newer construction techniques

7/27/2019 newer construction techniques

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Coffered slab


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This system consists of beams at regularintervals in perpendicular direction .

The beams are monolithic with a toppingslab

Each rib acts as a T-beam Pre-cast grid units of M-25 (1:1:2) can be

placed as shuttering units and over a thinslab can be cast in-situ.

Shapes of grid :

- Square grids

- Rectangular grids- Diagonal grids ( beams inclined at 45degrees )

- Circular grids ( different circles of differentradii are tied by straight members

Spacing of ribs 2-3M c/c

Width of ribs 200-250mm

Depth of ribs Span/20 to

span/25Thickness ofslab

65-10mm

Reinforcement 8mm dia @200mmc/c

Span range 10-30M*consider shorter span for depth

To design a suitable system :- Understand deflection pattern of the grids .- Understand deflections at junctions .- Compute load distribution .

Maximum bending moments develop at the center of the span . Maximum torsional moments are generated at the corner of the grid . Maximum shear force develops at the midpoint of longer side supports . For 66 m square floor plan, 5 ribs of 140 mm depth (overall depth 205 mm) isfound to be structurally most efficient for 3 kN/m2 live load intensity. For square floor plan of 8 8 m, the most efficient structural system is 9 ribs of240 mm depth for a live load intensity of 3 kN/m2


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Demerits :

Coffers need to be placed accuratelyand this is time consuming .

The formwork is expensive . Specialized and extensive propping

is required .

Soffits in this system are not flatwhich could b a disadvantage whenfixing electrical units and services .

Effective in sagging but requires slabin area of hogging .

The slab becomes very thich , hencean overall increase in building heightleading to more material usage andincreased cost .

NBCC house at Bhikaji Cama Place

Lab block ICGEB JNU campus , New Delhi

Hero honda factory Gurgaon

Parliament secreteriat extension building at NewDelhi

Applications : Coffered system is quite suitable for medium size floors. Coffered slabs provide a lighter and stiffer slab than an equivalent flat slab, reducing

the extend of foundations. They provide a very good form where slab vibration is anissue, such as laboratories and hospitals.

This system reduces self weight of the a structure and a column less space is achievedof longer spans .


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Load bearing structure of a recently built DEPARTMENT STORE IN PRAGUE consists of a flat

(double ribbed) reinforced concrete slabs supported directly on columns located within span distances 12 x12 m. Slabs above the first and second storey cantilever out by 3 m beyond the edge columns .

Plan view of the building of the gross area 78 53 m

Load bearing structure consists of reinforced concrete double ribbed slabs of the total thickness 0.45 msupported directly on columns of the cross section 0.5 0.5 m or 0.7 0.7 m located within span distances

12 12 m . The slabs are provided by hidden heads of the plan view dimensions 1,65 1,65 m (at the edge columns)

and 3.35 3.35 m (at the interior )

Design loads of the slab above the first floor considered in the original analysis consist of permanent part7.0 kN/m2 and temporary part 4.0 kN/m2

After few years in service serious performance insufficiencies of cladding, interior partitions, and othersecondary elements had been observed. Due to public perception of the observed faults the second storeywas closed and the whole building has been reconstructed. Detailed analysis has shown that serviceabilityfailure of the second storey was primarily caused by lack of consideration in design of deflection duepermanent load and shrinkage


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Flat Slabs


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Flat slabs are slabs that are supported directly on columnswithout any beams. They are highly versatile elements widelyused in construction, providing minimum depth, fastconstruction and allowing flexible column grids.

Span-For spans from 5 to 9 m, thin flat slabs are thepreferred solution for theconstruction of in-situ concreteframe buildings where a square or near-square grid is used.For spans over 9 m post-tensioning should be considered.

Advantages

Faster construction-Flat plate design will facilitate the use of

big table formwork to increase productivityReduced services and cladding costs-Flat slab construction

places no restrictions on the positioning of horizontalservices and partitions and canminimize floor-to-floorheights when there is no requirement for a deep false ceiling.

Flexibility in room layout:

Allows Architect to introduce partition walls any anywhererequired, allows owner to change the sizeof room layout, allows

choice of omitting false ceiling and finish soffit of slab withskimcoating.

Saving in building height:

Lower storey height will reduce building weight due to lowerpartitions and cladding to faade, approx.saves 10% invertical members, reduce foundation load.
http://www.concretecentre.com/technical_information/building_solutions/frame_elements/slabs/flat_slab.aspxhttp://www.concretecentre.com/technical_information/building_solutions/frame_elements/slabs/flat_slab.aspx


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Design considerations WALL AND COLUMN POSITION:

Locate position of wall to maximise the structural stiffness for

lateral loads

Facilitates the rigidity to be located to the centre of building OPTIMISATION OF STRUCTURAL LAYOUT PLAN

the sizes of vertical and structural structural members can be optimised to keepthe volume of concrete for the entire superstructure inclusive of walls and liftcores to be in the region of 0.4 to 0.5 m3per square metre

CRACK CONTROL

It is advisable to perform crack width calculations based on spacing ofreinforcement as detailed and the moment envelope obtained from structural

analysisgood detailing of reinforcement will restrict the crack width to within

acceptable tolerances as specified in the codes and reduce futuremaintenance cost of the building

FLOOR OPENINGS

No opening should encroach upon a column head or drop

Sufficient reinforcement must be provided to take care of stress Concentration

PUNCHING SHEAR

always a critical consideration in flat plate design around the columnsinstead of using thicker section, shear reinforcement in the form of shear heads,

shear studs or stirrup cages may be embedded in the slab to enhance shearcapacity at the edges of walls and columns

MULTIPLE FUNCTION PERIMETER BEAMS

adds lateral rigidity

reduce slab deflection


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Parking lots

Bridges

High rise buildingsTrump International Condo/Hotel Tower, Las Vegas

Aqua building in Chicago

Applications


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Vierendeel Girders


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Girder is the term used to denote the main horizontal support of a structure which supports smaller

beams .Girders often have an I-shape cross section for strength, but may also have a box shape, Z shape

or other forms. A girder is commonly used many times in the building of bridges, and planes.

Because deep trusses are particularly efficient for long

spans they are sometimes designed to be a full storey-

height deep, the top boom being within the upper floor zone

and the lower floor zone incorporating the bottom boom.

The Vierendeel truss/girder is characterized by having only vertical

members between the top and bottom chords and is a statically

indeterminate structure.

Elements in Vierendeel trusses are subjected to bending, axial

force and shear , unlike conventional trusses with diagonal web

members where the members are primarily designed for axial loads.

Connections :


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The utility of this type of structure in buildings is that a large amount of the exterior

envelope remains unobstructed and can be used forfenestrationand door

openings. This is preferable to a braced-frame system, which would leave some

areas obstructed by the diagonal braces.

Because of its floor-to-floor structural depth one Vierendeel truss can carry two

floors,thus reducing the number of perimeter beams

Advantages :
http://en.wikipedia.org/wiki/Windowhttp://en.wikipedia.org/wiki/Window


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The Vierendeel girder design is sometimes adopted in the design of footbridges. In traditional truss design,

triangular shape of truss is normally used because the shape cannot be changed without altering the length of

its members. By applying loads only to the joints of trusses, the members of truss are only subjected to a

uniform tensile or compressive stress across their cross sections because their lines of action pass through a

common hinged joint.

The use of this girder enables the footbridge to span larger distances and present an attractive outlook.

Disadvantages :

It suffers from the drawback that the distribution of

stresses is more complicated than normal truss structures

Limited usage due to higher costs compared to a

triangulated truss.


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YALE UNIVERSITY LIBRARY- CONNECTICUT


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Portal Frames


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P O R T A L F R A M E S C A N B E D E F I N E D A S T W O - D I M E N S I O N A LR I G I D F R A M E S T H A T H A V E T H E B A S I C C H A R A C T E R I S T I C S O F

A R I G I D J O I N T B E T W E E N C O L U M N A N D B E A M .

T H E M A I N O B J E C T I V E O F T H I S F O R M O F D E S I G N I S T O

R E D U C E B E N D I N G M O M E N T I N T H E B E A M , W H I C H A L L O W ST H E F R A M E T O A C T A S O N E S T R U C T U R A L U N I T .

T H E T R A N S F E R O F S T R E S S E S F R O M T H E B E A M T O T H EC O L U M N R E S U L T S I N R O T A T I O N A L M O V E M E N T A T T H EF O U N D A T I O N , W H I C H C A N B E O V E R C O M E B Y T H EI N T R O D U C T I O N O F A P I N / H I N G E J O I N T .

M A T E R I A L S Wood eg glulam

Steel reinforced precast concrete

steel

P O R T A L F R A M E S A R E D E S I G N E D F O R T H E F O L L O W I N GL O A D S :

roof load

wind load

W H I L E D E S I G N I N G , C A R E S H O U L D B E T A K E N F O R P R O P E R

Joints

Foundation

Bracing


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Usually the span of the structure is 20 to 30 m with column spacing of 6-12 m The maximum span of the structure is 80m with column spacing of 24m

An clear height (from the top of the floor to the underside of the haunch) between 5 and12 m

A roof pitch between 5 and 10 (6 is commonly adopted)

A stiffness ratio between the column and rafter section of approximately 1.5

Advantages

They are very efficient for enclosing large volumes, therefore they are often usedfor industrial, storage, retail and commercial applications as well as for agriculturalpurposes.

They can be erected quickly

steel has the highest strength to weight ratio, resulting in a building frame that will

not warp or crack.

It is also resistant to expansion and contraction due to changes in the weather,which helps ensure that the materials used to create the rest of the building will notcrack or buckle

Disadvantages

Skilled labour required

Metal needs to be galvanised before erection, increasing the cost


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Components of a portal frame

Deflection under horizontal loa

Deflection under vertical loads

Apex connection

Eavesconnection


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Cellular beam portal frameHayes garden centreUK

Pitched roof symmetric portal fraLancashire Waste Development

UK


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Portal frames at Chattarpurmetro station


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Lattice Girder


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Girder = a support beam in construction; Lattice = an arrangement of objects in a regular periodicpattern in 2 or 3 dimensions.

A lattice girder is a type of girder with a criss-crossed web design, such as in gardening lattices,between the two edges of the girder. The diagonal lines of steel give support in all directions, helping to

prevent the girder, which is one of the main support elements in a bridge design, from bending. A lattice girder can be used as is or covered in shot-crete for additional load strength. Modern lattice girders are most often used in the 3-bar or 4-bar configurations, and the diagonal

stiffening delivers load along the entire length of the bars for resistance. Often seen on older bridges or buildings, lattice girders are also widely used in mining tunnels for roof

support during excavations and can be erected quickly.All load-bearing elements are produced according to the particular demands in tunneling:High strength

Great deformabilityWell suited for weldingSteel grade: Reinforcing or construction steel

Lattice girder specificationsUpper longitudinal bar diameter variable = 8-14 mmLower longitudinal bar diameter fixed = 8-14 mmLattice girder height (min-max) variable = 70-400 mmLattice girder length (min-max) = 1000-12000 mmBending pin diameter fixed upper & lower = 20 mmPitch fixed = 200 mmStirrup wire diameter fixed = 6 mm

i ld f A li i


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Fields of ApplicationPassive support system for the excavated cross sectionProfile template for the excavation geometryBearing for pre-support elements

Advantages Immediate support in the excavation areaUtilization as a true-to-form template for shotcrete applicationEasy and quick assembling, simple handlingOptimum bond and interconnection with the shotcrete liningSimple adjustment and shaping to the excavation geometry Ideal bearing for spiles and lagging boardsSpiles may be installed both above or through the latticegirders

One of the most well-known examples of lattice girder design isFrance's Eiffel Tower, built in latticed iron in the 1800s.

3-bar configuration 4-bar configuration

Wall plate beams

b id d i d l d


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Motorway bridge near Grudzidz, Poland

The URW wheel allows moving to take place in a longitudinaldirection and a horizontal sliding support at the level of the girder.

LGS roofs can easily be moved by hand and are alsoequipped with brakes. This ensures a fast movingprocedure.

Roofing for the scaffold construction isprovided whereby the integrated sheetingoffers the best protection against wind andweather. The benefit of using sheeting is its

translucence and artificial lighting is notabsolutely necessary.

For the reinforcement and concretingwork, the weather protection roof ispushed over the permanently installedreinforcement tent. All requiredmaterials can then be craned in withoutany problems.

Two movable weatherprotection roofs spantwo incrementallaunching facilitiesfor the construction

of a bridgesuperstructure. Thisconstruction providesprotected workingareas and guaranteescontinuousoperations even inwinter as well as

resulting in the bestconcrete quality.

In this way, the contractor has been able to shorten theplanned construction period and has thus contributedsignificantly to the success of the project.


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Folded Plates


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Folded plates assemblies of flat plate or slab, inclined in different directions and joined along theirlongitudinal edges.

The main objective of this structure is to make long span column less spaces. The structural characteristics of folding structures depend on the shape of the folding

1. Prismatic : if they consist of rectangular plates. uninterrupted and linked folding edges where parallel

and skew up folds and down folds alternate.2. Prismoidal, triangular or trapezoidal.-several folds intersect like a bunch in one single spot

On its material1. Concrete2. Timber3. metal4. synthetics

on the design of the bearings.

Advantages- the material required is much less than in convention al sysytem. Construction cost is usually low The analysis and design of folded plate system is relatively easy. Aesthetically pleasing structures can be designed using variety of shapes and forms. The structure has high intrinsic rigidity, high load carrying capacity which makes it economical over large

span.

Disadvantages- Skilled labors required.

Prismatic Prismoidal


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Field of application-

Auditorium dome Bowling alley roof Bakery Gymnasium

Assembly hall Art academy

Different designs in folded plate system

Flat slab over a lager cloumn free span

Folded slab over a lager cloumn free span


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Folded plate roof for gymnasium and cafeteria-

There are windows between the tilted Z shaped elements to provide overhead natural light. There are twospans of about 50 and 60 ft. The supports at the ends of the elements are arranged to make it appear that theroof floats on its supports. The shell thickness is three inches

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