are kept of the concrete mix, casting date, striking date and curing regime for each individual beam.

▪ Beams are generally stripped the following working day once they have received a strength of 15N/mm2.

▪ Strength is checked with a Schmidt hammer on every beam adjacent to the lifting points.

▪ After strength, has been reached the beams are lifted using the cast in lifting points and overhead crane.

▪ Beams are inspected by the QA Foreman for concrete compaction and a further check on dimensional accuracy is undertaken.

▪ Beams that are acceptable are sent to the stock yard and added to the stock list.

▪ Beams that require cosmetic damage are moved to the finishing area.

▪ Beams that have any structural defects are condemned and scrapped.

▪ Once beams are called off for delivery they are loaded onto trailers by either overhead crane or side loading material handlers operated by qualified personnel. Transportation Beams are delivered to site in accordance with the agreed project programme and sequence and at times agreed with the site. Hauliers are provided with a delivery ticket in triplicate stating client, address, time and date required, a list of units on the load and the overall load weight. When the load is secured protective material e.g. rubber pads are placed between the security strap and the precast beam. On arrival to site the beams are checked off by an authorised member of site staff and the delivery ticket signed to acknowledge receipt along with the time on and off site. A copy of the delivery ticket is retained by the factory, the haulier and the site.