Structural Concrete

Table of Contents

1. Introduction .............................................................................................................. 2

1.1. Brief requirements .............................................................................................. 2

1.2. Structural Description ......................................................................................... 2

1.3. Group work......................................................................................................... 2

1.4. Construction Type and Techniques .................................................................... 6

1.5. Benefits of precast concrete over casting in situ ................................................ 7

2. Key Design Elements ............................................................................................... 8

2.1. Structural considerations .................................................................................... 8

2.2. Frame analysis ................................................................................................... 8

2.3. Aesthetics........................................................................................................... 8

2.4. Durability ............................................................................................................ 8

3. Structural Members .................................................................................................. 9

3.1. Columns ............................................................................................................. 9

3.2. Beams ................................................................................................................ 9

3.3. Slab .................................................................................................................... 9

3.4. Retaining wall ..................................................................................................... 9

4. Structural Arrangement .......................................................................................... 10

4.1. Basement ......................................................................................................... 10

4.2. Ground floor ..................................................................................................... 10

4.3. Remaining nine residential floors ..................................................................... 10

4.4. Lift and stair tower ............................................................................................ 10

5. Lateral Stability....................................................................................................... 10

5.1. Components contributing to lateral stability ...................................................... 11

5.1.1. Bracing ...................................................................................................... 11

5.1.2. Portal/sway frame ...................................................................................... 11

5.1.3. Diaphragm ................................................................................................. 11

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5.1.4. Shear cores/walls ...................................................................................... 12

5.2. Lateral stability strategies ................................................................................. 12

5.3. Lateral stability of Portland House .................................................................... 13

6. Load Flow Chart ..................................................................................................... 14

7. Design Considerations and Technical Approaches ................................................ 16

7.1. Load description ............................................................................................... 16

7.1.1. Dead load .................................................................................................. 16

7.1.2. Imposed loadings ...................................................................................... 16

7.1.3. Wind load ................................................................................................... 16

7.2. Critical load combination .................................................................................. 17

7.3. Design checks .................................................................................................. 17

7.3.1. Slab ........................................................................................................... 17

7.3.2. Beam ......................................................................................................... 17

7.3.3. Column ...................................................................................................... 17

7.3.4. Retaining wall ............................................................................................ 18

8. Site Erection ........................................................................................................... 18

8.1. Method statement ............................................................................................ 18

8.1.1. Columns .................................................................................................... 18

8.1.2. Beams ....................................................................................................... 18

8.1.3. Slabs.......................................................................................................... 19

8.1.4. Construction speed .................................................................................... 19

8.2. Erection stages ................................................................................................ 19

9. Design Calculations ............................................................................................... 20

References .................................................................................................................... 21

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1. Introduction

1.1. Brief requirements

The structural arrangement, analysis and design calculation for the precast concrete

building to be known as Portland House is presented in the following report. The structure is part

of an extensive redevelopment of a former industrial location in the city centre of a large UK

town. The building comprises underground basement parking, private leisure facilities on the

ground floor and a further nine floors of luxury residential accommodation. The functional

requirements and constraints make the structure interesting. The new building is to be developed

on the corner site as shown in Figure 1. The design is in accordance with the requirements of the

Eurocode 2 worked example of precast concrete (Eurocode 2), BS 8110 Part 3 and the precast

Eurocode 2 design manual by R.S. Narayana.

1.2. Structural Description

Portland House has a private car park in the basement with the soffit height of 2.4 m, as

shown in Figure 2. The ground floor is a leisure development with a gym and other facilities for

the residents with soffit height of 3.75 m, as shown in Figure 3. A further nine floors are

residential apartments with three apartments on each floor with soffit height of 2.75 m as shown

in Figure 4. The arrangement of vertical columns has some constraints; in the basement the

columns should not be between the parking bay and the drive in/out area; in the residential

apartments they should be in the line of the party walls for more flexibility in the internal fitting

out of the apartments to suit the customers’ requirements.

Further, the access to the car park in the basement will be provided by a ramp on the east

side of the building. This is also a restriction to providing a shear wall in in that corner. The

client wants to make the building iconic by providing some external façades. Providing solar

panel façades will improve the aesthetics of the building as well as make it energy efficient.

The stair and lift tower is a steel construction with glass as cladding to make the tower

lighter and reduce the significance of the load transferred onto the transfer beam.

The external cladding for the building is a concrete façade and the solar panel façades are

placed on alternate floors, to make the solar panels more efficient.

1.3. Group work

All the members of the group contributed to the project, with active participation of

everyone in all stages of the work.

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Figure 1: Site Plan

Figure 2: General Arrangement – Basement Level

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Figure 3: General Arrangement – Ground Floor Level

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Figure 4: General Arrangement – Typical Residential Floor Level

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1.4. Construction Type and Techniques

Planning considerations dictate that the cleat needs fast and easy construction, with the

use of faced precast concrete cladding panels on the façade. Also, the client wants to spend more

on the exterior façades and the visual appeal of the building. The most suitable technique for this

project is a precast concrete building. As there is no storage space on site, in situ construction

would be difficult. Further, it is a time-consuming process. To erect a precast structure, a crane is

important and its location on the site is equally important. The location of the crane, site office

and storage space is shown in Figure 5. It can be seen clearly on the site plan that the north and

west sides have a main road passing along the building. The east side has a space of 10 m from

the site boundary wall that can be utilised for the site office and storage. The south side has an

existing car park.

Figure 5: Location of Site Office, Storage Space and Crane

Tower Crane

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1.5. Benefits of precast concrete over casting in situ

Key Concerns Casting in situ Precast

Structural efficiency Casting in situ elements are designed with lower span to depth ratio, increasing the number of vertical elements. Higher dead load increases the size of structural members and foundations.

Precast elements can be designed with a high span to depth ratio, reducing the need for additional vertical elements. Can be designed with lightweight concrete to reduce dead load and decrease the size of structural members and foundations.

Viability Not viable for compact sites.

Quite easy for compact sites.

Prep work Setting and removing form works is a laborious task. Waiting for a concrete truck to arrive and concrete to cure. Perform strength tests and quality control measures on site.

Simply order the precast elements as per requirements.

Weather With casting in situ, rain, sleet or snow can delay the pour and the project. In cold temperatures and wet conditions, concrete sets slowly.

With precast, the structure is poured in a controlled environment so weather is never a factor and the project is never delayed by inclement weather.

Concrete Strength A number of uncontrollable factors can decrease the strength of freshly poured concrete including extreme temperatures, fluctuations in temperature and humidity.

Controlled pour conditions, strict quality control measures and factory strength testing ensure pre-cast concrete meets strength and durability specifications.

Durability Various defects can occur when casting in situ concrete like shrinkage, honey combing, creeping, etc. The durability of structure is decreased.

Strict quality control measures and screening of precast elements reduce the chances of defects, increasing the overall durability of the structure.

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2. Key Design Elements

Precasters can aid designers with a variety of key design elements that must be

considered beyond simply manufacturing the components, especially when using fully precast

concrete structural systems. These factors include the following:

2.1. Structural considerations

These aspects include placement of columns and beams, number of structural supports,

and other criteria. In parking structures, they also can include helping to devise the ramp

configuration and determining how many supported levels will be needed.

2.2. Frame analysis

With more stringent seismic requirements across the country, former structural designs

are no longer sufficient in some locations. The precaster can work with the engineer of record to

find a precast concrete connection system that will meet the seismic needs of the specific site.

2.3. Aesthetics

Precasters can provide a range of aesthetic options to eliminate significant budget

concerns by replicating more expensive materials such as laid-up brick, limestone, granite, and

cut stone. These options allow the budget to be allocated to other areas where it is needed. The

ability to cast more than one colour into an architectural panel and to use form liners to create

sculptural looks provides more flexibility while reducing costs. The different finishes on façades

can certainly improve the external aesthetics of the building. The solar panel façades will also

make the building more energy efficient

2.4. Durability

According to the design manual, the minimum cover should be 40 mm, the minimum

concrete grade should be C35/45 with a maximum W/c ratio of 0.50 and minimum cement

content of 340 kg/m3. According to the national annex (3a) the same cover can be used for both

reinforcement and pre-stressed steel, but the designer can increase it at his/her discretion. Crack

width limitation for concrete in compression is 25 mm from tendons.

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3. Structural Members

The structural members are designed to safely and efficiently transmit the loads applied

on the structure to its supports. In the tower, different members will have different roles and thus

different sections.

3.1. Columns

Columns typically support cross members such as beams, spandrels and panels.

Traditionally square or rectangular in profile, columns are usually cast as multilevel components

ranging in length from a single story to six or more levels.

L-shape columns will be constructed with the help of slip form work.

3.2. Beams

Beams, horizontal members that support deck components such as double tees and

hollow-core slabs, typically are considered to be structural components. Three types cover the

majority of uses: rectangular beams, inverted tee beams, and L-shaped beams. In this project we

are using L-shaped and inverted tee beans.

3.3. Slab

Hollow-core slabs, also known as planks, are used as floor/wall components in a wide

range of buildings. Hollow-core slabs typically measure 8 to 12 in. thick, but they can be made as

thin as 4 in. or as thick as 16 in. They are relatively light and easy to erect.

3.4. Retaining wall

The retaining wall is provided all around the basement to retain the soil. Waterproof

lining is provided all around the retaining wall and the basement slab to make the basement

waterproof.

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4. Structural Arrangement

The geometry of the building is predefined but the arrangement of the columns and the

beam arrangement is the important and challenging part. Because of the various constraints for

vertical elements, the internal column arrangement is to be done according to the requirements of

the client. The most critical part is to make the structure stable and durable.

4.1. Basement

The basement forms the personal car park for the residents (Figure1). The retaining wall

and foundation is cast in situ and the columns, beam and roof slab are precast. The floor for the

basement is a pre-stressed, precast hollow core slab. Before construction of the retaining wall

and floor slab, waterproof lining should be laid to make the basement waterproof, as the ground

water level is approximately 2.4 m below original ground level.

4.2. Ground floor

The ground floor will provide space for luxury leisure facilities (Figure 3). The floor to

soffit height of the ground floor is 3.75 m, which includes an appropriate allowance for services.

Access from the ground floor to the residential floors is via a lift or stairs in the tower located on

the north elevation of the building. Imposed load on the ground floor is higher than the other

residential floors. The precast slab and beam arrangement makes the erection fast and easy.

4.3. Remaining nine residential floors

Nine floors of luxury residential accommodations are to be constructed (Figure 4). The

floor to soffit height is 2.75 m, including an appropriate allowance for services. Access will be

via stairs or a lift in the tower. To enable internal fitting out of apartments to suit individual

customer requirements, internal columns/walls will be located only along the line of party walls,

while internal columns within the apartments are not permitted. The location of two service

routes is shown; these require a void in the floor slab of 0.75 m x 2.4sm within each zone.

4.4. Lift and stair tower

For the lift and the stair tower, four steel columns are provided and tied with the concrete

structure. The tower will be covered with glass to make it lighter.

5. Lateral Stability

The overall lateral stability of a concrete precast building depends upon the various

elements such as: bracing; shear cores and walls; portal frames; and diaphragms. These elements

are used in combination to achieve maximum possible stability of the structure.

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5.1. Components contributing to lateral stability

5.1.1. Bracing

This is one of the most well-known methods of providing lateral stability. Bracing (Figure 6)

consists of diagonal elements and acts in a similar way to a

cantilevering vertical truss, from the ground up. For this reason,

bracing should be present at every level of the structure down to the

founding level in order for it to be effective. If bracing is

discontinuous, significant lateral forces are generated and need to be

transferred from one bracing system to another. This can exert high

localised lateral loads onto elements of the structure. In addition, the

transfer system adopted for this purpose needs to have adequate

stiffness. It is not uncommon to see bracing working in conjunction

with other vertical elements to achieve the overall lateral stability of a

structure.

5.1.2. Portal/sway frame

The portal frame, also known as the sway frame, is

based on the concept of portal frames whose connections are

designed to withstand forces generated from lateral and

vertical loads (Figure 7). This negates the need for any

vertical bracing elements and therefore large clear span

spaces are created. This does, however, add significant

complexity to the construction of the frame as well as its

weight. This is because members tend to be larger than their

simpler construction counter-parts and connections become

more onerous in their design and installation. These practical

considerations limit the number of story sway frames that can

be constructed.

Further, the adoption of portal frames as a bracing

solution may require careful consideration of second-order effects within the structure. This adds

significantly to the complexity of its analysis and design.

5.1.3. Diaphragm

A diaphragm is an area of the structure that provides bracing in its plane

(Figure 8). Typically these are floor slabs and roof cladding, but can also

be in vertical cladding elements. If cladding is used as a diaphragm,

careful consideration must be given to the temporary condition of the

structure during erection. This is also true for the maintenance of the

structure if the cladding has a shorter design life than the structure.

Diaphragms are tied back to vertical elements of the structure that

provide lateral stability. They prevent structures from ‘racking’ or

rotating about an axis. There are instances where diaphragms are not

strong enough to resist the lateral loads that can build up within them. In

Figure 6: Bracing

Figure 7: Portal Frame with

One Bay Braced

Figure 8: Floor and Roof

Diaphragms. (All other

lateral stability elements

omitted for clarity.)

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such cases, the diaphragm is strengthened or horizontal bracing is installed to either supplement

or replace the diaphragm.

5.1.4. Shear cores/walls

Shear cores and walls are vertical elements within a structure that

provide lateral stability (Figure 9). The rest of the structure is framed

around them and they typically work in conjunction with floor plates

and rooves that act as diaphragms. They can also be paired with braced

base systems. Shear cores typically act as vertical access throughout

the structure via lifts and stairs and are usually located in line with the

centroid of the structure in an attempt to minimise torsional effects.

There will, however, always be a difference between the centre of

stiffness of the structure and the centroid of the applied wind load. As

a result, some torsion does develop within the shear cores due to

eccentric loading.

5.2. Lateral stability strategies

Before attempting to adopt a particular strategy for lateral stability, some appreciation of

alternative solutions (and their consequences) is required. Consider Figure 10:

Figure 10: Plan of Various Lateral Stability Solutions

Structure A does boast a solution that braces the structure orthogonally in both directions.

There is the risk that if one of the vertical bracing elements is subjected to accidental

damage, the entire structure will become unsafe.

Structure B is the most efficient solution, but is not normally architecturally sound. It does

however have redundancy in that if one of the vertical bracing elements were to fail, it would

not leave the building in an unsafe state.

Structure C is a poor solution as off-centre wind forces would generate significant torsion

within the structure. If the structure were made primarily from concrete, shrinkage within it

would also create significant stresses that could not be relieved.

Figure 9: Shear Core

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Structure D is slightly better than structure C, but is still susceptible to off-centre wind

forces and would also generate significant torsion within the structure.

5.3. Lateral stability of Portland House

In this structure, a combination of L-shaped columns (shear wall) and floor slabs are

used. The L-shaped columns are place at the three corners of the building which ensures enough

lateral stability against twisting and the sustainability of vertical elements for horizontal forces.

The fourth corner is a shear wall, which also helps the structure’s lateral stability.

The chosen method of achieving lateral stability of a structure is normally driven by both

geometry and the load transfer path of the structural elements to the foundations (vertical +

horizontal). This is particularly appropriate for this building where the pic-up columns are used

to transfer loads onto the beams and then the beams transfer the load onto the respective

adjoining members.

The L-shaped columns are well tied with the rest of the columns (pre-cast rectangular) with the

help of beams that complete the framing of the structure.

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Figure 11: Preliminary Plan for Lateral Stability Solution.

6. Load Flow Chart

Figure 12: General Flow Chart of Transfer of Load.

In Figure 11, there are two pick-up columns which are transferring all the floor loads of nine

floors to the transfer beam. Those two columns are provided to complete the framing of the

structure. The supporting beams are heavily loaded; so for distributing load, the beam grid is

formed. The loads from those beams are transferred to the primary beams or columns and finally

to the foundation, completing the load path.

Slab • All imposed loads and dead loads of slab are transferred to beam

Beam

• Beam transfers the beam from the slab and its self weight to the columns.

Column

• Load from beam is then directly transferred to the supporting element like foundation or transfer beam.

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Figure 13: Flow Chart for Transfer of Load from Pick-Up Column.

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7. Design Considerations and Technical Approaches

7.1. Load description

7.1.1. Dead load

All self-weight of structural elements as per element dimensions.

7.1.2. Imposed loadings

The following are the imposed loads as per the brief

1. Plant room (including roof) 10kN/m2

2. Other areas of roof not zoned for plant 0.75kN/m2

3. Residential floors, including corridors

and stairs (including allowance for

internal partitions but not party walls)

3.0kN/m2

4. Party walls 4.3 kN/m2 on elevation

5. Ground floor 4.0 kN/m2

6. Basement 2.5 kN/m2

These loadings include an allowance for raised floors, ceilings and services. A load of

2.4 kN/m2 for cladding is considered. The front (north) side of the building, the basement roof

should be designed for an imposed load of 10 kN/m2.

7.1.3. Wind load

Figure 14: Wind Load Direction on Regular Rectangular Building.

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Figure 15: Wind Co-efficient for Regular Rectangular Building.

7.2. Critical load combination

The critical load combination, according to the Eurocode 2, is 1.35 times of dead load

and 1.5 times the imposed load, i.e., 1.35 Gk + 1.5 Qk.

7.3. Design checks

The precast elements are designed according to the Eurocode 2. The following are design checks

provided accordingly:

7.3.1. Slab

a. Initial and final losses.

b. Initial and final deflection

c. Ultimate shear and resistance.

d. Check for splitting of hollow web.

7.3.2. Beam

a. Initial and final losses.

b. Ultimate shear and resistance (moment-carrying capacity).

c. Amount of effective and working pre-stress.

7.3.3. Column

a. Lateral stability

b. First and second effect due to lateral loads

c. Design with moment consideration.

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7.3.4. Retaining wall

a. Sliding

b. Overturning

c. Bearing capacity

d. Bearing pressure at ultimate limit state.

8. Site Erection

8.1. Method statement

The principles of site erection, the methods of making structural joints and the

specification for materials are all in accordance with the requirements of BS 8110. At the

commencement of each project a method statement confirming how the building will be

manufactured, transported and installed should be prepared. The headings covered in this

statement include:

Safety (including the mandatory safety statement)

Mould-work

Materials

Handling/cranage and transportation

Site installation (procedure or sequence)

The design for temporary conditions during erection should take into account the overall

frame stability and the stresses in individual frame components and joints. Load paths through a

partially completed structure may be different from those in a completed frame. An example is

the temporary state when floor units have been placed on one side only of an internal beam. Here

the connection should be checked for its resistance to torsion and if necessary, propped until the

slabs on the other side of the beam have been placed in position.

8.1.1. Columns

Columns are typically erected in pockets onto pre-levelled plates giving a clearance of 50 mm to

75 mm to the sides and bottom of the pocket. Temporary wedges are driven into the pockets and

together with diagonal props, are used to line and plumb the column. In situ concrete with an

early strength of about 40 N/mm2 at three days is compacted into the pockets to ensure that a

structural connection is made between the column and the foundation.

8.1.2. Beams

Beams are either grouted through sleeved connections between upper and lower columns or are

supported on billet-type corbels. Bolted cleat-type connections or billets with a welded or bolted

top fixing provide an immediate restraint to shear and torsional forces. Dowelled corbels and

billets that depend on an in situ top fixing do not provide adequate restraint and temporary

propping will be required to stabilise the beam. High strength grouts are used in all cases.

Strengths of 30 N/mm2 are required after eight hours.

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8.1.3. Slabs

Hollow core slab plans range from 0 to 7.5 m in length. Spreader cables from the lifting

hook crane to the plank should be a minimum of 6 m in length. The chokers for placing around

the plans should be 4 m in length with one choker at each end of the plank approximately 0.5 to

1m in from each end of the planks.

8.1.4. Construction speed

Installation times for precast units vary with each project, but the following rates of

installation (based on one erection crew) are indicative.

8.2. Erection stages

The erection of the building is done in stages using mobile cranes and one tower carne, as

shown in the Gantt chart. The first step involves the construction of foundations, L-shaped

columns up to ground level, the erection of the precast columns of the retaining wall, beams and

in situ casting of the basement floor slab. Then the slip formwork continues to move and the

ongoing construction of the L-shaped columns is carried out at different stages. The erection of

precast elements is carried out along with the construction activities. The next steps are similar.

The stepwise erection process and duration are shown in the Gantt chart.

Single storey columns 12−14 per day

Spine beams or edge beams 12–15 per day

Wall panels 12–18 per day

Floor slabs 350 m2 per day

Stairs and landings 12–15 per day

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9. Design Calculations

The hand-written design calculations are submitted in drop-box.

The following is the pan with the marking of all the critical elements designed in the

hand calculations.

Figure 15: Critical Beam Column Marking .

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References

1. Eurocode 2.

2. Reinforced concrete design to Eurocode 2 by Bill Mosley, John Bungery and Ray

Hulse.

3. Handbook of Reinforced Concrete by S.N.Sinha.

4. Worked example of precast concrete (Eurocode 2).

5. Design of structural elements by Chanakya Arya.

6. CCIP Concise columns graphs.

7. Reinforced Concrete Design Manual.

8. BS8110 Part3.

9. Precast Eurocode 2 Design Manual by R.S. Narayanan.