Concept of Crosswall Construction for Earthquake Hazards

1. INTRODUCTION

The aim of this report is to understand the basic concept of crosswall construction, and a proposal for ‘Armature Crosswalls’ to be used as earthquake hazard mitigation for reinforced concrete and masonry infill-wall buildings vulnerable to collapse. RC frame and infill construction is common throughout the world and often has proved lethal in earthquakes. The paper traces the history of masonry infill construction from pre-modern forms that have shown earthquake resistance in the past, to the early modern steel skeleton frame buildings that survived the 1906 San Francisco earthquake. the construction flow at real time in step by step procedures and quantifying the benefits and studying their applications. This is done by making a comparison in usage of the same technique in two different countries namely India and the United Kingdom. This comparison is aimed at producing an insight into the technique and to identify the areas of improvement.

2. Crosswall construction

2.1 Definition:

Crosswall is a modern method of building construction using division walls which transfers the floor loads through the building to the foundations. The name crosswall itself signifies the masonry connection is on either sides of the main wall. It employs concrete components like lift and stair cores, precision engineered (highly skilled), and precasted in the factories.

2.2 Applications:

This type of pre-cast single-skin or flat panel concrete construction is becoming more popular in commercial applications such as hotels, motels, prisons, military barracks and student accommodations. These types of structures up to 16 storeys have been finished in U.K using the crosswall technique. The use of such panels can result in fast, simple construction process on site followed quickly by finishing trades. Crosswall is mainly used for medium and high rise buildings.

2.3 Benefits:

Cross wall construction method reduces the wet trades and creates very early dry boxes for subsequent trades. This helps in speeding up the construction process without delay in work. Crosswalls also helps to eliminate internal cladding and other items such as party walls because the concrete crosswalls and floors do more than load carrying system. Crosswall technique reduces the labour onsite. A precast panel provides concrete frames without structural down stands.

Cross wall construction which has been developed for providing very fast and high quality in

repetitive

accommodation units such as hotels and the opportunity for multi storey apartments is rising, student accommodation buildings, stadiums etc. but the crosswall technique optimisation in low rise buildings is minimal(single houses) because the dimensions of each room may not be the similar.

The Crosswall construction is incorporated with a series of both horizontal and vertical ties which are designed in such a way to prevent the failures and collapses according to building specifications. Other precast work can be started simultaneously before the precast units are erected.

The crosswall buildings have less maintenance service and have good acoustics values. The acoustic performance of crosswall is excellent because of its mass and effective damping. Crosswalls reduces the risks of failing pre completion acoustic tests. (The Concrete Centre, 2006)

Pre cast walls and floor units are more than just a structure (highly effective in costs, speed, early start for other trades, and provision of fire separation panels), wall panels are provided with good finish and air tight tolerance. Tight tolerance enables fitting of bathroom pods, carpets and built in furniture. Flat pack construction is very quick and cost effective because it reduces the material waste on the site and the party walls are largely eliminated.

The separation of acoustics and additional finishes are reduced as the wall panels provide the sufficient airborne noise separation.

Thermal mass is provided by concrete and the thermal is utilised in crosswall constructions because the concrete widely spreads on the unfinished surfaces and the thermal mass reduces the risk of over heating in summer season by keeping the surrounding cool.( Doebber, Ellis M.W; 2005)

The main features incorporated in cross wall construction are

• Direct decorative finish to the walls with only minor pre decoration treatments.

• Solid room sized slabs pre finished for direct ceiling decorations.

• Reduced structural zone without downstands.

• Cross wall system generally utilises stair cores and lift cores for overall stability.

• Pre fitted windows eliminates internal cladding.

• Optimal methods of floor construction, allowing flexibility for individual client requirements.

Construction of stairs and lift cores can permit early access for subsequent trades. (The Concrete Centre, 2007)

2.4 Limitations:

In spite of several benefits offered by the precast panels, it has found that there is no wide acceptance of the precast panels in construction because of

• Highly skilled engineers or labours are required for manufacturing, placing and erecting the moulds on required positions.

• Lack of awareness and initiatives especially in Indian construction industry because of invariable labour intensive methods which leads to delay in the construction and that result in cost over-run and poor workmanship.

• Shortage of skilled and semi-skilled personnel availability may bring poor finishes, leakages in the buildings, corrosion of structural elements. These defects can be only minimised by the use of mass scale projects such as schools, offices, hospitals and other similar projects.(A.B.Shah,2005)

• Precast wall panel construction may be more or equal expensive as masonry construction because it mainly depends upon the transportation of offsite fabricated materials to onsite. (Havel.G, 2006)

• Difficult to transport heavy weight and size of the precast panels.

• The profit margin is very low in the small scale constructions.

• For single and two storey dwellings it would be typical to use single storey height walls panels between 90-300mm thick concrete for external walls and 70-100mm thick panels for the internal walls. The variation in designs leads to the problem and the single housing clients are not happy with the precast technique even though it provides a higher quality and good finish.

• This crosswall technique is mostly used for large repetitive structures may not be used for small scale construction like individual housing because the crosswall is more expensive tool when compared. (Glass.J, 2000)

2.4.1 Fire Conditions:

The building with large precast wall panels often has high ceilings and heavy fire loadings, as in retail stores, factories and warehouses. The fire in one of these occupancies require large volumes of water from large lines, if the fire is not controlled master stream appliances will be needed.

The fire in large precast buildings is likely to cause an early collapse of the roofs even if they are no weak connections. In this type of precast panel construction all the parts of the building are tied together as part of a structure, the roof collapses are more likely to tip the walls inwards and outwards.

As we know, the roof supports are more securely connected to the walls, the chances of roof collapsing is high before the wall connection fails.

The failure of any part of the structure may affect the stability of the other parts of the building. (Havel.G, 2006)

3. Historical background:

The precast concrete wall panel system was devised in England by William H.Lascelles (1832-85) of Exeter. In 19th century the pre-casting concrete for the structural purposes was started. Francois Hennebique (1842-1921) first introduced precast concrete into cast in-situ flour mill in France. White and Morris had given the historical accounts of the early development on precast concrete.

(Elliot.K.S; 2000)

Later in 1930s the use of precast as been expanded by companies such as Bison, Trent concrete and girling. Whilst precast concrete usage was stand at the first place it differs from country to country. One of the reason was the structural timber became more limited in some of the countries which led to development and improvement of precast usage. During the next 25 years the development in the precast frames, precast cladding as increased the market share to around 15% in industrial, commercial sectors. (Richardson.J.G, 1973)

Since the 1990s, a significant amount of research has been conducted on the seismic behavior and the design of precast wall paneled structures that do not emulate the behavior of cast-in-place reinforced concrete construction.

4.Development Of Precast Concrete Panel Frame Systems:

Precast panel frame systems have been successfully emerged from the research use of post tensioning between the precast beam and columns to achieve the lateral load resistance needed in seismic areas. (Seismic design of friction-damped precast concrete frame structure)

For the Docklands project on constructing student accommodation blocks in London, the concrete was prefabricated in Belgium and transported because the northern Europe was only able to cope up with the demands of the project and other small local markets were tightened.

Precast concrete wall panels in buildings speeds up the building process by adopting the precast concrete moulds. Decorative and light weight blocks have a great to offer visual values and technical values. The small store buildings from the precast industries offer excellent means of construction within the budget. The farm buildings, ware houses, industrial buildings are required to be constructed on exposed sites so the materials of standard precast frame components are supplied.

Precasting of simple lintels offers opportunities of time saving on the site. The schools, universities are built by using the precast modular components.

4.1

Comparison of Conventional and Large precast panel Construction

(Shah.A.B, 2005)

The above table clearly signifies the benefits of large precast construction over conventional construction methods in various fields.

4.2 Structural Superiority:

The precast panel structural system can be quiet efficient compared to other systems and it was tested and proven. (Fintel.M, July1991).

In cast in-situ concrete structures, the large stresses may built up in the structure due to the curing, shrinkage, creep, temperature etc. However no such stresses are built in large precast panel systems during casting. Due to these special characteristics of large panels it has proved its efficiency for more than 50 years in Europe, America and other developed countries. (Shah.A.B, 2005)

5. Precast Concrete Panel Manufacture Process

5.1 Casting:

Precast concrete panels can be manufactured by various casting methods.

5.1.1 Wet casting

: It is generally used for small number of units having similar specifications. The moulds are manufactured by heating them in the enclosed and covered zones. Skilled engineers design the type of moulds depending upon the requirement for the project and also under the guidance of trained supervisions.

As this casting is used for small units, it can be manufactured manually depending upon the number of units to be prepared. This casting can be provided for small housing. Wet casting provides the concrete in the cube strength ranging from 21 to 50N/mm, the slump varying from 0 to 175mm with a compacting factor varying from 0.8 to 0.97.(specifications are derived from CP 110 Unified code for structural concrete)

5.1.2 Gang casting:

It is usually used when the moulds are combined together into a large unit assembly. The gang casting is developed by the modification of wet castings. The gang casting could be used when the similar unit requirement is more.

For example: gang casting can be used in the production of stair case in the multi storied buildings.

The main factors for adopting gang casting process is because of the designed units, and the general components of concrete. The gang casting can be arranged horizontally and the outputs which are achieved from gang casting can be more enhance in the stack casting. The main advantage of gang casting is it allows the concrete to place faster and than the concrete is compacted with the help of immersion-type vibrators.

The greater accuracy of the component units are produced with the gang casting. The gang moulds are tied up in series so that the pressure loading on each individual unit is counteracted by the adjacent units, by this way it reduces the number of tie members. The gang moulds can be used for long horizontal spans, but the filling of gang moulds must be carefully controlled such that the intermediate moulds are not subjected to differential loading which may cause deflections and waves along the line of moulds. (Gibb.A, 1999)

5.1.2 Stack Casting:

Stack casting is the slight modification to the flat and gang casting units. In this casting process gang moulds are filled and hardened after the hardening process the divider plates are driven into the mould up to an appropriate depths so that the next layer will be ready for casting. Stack casting is used to produce ‘A’ frames and can be used in the repetitive structures such as prisons constructions. All the rooms are of same size and dimensions.

Precasters have found that the incorporation of through holes, barrels or by insertion of anchors allows fastening the mould sides and bring them to the subsequent positions with exact casting thickness.

5.1.3 Battery casting:

It is mostly used on wider scale, battery moulds has become more popular in large concrete wall panel constructions and the casting technique is also used for the manufacture of floor slabs and for decorative cladding components.

The battery casting can be used in the manufacture of ‘L’ shaped components such as balcony elements and lift enclosures. In this type, the units are generally cast in batteries of two or more. The battery mould can also have eight to twelve cells in the same mould. The changes on the casting process which provides a continuous casting, hardening, and curing schedule prior to de moulding. The battery mould which basically contains a series of plates that are spaced a part by the other mould members.

A Battery mould allows high density of casting to be carried in the available space. Care and proper supervision is required in the assembly operations so it results in securely tied moulds which will be impossible for the concrete member to get separate from the mould. (Richardson.J.G, 1973)

5.2. Direct Casting and Inverted Shell Procedures:

The precasted concrete panels can be obtained from the flexible formwork. The fabric formwork can be used to produce two basic types of concrete panels such as direct casting and inverted shells. In the direct cast panels, the concrete moulds are formed by the sandwiching fabric between the two rectangular frames. Firstly, the lower frame has the intermediate supports place inside it. The lower frame is X- shaped intermediate supports. The fabric membrane is than prestressed between the lower and intermediate supports.

Finally, the upper frame is placed over the membrane and than aligned with the lower frames. When the concrete is placed in the mould, the fabric form bends downwards and creates three dimensional tension curves between the available supports. Using the direct casting method, a single membrane can be moulded to form different varieties of designs by simply changing the design of the intermediate supports. The produced precasted frames with different designs can be used as various building components. The direct cast panel can be used as moulds to produce light weight shaft panels with compressed shells are caste from the mould. The panel has a minimum thickness of 38mm at the apex and a maximum thickness of 127mm at the perimeter and the diagonal X beams.

The two glass fiber reinforced concrete panels can also be produced by optimising the same process of tilting the direct cast panel to produce a compression shell panel. The final obtained glass fibre reinforced concrete panel cast from the mould which significantly varies in thickness from 13 to 38mm.

Unlimited desired number of different pattern or designs can be produced from these methods. Each intermediate supports produces its own set of compression shells. The moulds can be produced by providing the compression resistance to different load patterns by the changing the loading which is used in direct cast mould productions.

This method helps in developing an architectural quality concrete finishes using the industrial concrete mixing. Also expands the architectural potential of concrete constructions. But the problem was identified that there was no suitable method for predicting the magnitude of deflections in the formwork membranes under the variety of loading and also structural behaviour of some of the structures are not examined, precise engineers may solve the problem of structural behaviour in the precast concrete panels.

(West.M, April 2004)

6.Crosswall Construction Procedures

Precasting the elements such as foundations, wall panels, floors, stairs, chajjas, water tanks are manufactured at the factory and they are directly installed on the site.

6.1

Foundations:

The foundations along with the walls up to the plinth levels can be cast by using M20 grade concrete in the factories and further construction can be processed by using precast panels.

6.2 Precast

Wall Panels:

The precast wall panels can be made of concrete with the reinforcement provided as per the specifications. The wall panels can be cast in horizontal positions and than they are lifted from the casting beds after the concrete attains minimum required strengths. They are three main types of wall elements that are solid panels, panels with door openings and panels with window openings.

6.2.1 Sandwich Panels:

The sandwich panel which involves a precast concrete outerleaf, and the choice of simulated stone finishes or facings, insulating layer and a blacking leaf of plane grey concrete. The insulation which is installed under the factory conditions is well protected by the concrete. The thickness of the insulation contained in the sandwich panels can be varied to achieve the required U-values.

The precast concrete sandwich panels are often used for the building exteriors cladding and also serves as shear walls.

The two sandwich layers are generally connected by the stainless steel connectors, which may consists of wind and the shear connectors. Several insulation types such as mineral fibre insulation materials can be used. A cavity can also be introduced if necessary. Mineral fibre insulation is environmentally friendly, fire resistant when compared to the expanded polystyrene products.

The sandwich panels may support floors, slabs and beams. The main advantage of the load bearing panels is they may not require perimeter columns and instead increases the floor area and gives flush wall profiles. The applied finish panels may include terracotta, glazed bricks, tiles, granite, and limestone. A panel may incorporate more than 1000 bricks or 100 stones.

(Dawson.S, 2004)

The sandwich panel system which includes polystyrene insulation sandwiched between the two concrete walls. The interior of the sandwich panel is thicker because several studies as shown the thermal capacitance of benefits are greater when the thermal capacitance is within the insulation barrier

(Kossecka & Kossny, 2002) The two types of precast concrete panel systems are One is “waffle” precast concrete panel system which is currently used in the light commercial and residential industry and the other is “sandwich” precast concrete panel system which represents the available wall technologies which has greater thermal performance than the waffle panel system.

The inner leaf of the sandwich panel may be used as a load bearing structural element for giving support to the floor units. This provides more efficiency to the construction process and minimises the need to integrate different trades.

Techrete Company has manufactured the load bearing sandwich panels for Dublin’s city centre; the city centre was designed by O’Mahony Pike and for the first time the sandwich panel was used in a structural capacity on residential development. The sandwich panel provides very strong, durable, energy-efficient and fire resistant cladding systems.

All the panels are manufactured in the factory and they are ‘Just in time’ delivered to the site, they are enabled to provide very high quality finishes. Construction will be much faster and the load bearing walls panels they provide both structural support and external finish, the labour on site is minimised.

(Taylor.P.J, 1992)

Most of the precast concrete cladding system comprises of a single layered structural concrete panels which are manufactured in the factory and than installed on multi storey buildings with a weather resistant external finishes. Sandwich panel generally contains insulated material between the two precast layers.

Techrete

is one of the leading precast concrete manufacturers which has expanded the range and potential of the sandwiched panels. The Techrete company has introduced the precast panels in stone and bricks. The air cavity can be fixed in between the panels and they can be integrated as a part of load bearing structures.

6.2.1.1 Two-Wythe Sandwich Panels:

The precast concrete slabs are constructed by two Wythes of concrete which are separated by thermal insulation layer. Two Wythes panels are provided with strong concrete which enables both lifting and handling. The solid concrete may also have catastrophic impact on the thermal performance on the precast concrete panels. The research was mainly directed towards the development of precast concrete three-Wythe sandwich panel with the improved thermal performances. Often, both the concrete Wythes are of same thickness and the surface of the exterior Wythes may include the architectural panels. The panels with two concrete Wythes and one insulated layer are referred to as two-Wythe panels.

(Lee B.J,Pessiki; 2006)

6.2.1.2 Three-Wythe Sandwich Panels:

The three Wythe panel which usually as three concrete Wythes and two insulated layers, those are connected by solid concrete and they are staggered in location so that no concrete path extends directly through the entire thickness of the panel. In practice, the three-Wythe panels are evaluated by estimating the thermal resistance (R-value) using the finite element methods.

Three Wythe panel was developed to reduce the thermal bridges which were produced by solid concrete. Generally the thermal performance of three Wythe panel is evaluated by estimating the R-Values (thermal resistance) using finite element methods.

The benefits of three Wythe panels are:

The concrete connection between the Wythes allows improving the thermal performance over the two Wythe panel. The increased overall panel thickness may lead to increased span capability, this how which increases the usage of sandwiched panels. But the three-Wythe sandwiched panel may not be applicable for all scenarios because it increases the production time and production costs when compared to two sandwiched panels.

On the whole three Wythe’s panels provide greater advantages in thermal performance than two Wythe panel but with higher cost and time productivities.

(LEE, PESSIKI; 2003)

6.2.2 Tilt-Up Panels:

The tilt up technique which combines the advantage of precast walls with other benefits of site casting, the size and the thickness of the panel is reduced. The tilt up construction had grown more rapidly with respect to the increase in the demands for more durable and economical buildings.

The use of WWR(wire welded reinforcement) in the tilt up panels is relatively new concept. According to concrete international there are inherent advantages and disadvantages to the use of the WWR. (Griffin.J, 2003)

WWR (wire welded reinforcement) mats are manufactured in the plant-controlled environment, which gives the correct number of bars that to be placed in the panels depending upon the additional drawings. In the fields the prefabricated mats give assurance that the bars do not bunch or free float together in the plane of reinforcement, “step- through” meshes are well maintained which offers the workers the ability to step between adjacent bars, reduction of labour on the site.

Tilt up panel can be reinforced in less time because of the labour reduction. This wire welded reinforcement(WWR) may be used in joining the precast concrete walls and may finish the work very quickly without the need of excess labour and has an advantage in reducing size and thickness of the walls in multi-storey structures.

6.2.3 Double wall precast panels:

Double wall building technology means that both sides of the wall and the floor components are form finished. The interior surfaces of the walls are dry and smooth, only single coat of paint may be required to achieve the look and feel the drywall finish. The exterior surfaces of the walls can be produced with variety of finishes and surface treatments.

Dukane Precast Company has used the double wall technique for the low-rise residential and non residential constructions. This company has built a plant geared for low cost production of roof and wall that created safest, durable and most energy efficient building systems. The double wall building method may offer significant energy conservation by recognising thermal mass properties.

The benefits of double wall panels:

The double wall panel design may be a good choice for the home buyers looking primarily in cost, comfort, health benefits. The high degree of insulation provided by the panels can permit the use of smaller heating and air conditioning units, thus may save monthly operating costs of the house.

(Concrete Products Staff, 2002)

6.3 Floors/Roof Panels:

6.3.1 Hollowcore Floors:

Hollowcore and prestressed floors are also commonly used as floor slabs in multi-Family housing, schools, and hotels, offices which may take an advantage of span to depth ratios, high load carrying capacity, fire ratings and speed of construction. The hollow unit reduces the self weight of the slabs. These floor units may be available in 1200mm in widths & depths from 110 to 400 mm.

The hollow core slabs for the residential buildings may have very good span capabilities (short & long span). The long span is used for the car parks and office constructions and they can exhibit upward cambers. The short spans can also be provided with a layer of the expanded polysterene on the soffit to provide the insulation for the ground floor situations.

The hollowcore slabs with reinforcement can be generally 225mm deep and 1200 mm wide Termodeck Company is more specialised in providing hollow core units.

(Borghoff.M, 2006)

The hollow core wall panels can be installed with or without insulation. The floor units can be provided with the polysterene or poly-isocyanurate (PIR) insulation material.

The benefits of using hollowcore floor units are as follows:

It may include high load carrying capacity, long spans, durability, erection speed, providing instant working, and very good thermal and sound insulation, providing the floor with fire resistant properties without the need for the fire protection treatment.

Hollow core units may be the ideal building material for the construction of ware houses, manufacturing plants, schools, retail stores, office buildings and administration buildings. The use of pre stressed hollow core units and the solution which may enables fast construction and it is cost effective because the secondary fire resistance treatment is not required

Precast hollowcore floors are designed with up to 4 hours fire resistance by using tabulated data that gives minimum dimensions for the depth of concrete cover to the prestressing strand or wires as well as overall depth of the floor slab to be used. (Norman E Brown, Head of Engineering Services – British Precast
Secretary – Precast Flooring Federation)

6.4 Transportation:

The precast concrete panels can delivered to the site over the highways by semi trailer trucks. A few can be shipped by rail or other modes of transportation depending upon the feasibility of resources. The precast plants may not restrict the size and weight of the precast panel production if there is ease of transportation. The use of light weight aggregate concrete panels can minimize the impact of weight on shipping, handling and