long span structures in concrete and steel

Long Span Structures

Garima RajputRithika Ravishankar

Roshani Tamkhade

Rasika DongareAishwarya Khurana

Nilesh ManeLong Span Beams Long Span Trusses Portal Frames

Long span structures create unobstructed, column-free spaces greater than 30 metres (100 feet) for a variety of functions.

Long Span Beams Long Span Trusses Portal Frames

Visibility Flexibility Large Scale Storage

AuditoriumsStadiums

Exhibition hallsManufacturing facilities Aircraft hangars

Structural systems forLong Span Buildings


Subject to Bending

Funicular Structures

Both tensile and compressive

forces

Pure tension or pure

compression1. Girder2. Truss (Depth

to span ratio – 1:5 to 1:15)

3. Two-way grid4. Two-way truss5. Space truss

(Depth to span ratio – 1:35 to 1:40)

1. Parabolic Arch2. Tunnel vault3. Domes4. Cable stayed

roof5. Bicycle wheel6. Warped

tension surfaces

Structural systems forLong Span Buildings


Subject to Bending

Funicular Structures

Both tensile and compressive

forces

Pure tension or pure

compression1. Girder2. Two-way grid3. Truss (Depth

to span ratio – 1:5 to 1:15)

4. Two-way truss5. Space truss

(Depth to span ratio – 1:35 to 1:40)

Pure Compression:1. Parabolic Arch2. Tunnel vault3. Domes

Pure Tension:4. Cable stayed

roof5. Bicycle wheel6. Warped

tension surfaces


Some of the oldest long span structures dated back to the Roman civilization. However, most long-span buildings then were single level constructed using vaults and domes.

By the late 20th century, durable upper limits of span were established for these types: the largest covered stadium had a span of 204 meters (670 feet),

the largest exhibition hall had a span of 216 meters (710 feet),

and the largest commercial fixed-wing aircraft had a 75–80 meter (250–266 foot) span hangar.

The major evolution in long span section-active structures has occurred in the aspect of shift from in-situ to precast construction.

History and Evolution

Old-to-New long span structures with their height

and spans

Another method of classification of long span structures is as follows


Form - Active

Systems of flexible, non-rigid matter, inwhich the redirection of forces is effected by particular form design and characteristic form stabilization

Systems of rigid, solid, linear elements, in which redirection of forces is effected by mobilization of sectional forces

Systems of short, solid, straight lineal members, in which the redirection of forces is effected by vector partition, i.e. by multidirectional splitting of single force simply to tension or compressive elements

Section - Active Vector - Active Surface - Active

Cable StructuresTent Structures

Pneumatic structuresArch structures

Flat TrussesCurved TrussesSpace Trusses

Beam StructuresFramed structures

Slab structures

Systems of flexible or rigid planes able to resist tension, compression or shear, in which the redirection of forces is effected by mobilization of sectional forces

Plate StructuresFolded StructuresShell structures


IntroductionBeams greater than 30 meters in span are said to be LONG SPAN BEAMS.

The use of long span beams results in a range of benefits, including flexible, column free internal spaces, reduced foundation costs, and reduced steel erection times.

Many long span solutions are also well adapted to facilitate the integration of services without increasing the overall floor depth.

The design of long span steel and (steel - concrete) composite beams is generally carried out in accordance with the IS.

>30 meters

Conventional beam Long span beam


TypesMost common type of long span beams used today are: Plate Girders, and Beams with Web Openings.The popular construction methodology is composite construction (steel + concrete)

The types of long span beams are:

1. Parallel beam approachThe parallel beam approach is effective for spans up to around 14 m. Floor grids comprise two layers of fully continuous beams running in orthogonal directions. Services running in either direction can be integrated within these two layers, so that services passing in any direction can be accommodated within the structural floor depth. A further benefit is that, being fully continuous, the depth of the beams themselves is reduced without incurring the expense and complexity of rigid, full strength connections .


Types2. Composite Beam with Web openingsWeb openings are typically formed in beams to allow services to pass through the beam, reducing the effective overall depth of floor construction for a given spanning capability or for aesthetic reasonsSpan: 10 to 16 m.

The alternative way of forming the web openings is simply to cut them into the plate used to form the web of a plate girder, or the web of a rolled section.

The openings introduce a number of potential failure modes not found in solid web beams. Large openings may require stiffening to avoid instability (buckling) of the web posts.

Failure in

cellular beam


Introduction

Failure in cellular beam


With stiffened web openings


Types3. Tapered GirdersTapered girders can be a cost effective solution in the span range 10 m to 20 m. They are another solution that allows services to be accommodated within the structural floor zone.

The depth of the girder increases towards mid-span, where applied moments are greatest, and thereby facilitating hanging services under the shallower regions near the beam supports. It is also possible to form web openings in tapered girders in regions of low shear, towards mid-span. These provide more options for service integration .


Types4. Stub girdersStub girders are a Vierendeel form of truss. The bottom chord is typically formed from a shallow open section (H-beam), on which sit short lengths (stubs) of deeper I-sections.

The number of elements/surfaces associated with a stub girder may increase the cost of fire protection compared with simpler solutions.

Spans in excess of 20 m can be economically achieved. Services and/or secondary beams can pass through the gaps between the beam stubs, reducing overall construction depth.


Types5. Haunched Composite beamsHaunches may be added at the ends of a composite beam to provide moment continuity. The stiffness and strength of the connections mean that the rest of the span can be shallower (the bending moment diagram is 'lifted' and the effective stiffness of the beam substantially increased), and services passed under it. In buildings where the services are likely to need frequent replacement (for example in hospitals ), hanging the services under the beams can be advantageous.

Spans in excess of 20 m can readily be achieved.


Types6. Composite trussesComposite trusses, which use the concrete slab as the upper chord in the final state, can achieve spans in excess of 20 m. This means they have been used when very long spanning capability was needed. The main disadvantages are that during the construction phase the truss may be rather flexible (laterally), and that in the final state the costs of fire protection can be high given the large number of surfaces to protect. Clearly one of the prices to pay for the spanning ability is that fabrication cost is higher than for a plain beam. Services can be passed through the gaps between the truss members to reduce overall floor depth.


Construction drawingsPrecast concrete beam sections

Reinforced Concrete(In - situ / Precast)

Timber, Laminated Timber

Glue-laminated timber can be prefabricated using metal connectors into trusses that span up to 45 metres (150 feet)Most economical forms: the pure compression shapes of the multiple-arch vault, with spans up to 93 metres (305 feet), and ribbed domes, with spans up to 107 metres (350 feet). Used as industrial storage buildings for corrosive materials

MetalStructural steel

(Cut on site / Prefabricated)Bending structures originally developed for bridges, such as plate girders and trusses, are used in long-span buildings. Plate girders are welded from steel plates to make I beams that are deeper than the standard rolled shapes and that can span up to 60 metres (200 feet)


Materials

Cleddau Bridge

• The Cleddau Bridge is a toll bridge on the A477 road that spans the River Cleddau between Neyland and Pembroke Dock, Wales.

• Errors in the box girder design caused it to collapse during its construction in 1970. Long Span Beams Long Span Trusses Portal Frames

Case study: Failure

• It failed during its erection by cantilevering segments of the span, out from the piers.• The bridge was designed as a single continuous box girder of welded steel. • The span that collapsed was the second one on the south side. The boxes were

fabricated in sections and moved over the previously built sections, aligned in place and welded.

• The collapse occurred when the last section of box for the second span was being moved out along the cantilever.

• This section slid forward down the cantilever buckled, at the support and collapsed into the river (Fig 2), killing four men, including the site-engineer.


• Investigation of collapse showed that the collapse was due to the buckling of the diaphragm at the support (i.e., at the root of the second span being erected).

• The diaphragm was torn away from the sloping web near the bottom. This caused reduction in the lever arm between flanges resisting negative bending moment at the support.

• The tendency of the bottom flange to buckle was inevitably increased by the reduction of the distance between the flanges, as this increased the force needed in each flange to carry the moment with the reduced lever arm.

• The support diaphragm was, in effect, a transverse plate girder, which carried heavy loads from the webs of the plate girder at its extreme ends and was supported by the bearings as shown in Fig 3.


Modern Techniques For Long Span Beams –Precast Concrete

PRECAST BRIDGES

• BENEFITS TO OWNERo Reduction in the duration of work zoneso Reduced traffic handling costso Reduced accident exposure riskso Less inconvenience to the traveling

public• BENEFITS TO OWNER

o Reduced exposure to hazardso Reduced accident exposure riskso Fewer weather delayso Lower costs


Benefits of using precast concrete beams

Quality and Corrosion Resistance

Immediate Delivery and Erection

No Curing Time


Long Span Beam + Truss

Typical multi girder system with x-type intermediate cross frames and stay-in-place formwork used for constructing a

deck-slab

Curved roof trusses can be used to support structural decks with a suspended ceiling. The natural open web of the steel truss allows for the simple passage of services.

Long Span Beams Long Span Trusses Portal Frames+

Cross – over 1

Long Span TrussesLong Span Beams Long Span Trusses Portal Frames

• A roof truss is a structure that includes one or multiple triangular units that include straight slender members with their ends connected via nodes.

• Trusses are frame works in which the members are subjected to essentially axial forces due to externally applied load.

• Bending leads to compression in the top chords (or horizontal members), tension in the bottom chords, and either tension or compression in the vertical and diagonal members, depending on their orientation.

External loads on the nodes Tension & Compression members


Introduction

Pitched Roof Truss

• A pitched roof truss has a bottom chord with two inclined top chord connected through gusset plates or panels. Extra supports in the form of struts are also added as per the requirement.•These trusses have a greater depth at mid-span.

Pitched roof truss


Categories

• A pitched roof truss has a bottom chord and a top chord that run parallel to each other. Extra supports in the form of struts are also added as per the requirement.

Parallel chord truss


Categories Parallel Chord truss

King post truss:

A king post is a central vertical post used in architectural or bridge designs, working in tension to support a beam below from a truss apex above

Queen post truss:

A queen-post bridge has two uprights, placed about one-third of the way from each end of the truss. They are connected across the top by a beam and use a diagonal brace between the outer edges.


Types of trusses

Pratt truss:

•In Pratt trusses, the web members are arranged in such a way that under gravity load the longer diagonal members are under tension and the shorter vertical members experience compression.

•These trusses can be used for spans that range between 6-10m.Howe truss:

•The converse of the Pratt is the Howe truss. This is commonly used in light roofing so that the longer diagonals experience tension under reversal of stresses due to wind load.

•These trusses can be used for spans that range between 6-30m.


Types of trusses

Fink truss:

Fink trusses are used for longer spans having high pitch roof, since the web members in such truss are sub-divided to obtain shorter members.

Fan truss:

Fan trusses are used when the rafter members of the roof trusses have to be sub-divided into odd number of panels.

Scissor truss:

Scissor roof truss can particularly be found in cathedrals. The upside here is that the ceiling gets vaulted and you receive more space in the attic.


Types of trusses

Warren girder:

•Parallel chord trusses uses webs of the same lengths and thus reduce fabrication costs for very long spans.

•Modified Warren is used with additional verticals, introduced in order to reduce the unsupported length of compression chord members.

Lattice girder:

•It is commonly made using a combination of structural sections connected with diagonal lacing. This member is more correctly referred to as a laced strut or laced tie.


Types of trusses

Vierendeel truss:

The Vierendeel truss is a structure where the members are not triangulated but form rectangular openings, and is a frame with fixed joints that are capable of transferring and resisting bending moments.

K- type truss:

In the case of very deep and very shallow trusses it may become necessary to use K patterns for web members to achieve appropriate inclination of the web members.

North light truss:

In the north light truss, skylights or openings are provided to allow north light inside the structure.


Types of trusses

Types Of Loads

The following are the various types of loads to be considered while calculating the stresses.

• Dead Load • Live Load• Longitudinal Force• Horizontal Forces• Wind Load• Seismic Load

Direction of load transfer in Trusses


Load Analysis

Assumptions Behind Truss Analysis

• Truss members are connected at their ends only, and they are connected by friction-less pins.

• So you don't have to consider any secondary bending moment induced do to force of friction.

• Truss is loaded only at joints.• Weight of truss members can

be neglected, compared to load acting on the truss.


Load Analysis

• Force developed in a truss member is always axial. It can be either tensile, or compressive.

• If a member is under tensile load, this will be the direction of internal force developed .

• So you can notice that, under tensile load, internal force developed in the member is directed away from the joint.

• Similarly in case of compressive force, the internal force developed in the member is directed towards the joint.

Nature Of Load In Truss Members


Load analysis

Wood

Metal - Steel


Materials

Concrete – Precast / Prestressed

Bamboo


Materials

While bamboo has been used for centuries, the traditional methods of lashing bamboo together are not appropriate for the design of long span trusses.

• These lashed connections don’t fully utilize the full strength of bamboo member.

• They rely solely on friction, the load transfer between members is limited and thus structures require more members to do the same job that one could if it were well connected.

• Finally complex geometries with many members framing into one node or three dimensional space frames are difficult if not impossible to construct.

Traditional Bamboo ConnectionLong Span Beams Long Span Trusses Portal Frames

Alternative Materials Bamboo connections

• Since all fibers in a bamboo run parallel once a bolt is placed through it and the connection loaded in tension, the bolt acts like a wedge and splits the bamboo.

• Also the puncture allows moisture to enter the culm and accelerate decay

• Based on the proprietary nature of the hubs, their installation requirements, and the desire to develop cost effective, simple connections, the research focused on an alternate connection type to eliminate these challenges. Modern Bamboo Connection

These connections solve the issues of complex geometries by joining the members at a central hub. While they provide a standardized connection throughout a project, they are not readily available.


Alternative Materials Bamboo connections

• This connection requires filling several hollow cells of the bamboo with concrete and embedding a threaded rod.

• The new connection involves embedding a common steel reinforcing bar into a mortar filled bamboo culm and fillet welding several of these members to a steel gusset plate.

• The inner surface of the bamboo is roughened to provide a bond between the mortar and the bamboo while avoiding puncturing the member.

• Because the rebar is embedded in mortar, the load is transferred evenly across the member’s cross section and can transfer high axial loads to the bamboo.

• Finally, the incorporation of the steel gusset plate makes the bamboo easy to connect in any configuration desired

BAMBOO TRUSS

STEEL GUSSET PLATE

SECTION OF CONNECTIONLong Span Beams Long Span Trusses Portal Frames

Modern techniques Bamboo connections

Gatton Railway Bridge, AustraliaPratt truss design

Powerhouse roof, BoiseFink truss design


Applications

Church roof, AmericaScissor truss design

Industrial shed, England North light truss design


Applications

Convention centreWarren girder

Vierendeel bridge, BelgiumVierendeel truss design


Applications

• Quick Installation- The primary advantage of a truss is that it can be installed quickly and cost-effectively, even without heavy equipment to lift it into place.

• Increased Span- The unique properties of a triangular object allow trusses to span across longer distances.

• Load Distribution- The shape of a triangle allows all of the weight applied to the sides (or legs) to be redistributed down and away from the centre. In trusses, this transfers the entire weight of the roof to the outer walls.

• Accessibility- Since the bottom rail of a truss is typically the ceiling of the rooms below, the triangular spaces of the trusses themselves form accessible paths for the installation of HVAC, electric and other utility applications. The central void of a truss system is generally the attic of a home, with the slope of the roof forming the legs of the triangle.


Advantages Roof

• Transportation- Sometimes they are too big for a truck. In such cases, specially designed truss trailers have to be used to haul the structures around.

• Metal roofs- I. Skilled labour is required to install metal roof trusses.II. They are not energy efficient since they allow more heat

to escape from the structure.III. When the metal is cut, drilled, scratched or welded, rust

can become a problem.

• Wooden roofs-I. Wooden roofs are susceptible to fire.II. Wood can rot or become infested with bugs if not

maintained and treated properly.


Disadvantages Roof

• They are light, but strong- As they use small timbers or beams of metal, the trusses would be light, but are strong enough to handle loads thanks to the ridged triangles.

• Accessibility- They allow placement of roadways on the structure itself, such as a rail, to be placed straight across it.

• Material usage- Because of its design, it makes good use of limited construction materials to achieve strength that far outweighs its cost.

• Can be constructed in difficult site conditions- These types of bridges can be built quickly in places where many other types cannot, linking areas that other types will not work in.


Advantages Bridges

• They require high costs- While it is said that these bridges’ design efficiently uses materials, it does use a lot of them. Building a truss bridge can be costly, and its upkeep requires time and money.

• Wastage of materials- Without the proper design and work practice, constructing a truss bridge can result to waste of materials.

• Maintenance- Because of the amount of materials they use, these types of bridges require a lot of upkeep.

• Complicated Design-The design of truss bridges can become very complicated depending on the situation. The triangles have to be the perfect size and there has to be the perfect amount in order for the truss bridge to be safe.


Disadvantages Bridges


Case study

Howrah Bridge is a cantilever bridge with a suspended span over the Hooghly River in West Bengal, India.

• Address : West Bengal• Total length : 705 m• Opened : February 3, 1943• Construction started : 1935• Location : Howrah, Kolkata• Architect : James Meadows

Rendel• Material : James Meadows

Rendel


General Information Howrah Bridge

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• Bridge type : Suspension type Balanced Cantilever• Central span :1500 ft between centers of main towers• Anchor arm : 325ft each• Cantilever arm : 468ft each• Suspended span : 564ft• Main towers are 280ft high above the monoliths and 76 ft apart at the top

1500 ft325 ft 325 ft

468 ft 564 ft 468 ft280 ft

Anchor Arm Cantilever Arm Suspended Arm Cantilever Arm Anchor Arm


General Information Howrah Bridge

• All members of the super structure comprise built up riveted sections with a combination of high tensile and mild steel. No nuts and bolts.

• Road way beyond the tower is supported on ground leaving anchor arm free from deck loads• Bridge deck comprises 71 ft carriage way and 15 ft footway projecting either side of the trusses

and braced by a longitudinal fascia girder.• The deck system consists of cross girders hung with pinned connection.• They support a continuous pressed steel system over which deck concrete is laid out.

Construction Howrah Bridge



A structure at least one portion of which acts as an anchorage for sustaining another portion which extends beyond the supporting pier. • A simple cantilever span is formed by two cantilever arms extending from opposite sides of an obstacle to be crossed, meeting at the center.

• In a common variant, the suspended span, the cantilever arms do not meet in the center; instead, they support a central truss bridge which rests on the ends of the cantilever arms.

• The suspended span may be built off-site and lifted into place, or constructed in place using special travelling supports.

Cantilever Bridges Function

TEMPORARY PIER CLOSURE ENDLong Span Beams Long Span Trusses Portal Frames

TYPICAL CONSTRUCTION SEQUENCE

• Some steel arch bridges are built using pure cantilever spans from each side, with neither falsework below nor temporary supporting towers and cables above.

• These are then joined with a pin, usually after forcing the union point apart, and when jacks are removed and the bridge decking is added the bridge becomes a truss arch.

• Such unsupported construction is only possible where appropriate rock is available to support the tension in the upper chord of the span during construction, usually limiting this method to the spanning of narrow canyons.

Construction

International airport, China


Long Span Truss + Portal Frame

Long Span Beams Long Span Trusses +

Cross – over 2

Portal Frames

Reticular Loom




Cross – over 2

Portal Frames

Denver Union Station




Cross – over 2

Portal Frames

Portal FramesLong Span Beams Long Span Trusses Portal Frames

Portal frames were first developed during the Second World War and became popular in the 1960‘sThey are now commonly used to create wide-span enclosures, where a clear space is required uninterrupted by intermediary columns. They were originally used because of their structural efficiency, meaning that large spaces could be enclosed with little use of materials and for a low cost.They tend to be lightweight and can be fabricated off site, then bolted to a substructure.The portal frames themselves may be left exposed to the internal space, and if carefully designed can be very beautiful. Materials used for portal frame is Steel or steel reinforced precast concrete although can also constructed using laminated timber such as glulam


Curved portal frame (cellular beam)

Duo-pitch portal frame

Portal with crane

Types of Portal frames




Two-span portal frame Portal frame with external mezzanine

Where a pitch is required, portal frames can have a mono pitch, or can have a double pitch with a rigid joint at the apex.

Tied portal frame

Mono pitch portal frame



Other forms include; tied portal frames, propped portal frames and multi-span portal frames which can cover very large areas.

Where the portal frame includes a pitch, the wider the span of the frame, the higher the apex. To reduce the overall height, a curved rafter might be adopted, or a mansard form.A curved, or mansard form increases the pitch of the roof towards the eaves, where the runoff is likely to be at its greatest.



Portal frames are a type of structural frame, that, in their simplest form, are characterized by a beam (or rafter) supported at either end by columns.

Portal frame structures are often clad with prefabricated composite metal panels, incorporating insulation. Masonry cladding may be provided at low level to give greater resilience and security.

A secondary framework of purlins fixed to the rafters and rails fixed to the columns provides support for cladding.Generally, a building structure will be formed by a series of parallel portal frames running down the length of the buildings, typically 6 to 8m apart.

Single Skin Trapezoidal SheetingDouble Skin Trapezoidal Sheeting

Typical Portal frame


http://www.designingbuildings.co.uk/wiki/Column

http://www.designingbuildings.co.uk/wiki/Structure

http://www.designingbuildings.co.uk/wiki/Insulation

http://www.designingbuildings.co.uk/wiki/Masonry

http://www.designingbuildings.co.uk/wiki/Cladding

http://www.designingbuildings.co.uk/wiki/Purlins

http://www.designingbuildings.co.uk/wiki/Column

http://www.designingbuildings.co.uk/wiki/Cladding

http://www.designingbuildings.co.uk/wiki/Structure

Components Of Portal FrameTypical Portal frame


Portal Frame Connections

The major connections in a portal frame are the eaves and apex connections , which are both moment-resisting.

Portal frames are generally low-rise structures, comprising columns and horizontal or pitched rafters, connected by moment-resisting connections.Members of portal frames are jointed by means of welding and bolting so the joints of the frame could transfer moments also in addition to the axial load

Construction


The legs or stanchions of the portal frame need connecting at the bottom to a foundation.

Base Joint for Portal FramesConstruction


It is important that this joint is strong hence the use of wedge shaped pieces called gusset pieces to strengthen and increase the bolt area.

Ridge Joint for Portal FramesConstruction


It is important that this joint is strong hence the use of wedge shaped pieces called gusset pieces to strengthen and increase the bolt area.

Haunch Joint for Portal FramesConstruction


To help strengthen the framework and prevent movement diagonal bracing is used.


Diagonal Bracing for Portal FramesConstruction

Imposed loads on roofs depend on the roof slope. A point load, which is used for local checking of roof materials and fixings, and a uniformly distributed load, to be applied vertically.

In portal frames heavy point loads may occur from suspended walkways, air handling units etc. In certain situation it will be more appropriate to use truss or lattice girder rather than a portal frame.

Cranes impose both horizontal and vertical loads on the structure, part of loading is due to dynamic effects. The vertical load will be composed of a load due to weight of the crane bridge. The horizontal load due to crane surge and reaction from the wheel.

DEAD LOAD (self weight)

SERVICE LOADS

CRANE LOAD

Types of LoadsLoad analysis


When a portal frame is close to the boundary,there are several requirements aimed at stoppingfire spread by keeping the boundary intact:• The use of fire resistant cladding• Application of fire protection of the steel up

to the underside of the haunch

Two kinds of accidental loads are to be considered• Impact of unusual loading• Drifted snow• The opening of the dominant opening

which was assumed to be shut.

Wind uplift may be important in terms of rafter stability , but provided that adequate restraint can be provided to stabilize the bottom flange of the rafter near the apex

WIND LOAD

WIND LOAD

ACCIDENTAL LOADS

FIRE LOAD


Types of LoadsLoad analysis

Rafters are subject to high bending moments in the plane of the frame, that vary from a maximum ‘hogging’ moment at the junction with the column to a minimum sagging moment close to the apex. They are also subject to overall compression from the frame action. They are not subject to any minor axis moments.Although member resistance is important, stiffness of the frame is also necessary to limit the effects of deformed geometry and to limit the deflections.

Asymmetric or sway mode deflection Symmetric mode deflection

Bending MomentLoad analysis


Bending moment diagram under asymmetric loading


Bending moment diagram under symmetric loading

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long span structures in concrete and steel

Engineering