hollow sections for multi storey columns

Corus Tubes library publication

The contents of this publication are current, when republished it will be in the new Corus housestyle.

Hot Finished SHS forMulti-storey Columns

Contents

01 Structural Hollow Sections

01 Steel and the Environment

02 Moda in Casa Store

04 Fleet Place House, London

06 Montevetro

10 Rank Xerox Facilities Building

12 Queensberry House, London

14 Tay House, Glasgow

16 Putney Bridge Restaurant

17 Tutorial Block, Manchester College of Art & Technology

18 Westside Office Building, Apsley

19 Buttercrane Centre multi-storey Car Park, Newry

20 Connections

26 Structural Design

28 Fire Resistance

30 Cost Comparisons

32 Bibliography

Structural hollow sectionsmake beautiful and efficient buildings

Steel and the Environment

Structural hollow sections make beautiful and efficient buildings. This is evident inmany single storey buildings where SHS has allowed Architects to give full vent toexpressing the stucture in creative, exciting architecture. Following recentdevelopments in beam to column connections, and in composite and fire resistantconstruction, Architects have seized the opportunity to exploit the structural andaesthetic advantages of SHS in multi-storey buildings.

They present exciting design opportunities regardless of the number of storeys. When used in compression, as building columns, SHS are more efficient than other column types. The resulting reduction in structural weight presents the opportunity to make storey-by-storeycost savings.

SHS, with or without concrete filling, provide smaller column footprints than other designsolutions, increasing lettable floor area. Further, because each serial size of SHS has aconstant external dimension, the same column size can be maintained throughout the fullheight of the building - simplifying architectural details and ensuring economy in fabrication.

This all adds up to great news. Great news for building Owners, Developers and FacilitiesManagers as well as Architects, Engineers and Design & Build contractors.

Steel is the most recycled material in the world. Most scrap steel simply needscompressing or cutting up before being remelted. The quantity of steel remelted to make new products currently constitutes nearly 50% of the world’s annual steel production.

The production of steel meets increasingly stringent environmental standards. Some of thesestandards are imposed by law, others by the companies themselves. Modern steelworksnow incorporate dust and fume collection, water recirculation systems, noise control and, in many cases, odour control as well. Almost all of the liquid steel produced by British Steeluses the Basic Oxygen process and the continuous casting process, which takes 77% less energy than the traditional ingot method. The principle by-products of the steelmaking process are recyclable too. For example, slag, which is non-pollutant, can be recycled for use in slag cements and as a sub-base in road construction.

The use of steel sections in buildings gives unparalleled opportunities for remodelling ofbuilding frameworks representing a major cost and environmental advantage over thosecases where demolition and reconstruction are required. If necessary, steel-framed buildingsmay be designed for demounting and reassembly.

Hot Finished SHS for Multi-storey Columns 1

View of access stair and glazing panels

Side Facade Column showing attachments for glazing support

2 Hot Finished SHS for Multi-storey Columns

Case Study

A 3D grid allowingviews throughshimmering panelsof glass

Moda in Casa StoreArchitect: E NortonStructural Engineer: Prof dr ir M EekhoutFabricator: Octatube Space Structures BV

Detail of glazing panels


Moda in Casa Store

Circular hollow section columns are usedboth as principal load-bearing elements andas glazing supports in this showroombuilding which faces out onto the highstreet. The merchandise, high-techfurniture, is displayed within a pure three-dimensional grid made up of slender roundcolumns, glass walls and floors and lightcross-bracing. The building is three-storeyshigh, 45 metres deep and between 15metres and 18 metres wide, being set outon a 3 metre module. It is steel-framed withconcrete floors except within the front 3metre module where the floors are glass,supported by underslung, pyramid-shapedsteel rods. The front part of the building isstabilised against earthquake forces bysteel cross-bracing on all three sides. A 200mm wide flexible rubber zone, on oneside of the building allows up to 200mm ofmovement to occur between the front

module and the wall behind which serves tostabilise the main part of the building. On the opposite side, cross-bracing is usedto stabilise the facade. The interior CHScolumns are connected to beams by flangeand web plates which are designed todevelop moment and shear forces. All glazing panels on the facades aresupported at their four corners by brackets.On the front facade, the glazing panels havedimensions of 1.5m x 1.5m and thebrackets are either welded to supportingcolumns, which are full-height 90mmdiameter CHS, or are supported by a cat’scradle arrangement of diagonal rods behindthe glass line. These rods double up aslateral bracing. On the sides, the glazingpanels have dimensions of 3m x 1.5m withall brackets being welded to the CHSsupporting columns, which are 193.7mm in diameter.


Case Study

Steel tubularcolumns and rakingstruts support an eight storey office building

Architect: Skidmore Owings & Merrill Structural Engineer: Waterman PartnershipClient: Heron Property Corporation Limited

Axonometric view of building

Vertical section on beam to column connection detail

Plan on beam to column connection

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Plan at typical floor level

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FFL

323.9 CHS column with concrete infill

cladding line

17.5 m span asymmetric floor truss

truss end block connection with welded side plates

60 diameter clevis pin

solid block column bracket

323.9 CHS column with concrete infill

450 x 250 x 16 RHS edge beam


Fleet Place House, London

Built on one of the last sites to be developedon Fleet Place, this new office building isfitted onto a restricted city centre locationand placed astride a railway line immediatelybelow. The office is eight storeys high, with asuperstructure plan area of 71 x17 metres,below which is a car park in the basementlevel with plant space and equipment at thesub-basement level. It has been ingeniouslydesigned to re-use the existing pile capfoundations of the previous building on thesite. There are no internal columns. The 323.9mm diameter hot-finished steelCHS columns are placed outside thecladding line and support steel truss beamswith a composite deck spanning 17.5metres across the full width of the building.At ground floor level, heavy raking CHScolumns effect the difficult transitionbetween the positions of the existing

foundations and the column positions for theoffice superstructure. A horizontal steel trusscast into the first floor slab enables the largeforces at the knuckle joints at the top of theraking columns to be balanced. The steelsuperstructure is separated from thesubstructure by acoustic bearings. Shearkeys with vertically mounted thrust bearingsprevent sliding at the joint. The walls are tiedacross the acoustic joint by high tensile steelbars. The required fire rating of internalstructural elements is generally two hoursand one hour in the basement. However afire load analysis test of the external tubularcolumns indicated that, for these elements,a fire rating of 35 minutes would be sufficientbecause of their distance from the seat ofany fire. The columns are filled with concretewhich achieves a fire rating of 45 minutes.


Case Study

A twenty storeyapartment buildingby the River Thameswhere residentsenjoy the view of London’sspectacular skyline

Montevetro Apartments, London.Architect: Richard Rodgers PartnershipStructural Engineer: Waterman Partnership


Montevetro

The Montrevetro apartment building occupiesa prime position on the bank of the RiverThames and has fine views out. A consciousattempt has been made not only to takeadvantage of its riverside setting but minimisecapital outlay per unit by designing to a largescale. The shape of the building is distinctive,a block tapering from 20 storeys at the endnearest the Thames to 3 storeys at the otherend. The structure of the building comprisessteel and concrete columns and load-bearingconcrete walls, which all support concreteflat-slab floors at each level. The steelcolumns are CHS and are used along thewest facade so as to give minimum visualobstruction; concrete columns in this positionwould have been too bulky.

The CHS columns are spaced at 3.6 metrecentres inside the cladding line and providesupport to the external edge of the concreteflat slab; they support between 3 and 20storeys but have been able to be standardisedin the typical case to 244.5 CHS columns withwall thicknesses varying from 10 to 16mm,depending on the plan position and the floorlevel. The maximum size used is a 355.6 CHScolumn 16mm thick. The steel column headplates were sized so as to avoid the need toprovide shear link reinforcement in theconcrete slabs.


MontevetroCase Study

Montevetro Apartments, London.Architect: Richard Rodgers PartnershipStructural Engineer: Waterman Partnership

The fire rating required for the columnsvaried between one and two hours and wasachieved by combining concrete filling ofsteel columns with use of an intumescentpaint. The paint thicknesses vary from0.97mm to 2.42mm, being optimisedagainst column wall thicknesses. No column reinforcement was used. The columns were brought to site in three-storey height lengths. It was found possibleto dispense with the need for temporaryerection brackets at column splices bygrinding flush the ends of the CHS columnsover part of their thickness and welding theremaining prepared faces. RHS columnswere used, as well as CHS columns, in locations where their shape better suited the surrounding construction.


Vertical section at typical shearhead connection to floor slabs

Plan on shearhead connection

244.5 x 16/ 355.6 x 16 CHS

with shop applied intumescent paint

zone for site applied intumescent paint

250 or 275 concrete slab

cruciform locating spigot

butt joint

20 or 30 steel soffit plate

Vertical section on typical column baseplate connection

Plan on part of building at fifth floor level

vertical tab plates

244.5 x 16/ 335.6 x 16 CHSwith shop applied intumescent paint

lacer bar


Rank Xerox Facilities BuildingCase Study

A three storeycolumn-freebuilding centredaround a naturallylit atrium

Rank Xerox Facilities Building, Welwyn Garden City.Architect: Nicholas Grimshaw & Partners Engineer: Ove Arup & Partners

When Rank Xerox commissioned thiselectronic research and development facility,they asked for flexible column-free space withgood circulation between areas. A threestorey open plan building centred around anaturally lit atrium has achieved this, togetherwith energy efficiency and construction costsat the lowest end of the expected range.Circular hollow sections were chosen ascolumns and are spaced at 3 metre centresaround the perimeter and at 6 metre centresaround the central atrium, serving both asmain structure and as supports for claddingand glazing. With this arrangement, thedesign achieved a 42 m x 30 m floor platearea with only eight internal columns. The floor beams are composite, spanning 12 metres from the atrium directly onto theperimeter columns, so that only light tiebeams are required between columns at

these positions. The relatively close spacing ofthese columns kept foundation costs to aminimum and allowed simple stripfoundations. The cross-braced framesproviding lateral stability, the escape stairs,goods lifts and service risers are located inpods outside the main space. In general,cable and electrics are taken in the raisedfloor zone and air-conditioning ducts in theceiling void. Two air-handling units are locatedexternally above the pods on the north andsouth sides with ducts dropping down on theoutside to serve each floor. Because thecooling load in this building is high, externalshading is provided, in particular by motorisedhorizontal louvre blades cantilevered from theperimeter columns; these also supportcatwalks. The cantilevered supports for themotorised louvres were welded to thecolumns in the fabrication shop, this giving


Vertical section through wall at perimeter

219.3 x 6.3 CHS column

457 x 152 x 67 floor beam

external catwalk

10mm fin plate bracket welded to column

CHS columns

Horizontal section through 219 x 6.3mm CHS showing cladding fixings

Plan at first floor level showing 12 metre clear spacesurrounding atrium; service pods and stairs are outside

Louvre shades

Corner detail

Model of building

support brackets

motorised louvresupports

internal cassette and externalrainscreen cladding

internal partition 219.3 x 6.3 CHS column with 30 fire protection

glazing frame

30 insulationrainscreen cladding

internal cassettepanels

substantial cost savings over the alternativeof supplying them as part of the facadesystem. On dull days the louvres can beorientated to reflect light onto the ceiling. All steelwork members were within 12 metres overall length, thus facilitatingtransport. The benefits of the use of SHScolumns on this project derive directly fromthe basic properties of the tube: it is efficientagainst lateral buckling; its shape mirroredthe architectural details and gave theminimum overall mullion size; it provided asuitable corner detail both at the perimeterand within the atrium; and SHS have lowHp/A ratios, so minimising fire protectioncosts. The total all-up weight of thesteelworks including all connections is 45 kgper sq metre of floor area.


Queensberry House, LondonCase Study

Shafts of naturallight illuminate thecentral atrium ofthis six storey office building

Queensberry House, London.Architect: RHWL Structural Engineer: Buro Happold

To an increasing degree, property managersare finding that only the clear spaceprovided by long span beams can properlymeet the requirements for varying floorlayouts and a simple distribution of services.This six storey commercial building, which is30 x 50 metres on plan, is an elegantexample of the type. In section, it providesclear spans of 12 metres on each side ofthe office divided by a 6 metres wide atrium,which contains all the internal columns. Thecolumns make use of an innovative tube-in-tube system, in which one steel circular

hollow section is placed inside a larger one,with all the voids grouted up after erection.The columns were designed as compositein accordance with Eurocode 4, with the fireresistance undertaken to Eurocode 4.1. Thecolumns not only have a smaller size whencompared to the equivalent fire protectedsteel column but have much higher capacitythan similarly sized reinforced concreteelements because of the steel tubes andthe high capacity of the concrete confinedwithin the tubes. The columns weredelivered to site in two three-storey lengths


and joined by means of an in-situ concretejoint in the inner tube and by bolting andwelding on the outer tube. The floor zone hasa total depth of 510 mm and accommodatestwin cellular beams placed each side of thecolumns, with services passing through thebeam. In order to save depth, the concretefloor slab, which is directly supported by thesecondary beams, is lowered so as to belevel with the top of the main cellular beams,rather than being placed on top of it. The topflange of the main beam is fire protected onits upper side.

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Vertical section through cellular beams

Horizontal section through typical column showing connection to beam, not showing floor

Plan at typical level

323.9 x 6.3 CHS inner tube

130 composite rc slabon 203 x 133 x 30 UB

twin 513 deep cellular beams

457 x 10 CHS outer tube

twin 513 deep cellular beams

grout infill

40 thick throughfin plate

203 x 203 x 30 UBcompositesecondary beam

14 SHS Welding

Tay House, GlasgowCase Study

Strongly modelledelevation for aseven-storeyoffice building

Tay House, GlasgowArchitect: Abbey Holford Rowe Structural Engineer: Ove Arup & Partners Scotland

This seven-storey office building is placed ina highly prominent position. The designershave addressed this fact by making thebuilding a reference point on its corner, thehinge between a busy city street and thetangent road for the motorway going South.The facade is layered and given strong reliefby the use of external twin tubular columnsat each structural bay. This not only allowscolumn-free space inside the building butalso breaks down the facade and helps it tomatch the existing street frontage to oneside. A tapered steel beam spans 18.2 mfrom front to back of the building. Beingonly 500 mm deep near the facades, but1000 mm in the centre, there is space forservices to run under the beam ends nearthe cladding line.

The beams are generally not painted but aretreated with a vermiculite spray to provideone and a half hours fire resistance. Floor beams are given a pre-camber of 0.3per cent of their span to allow for live loaddeflection. The weatherproof facade ismade up from a standard cladding systemand is built around the ends of the floorbeam which penetrates it. It consists oftinted anti-sun glazing with glazed insulationinfill panels. The accessways outside whichare supported by the floor beams at eachlevel also serve as sunshades. The exteriorCHS columns are protected againstcorrosion by an epoxy paint system whichis overpainted with an intumescent paint toprovide a nominal one and a half hours fireresistance.

SHS Welding 15

Close-up view on entrance

View on main beam to column connection

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Vertical section on beam to column connection

Plan on beam to column connection

Plan at typical floor level

twin 323.9 CHS columns

twin 323.9 CHS columns

18.2m span tapered floor beam

250 x 250 x 16 RHS cross-beam

27 diameter studspassing throughbearing and welded to cross-beam

250 x 250 x 16 RHS cross-beam

cladding line

125 concrete composite floor slab

18.2m span tapered floor beam

16 SHS Welding

Putney Bridge RestaurantCase Study

A sparklingaddition to thepiecemealdevelopment of the cityscape

Putney Bridge RestaurantArchitect: Paskin Kyriakides SandsStructural Engineer: Alan Conisbee & Associates

It is a measure of the success of its designthat this small restaurant building is alreadya landmark, a sparkling addition to thepiecemeal development of the cityscape.The building provides for customers asense of enclosure but with good views outacross the River Thames. By night, it istransformed into a beacon of light andsocial activity. The ground floor is clad inprecast concrete with marble-like finish,while the first floor, being completely glazed,looks over the river. The first floor and theoversailing roof above it are supported byhollow section columns. At the back of thebuilding, the columns are vertical.Elsewhere, however, the columns arevertical at ground floor level only and, at firstfloor level and near the entrance, thecolumns are jauntily raked around the open

glass sides. The vertical columns consist oftwo 200 x 100 x 6.3 RHS welded togetheron a hit and miss pattern or, alternatively, asingle 250 x 250 x 6.3 RHS. The rakingcolumns are 168.3 x 6.3 CHS and, ingeneral, attach to a series of open trusses,spanning the width of the building. These trusses are made up of steelroundels, 88.9 x 5 mm CHS and 40 mmdiameter solid rod and, as well as theirunusual appearance, have good torsionalstiffness. The suspended first floor isconcrete and consists of 150 mm deepprecast concrete planks with a 75 mmtopping spanning between standard UB or UC steel sections. No fire protection was necessary for the raking columns.

Vertical section at rear column showing connection tobeam

2no 200 x 200 x 6.3 RHS hitand miss welded

75 screed on 150 precast slabsupported on UC bottom flange

203 x 203 x 46 UC beam

fin plate connection

250 x 250 x 6.3 RHS column

SHS Welding 17

Tutorial Block, Manchester College of Art & Technology

Case Study

A four storeytechnologyteaching block built in 26 weeks

Tutorial Block, Manchester Collegeof Arts and Technology.Architect and Engineer: Terrapin Limited

Design and build, as a method ofprocurement, is increasingly becoming thefirst choice for building owners and, in someapplications, this threatens to displacetraditional methods of building entirely. All-round design standards of buildings withprefabricated components are improvingand the speed of construction isunsurpassed, in spite of the infrastructurework that is still required. This teachingblock for technology students was largelypre-fabricated off-site. It has a total floorarea of 2,368 square metres over fourstoreys but was completed within a totalsite time of 26 weeks to meet term dates.The heart of the system is a structural steel

frame consisting of 200 x 200 RHScolumns and standard 533 deep mainbeams which in turn support twin 424 x 90x 3.2 cold formed steel beams carrying thefloor. The hollow section columns all havethe same outer dimensions but vary in wallthickness through the height of the building,depending on load. The wall claddingconsists of prefabricated panels on theinner skin and brickwork or insulated panelson the outer skin. The hollow sectioncolumns are fire-protected by plasterboardpanels and the beams by enclosurebetween fire-resistant floor and ceilingconstructions.

RHS columns

Vertical section at column position

200 x 200 x 10 SHS columnwith plasterboard fire casing

stub connecting piece welded to column

20mm splice plates with 8 M20 bolts


Westside Office Building, ApsleyCase Study

A two storey commercialbuilding designed toprovide a light and airy interior

Westside Office Building, Apsley.Architect: Aukett AssociatesStructural Engineer: Anthony Hunt AssociatesClient: Royal and Sun Alliance Property

Westside is a slickly planned and executedoffice building with low-maintenancefinishes, clean surfaces and light and airyinteriors in the best traditions of commercialpractice. The building is two storeys highwith two interior courtyard areas, the largerone being roofed over at first floor level toprovide a restaurant area and meetingplace. A structural steel frame is used. The columns are all hot-finished steel hollowsections, the perimeter columns being 200x 200 x 8 RHS and all internal columns 250x 250 x 8 RHS. At first floor level, 610 mmdeep secondary steel tubular lattice trussesspan 12 metres from the perimeter columnsto the internal column lines; they support aprecast concrete floor with topping.

The primary beams are similar trusses ofthe same depth but span 6 metres.Castellated beams are used in the roof andalso as perimeter beams at first floor level.The building is clad with curtain walling fromground to roof, which is supported directlyoff the columns or from horizontal tiemembers at roof and first floor level. Steel T section brackets are used withslotted holes at their ends to allowadjustment of the position of the claddingmullions. The steelwork, including columns,was required to have a one hour fire rating;this was generally achieved by use ofintumescent paints.

Vertical section through floor at external column

Plan at ground floor

cladding mullion attached to columnwith adjustable bracket

200 x 200 x 8 RHS column

610 deep tubular truss

end plate bolted to plated UCintermediate section

Buttercrane Centre Multi-storey Car Park, Newry

Case Study

A five level well-litmulti-storey Car Park

Buttercrane Centre Multi-storey Car Park, NewryArchitect and Structural Engineer: WDR & RT Taggart

This multi-storey car park has 5 levels in thehighest part and is notable for its provisionof a floor to floor height of 3.0 m, givingwell-lit and ‘safe’ spaces for users. Theheight chosen has the further advantage ofallowing the use of deep cellular beamswhich gives column free interior space overthe full 15.6m width of the parking area, andgood cross-ventilation in the event of fire.The main car park areas are placed eachside of a row of columns along the centralspine. The steel columns support 690 x229/191 asymmetric cellular steel beams at4.8 m centres which act compositely with aconcrete slab. All the interior columns alongthe central spine are 355.6 CHS columns

spaced at 4.8m centres to support the floorbeams on each side. The CHS columnsoccupy minimum volume, are withoutobtrusions and have good resistance tovehicular impact. In this case, it was notconsidered necessary to increase theimpact resistance of the hollow sections byconcrete filling. The CHS columns weresupplied in single lengths and did not needto be tied together at the top in thetemporary condition. The car park has afloor area of 12,300 sq metres and was builtin a period of 30 weeks. The total weight ofsteel used averaged 40 kg per sq metre offloor area.

Buttercrane Centre Multi-storey Car Park, Newry

Vertical section through floor at internal column

115mm concrete topping on 50mm precast concrete plate

693 x 229 cellular beam with shear studs

500 x 150 x 15 fin plate connection


355.6 x 12.5 CHS column


Connections

Hollow section structures, whether simple elements or complete weldedassemblies, have a slenderness and clarity of line that is unsurpassed. Weldedjoints, usually undertaken in a steelwork fabrication shop, allow a continuity to beachieved between the lines of the different CHS or RHS elements which are joinedtogether. The complement to this is the bolted connection, almost always part ofthe site operation, and this has its own vocabulary, which provides a means ofachieving elegant connections on site. A selection of some of the most commontypes of bolted connections are presented on the following pages.

The most common arrangements for beam to column joints make use of bolted connectionsvia attachments welded to the faces of the hollow section column. By far the most commonconnection of this type is the fin plate connection, using a flat plate welded to each columnface. For RHS columns, an alternative is the web cleat connection, using single anglesections or T-sections welded to the column face. The use of double angles is a furtheroption and provides greater capacity than a single angle would. An increasingly popularoption for CHS or RHS columns is the use of the reverse channel connection. The aboveconnection types can all be classed as simple connections. Of these the most economic isthe fin plate connection or the web cleat connection using a single angle.

Simple steel connections using flexible end plates or double angle cleats, which are bolteddirect to the column, are also possible with RHS columns. These joints use either expandingbolt types, such as Hollobolt, or fully threaded bolts in tapped holes produced by theFlowdrill system.

Rigid or semi-rigid moment connections are feasible with all types of hollow section column.These may use flange plates or beam stubs, which are usually of the same section as thebeam being connected. Through-plated connections are another popular type of momentconnection. This is similar in appearance to the fin plate connection but has slots in thecolumn to allow a single plate to be taken thorough it. In almost all cases momentconnections are more expensive than simple connections but the extra cost of theconnection can be more than offset by savings in beam sizes or by provision of more usablefloor space.

Connection details can be applied to both unfilled and composite concrete filled hollowsection columns. For all these, except Flowdrill or Hollobolt connections, the filling can be in-situ or pre-filled off-site. The latter will give the shortest construction times.


Beam to Column ConnectionsSimple ConnectionsSimple connections are normallyassumed to give vertical support but toprovide only limited restraint againstrotation. The connections are assumedto be able to rotate without damage.

Reverse channel connection Section through floor with steel beamsconnected to CHS columns by a channelfixing; a standard flexible end plate is normalat the beam end which then develops onlynominal moments. A similar detail is usedfor RHS columns.

Web cleat connectionSection through floor with secondary andprimary beams which are connected to RHScolumns by a T-section web cleat; thisconnection has greater stiffness androbustness than the equivalent fin plateconnection.

Fin plate/Shear tab connectionSection through composite deck floor withsteel beam connected to RHS or CHScolumn by a finplate; a very economic andductile joint; a seating cleat may be used tohelp erection, for removal afterwards ifrequired.

Hollobolt or Flowdrill connectionSection through floor with steel beamconnected to face of RHS column byHollobolts or fully threaded bolts in Flowdrillholes; only nominal moments develop withflush or partial depth flexible end plates;thicker plates can develop between 10 to15 per cent of beam capacity.


Moment ConnectionsConnections which are assumed to givevertical support and to provide a degreeof restraint against rotation and todevelop some moment capacity.

Composite beam moment connectionSection through floor with composite beamconnected to RHS or CHS columns by finplates and bottom flange plate to providecontinuous or semi-continuous joints; thetop reinforcement in the concrete slab isdesigned to provide the other arm of themoment couple.

Steel beam moment connectionSection through floor with secondary steelbeams on top of primary beams which areconnected to RHS or CHS columns by finplates and flange plates to providecontinuous or semi-continuous joints.

Stub connectionSection through floor with stub beamsections welded to face of RHS columns;moment capacity usually limited by yieldingof column face; large moments requireinternal diaphragms or external flangeplates.

Through-plate connectionSection through floor with through-platepassed through slots and welded to eachface of RHS or CHS column; the throughplate connection allows significant axialforces and bending moments to betransferred from the beam, if this is required.

Shearhead ConnectionsConnections providing shear andmoment transfer between concrete flatslabs and steel RHS or CHS columns

Centre stub shearheadSection through concrete flat slab floor withcolumn stubs welded to RHS or CHScolumns; stiffening collar is required aroundthe column where there are large momentsfrom unbalanced loads.

Grid shearhead Section through concrete flat slab floor withgrid welded to faces of RHS columns; theconnection is designed for transfer of tensileand compressive forces from slab toshearhead elements.


Shearhead Connections to Slabs

Baseplate with loose boltsSection through base with cast-in bolts in boxes; bolts loosened soon afterconcrete has set; underside of base andbolt boxes are grouted after baseplate ispacked up and column is plumb.

Baseplate Connections

Column Splice Connections

Flange plate connectionSection through floor with flange plates onRHS or CHS columns cast into concrete slab.The bottom plate has projecting studs weldedto it; for flange plates connected above theslab, in a raised floor space, a normal boltingarrangement may be used. In general theflange plate connection provides transfer ofaxial forces but only limited moment transfer.

Welded splice connectionElevation on RHS column with weld backingstrip and angle brackets for erection; bracketsare removed after some welding completed;backing strip also used as alignment spigot.For CHS Columns, vertical plates are usedinstead of brackets.

Welded splice connection with long bolts The use of long bolts with double nuts permitsaccurate alignment of the SHS and backingmaterial before site welding.

Note:When adopting concrete filled columns special details forsplice flange plates and column cap plates are required -see British Steel Tubes & Pipes brochure ‘Design Manual for SHS Concrete Filled Columns’ TD 296.


Column Splice Connections


Structural Design

The choice of design options available to the engineer when hollow sections are used isunique. Not only is there an unparalleled range of section sizes to choose from, there arealso options to increase the capacity of the column by composite action with a concrete infill.Yet a further option exists for the infill to be reinforced or left unreinforced.

Combination of options means that these options are available for use on the sameproject. Composite columns can be used on the lower, highly loaded columns, with unfilledcolumns being used in the upper storeys.

Substantial cost savings have been shown to be possible following tests in whichcomposite concrete filled hollow section columns were coated with intumescent paint. The results indicated that significant reductions were feasible in the weight of intumescentpaint specified.


Column Design Options

Three basic design options are possiblefor column design of SHS in multi-storey buildings:

Option 1Columns are designed on a floor by floorbasis or by grouping two or three storeystogether. The lightest steel section isselected for each column lift. This optionproduces the minimum weight column withsizes reducing through the height of the building.

Option 2Columns are designed on a floor by floorbasis or by grouping two or three storeystogether but have constant externaldimensions throughout the height of thebuilding. The column at the lowest level isdesigned for the least weight solution and itis the external dimensions of this sectionthat are used for all other sections at higherlevels. At the higher levels, the wallthickness of the hollow sections areprogressively reduced.

Option 3Columns are designed on a floor by floorbasis or by grouping two or three storeystogether and have constant externaldimensions throughout the height of thebuilding, but the type of column selected isoptimised for all the columns over the heightof the building, rather than just the groundfloor, as in Option 2. Generally this meansthat the ground floor column is smaller andthicker than that in Option 2 but the columnserial size chosen allows the overall weightof columns to be reduced.

Options 1

CHS columns filled withconcrete off-site

RHS columns filled with concrete on site

Options 2

Column weight

Options 3

1104 kg 1310 kg 1213 kg

323.

9 x

8.0

193.

7 x

10.0

168.

3 x

8.0

323.

9 x

8.0

323.

9 x

6.3

323.

9 x

6.3

193.

7 x

16.0

193.

7 x

10.0

193.

7 x

8.0

1st

2nd

3rd

4th

5th

6th

Roof


Fire Resistance

All fire resistance options that are commonly available for steel sections are alsoavailable for SHS columns. However hollow sections allow additional methods for fire protection including concrete or water filling. The following paragraphssummarise the main options available for the fire protection of SHS columns. It should be noted that the choice of protection system can have a significant effect on the final column cost.

EXTERNALLY PROTECTED COLUMNS

Unfilled Hollow Section ColumnsA wide range of options are available including protection of the SHS columns byplasterboard, traditionally one of the cheaper methods, or by use of cementitious sprays,intumescent coatings or preformed casings, including the use of tube-in-tube systems. In all cases, the fire-protected SHS columns will have the minimum area compared to allother similarly loaded columns in other materials.

Composite Concrete Filled Hollow Section ColumnsA composite column consisting of the hollow section with structural grade concrete fillingcan be designed very simply. In the first instance, the column is checked for roomtemperature loadings. The fire resistance is then checked and, if required, an externalprotection system added. This method has been found to give economic solutions as it bothminimises the wall thickness of the tube, because of the presence of the concrete, and asshown in recent fire tests, gives a reduction in the external protection system markedly belowthat of the unfilled section. Load ratio design methods can be used to check that the limitingtemperatures are not exceeded for the particular construction chosen.

Internal steelwork

Externally protected

Unfilled Concrete filled


INTERNALLY PROTECTED COLUMNS

Plain or Bar Reinforced Concrete Filled Hollow SectionsInternal protection enables steel hollow section columns to be used without any fireprotection whatever. In a fire, after approximately 30 minutes, the steel shell becomesunserviceable and sheds its load to the concrete. The core is then designed to carry thewhole of the load at the fire limit state. Plain concrete is suitable for mainly axially loadedcolumns. Bar reinforced concrete is required for columns with significant moments. In bothcases, as the concrete is the principle load-carrying element in fire, the resultant columns arebigger if internal, rather than external protection is chosen.

Comparison of Column SizesThe following table provides a guide to the practical effects of the different fire designmethods. It compares columns designed for either external or internal fire protection, all carrying the same load and with a one hour fire rating.

Externally Protected

UnfilledBoard Protected

Composite ConcreteFilled intumescent coated

Internally Protected

Non composite plainconcrete filled

Non composite barreinforced concrete filled

Table comparing sizes of RHS columns for various options; the Externally Protected columns are assumed to actcompositely in a fire; the Internally Protected columns are not assumed to work compositely in a fire.

200sq 180sq 400sq 300sq

230sq

External steelwork

Internally protected

Concrete filled Water filled

Fire Design Paths for hollow sections


Cost Comparisons

SHS column costsSHS columns offer a competitive first cost solution for columns in multi-storey buildings, aposition established from the results of a cost comparison based on a typical internal columnin a 7 storey building. The comparison looked at a completed, fire protected column with aone hour fire rating, and established the relative cost difference between hollow sectioncolumns and other steel sections. The study also highlighted that comparison costs for thesteel only solution may result in a false economy.

THE STUDY MADE THE FOLLOWING ASSUMPTIONS:

SteelworkThe study compared options for a typical 7-storey internal column carrying a loading of 6 kN/m2 on a grid layout of 7.2 metres by 6 metres. The study looked at floor arrangementsboth where primary and secondary beams were used and where only primary beams were used.

All three column design options referenced in the previous Structural Design section wereused in combination with the fire protection given in the table on page 31. Where possiblesteel of grade S355 was used as, in general, this gives the most economical solution forstructural steelwork. The costs used, to include supply, delivery and erection, were £975 pertonne for UC columns of grade S355, £1,350 per tonne for SHS columns of grade S275 and£1,400 per tonne for SHS columns of grade S355.

ConcreteThe following costs were assumed for the concrete filling material for both composite andnon-composite design. Non-composite design is used for internally protected concrete filledcolumns. The costs used, to include for supply, delivery and placing, were £158 per cubicmetre for plain concrete and £266 per cubic metre for bar reinforced concrete.

Fire ProtectionIn the case of internal protection, plain or bar reinforced concrete is assumed. In the case ofexternal protection, fire resistant boards were assumed for non-circular columns, such as UCand RHS sections, with intumescent paint used for both CHS and RHS columns. Recentresearch has indicated that significant savings can be had by using intumescent paint oncomposite columns as shown in the table below. The fire resistant boards were assumed tobe British Gypsum 15 mm Glasroc S, supplied and fixed at £19.50 per square metre. The intumescent paint was assumed to be Nullifire S605 ENOB applied off-site at £10.7 persquare metre for 400 µm thickness, £15.9 for 750 µm thickness, £21.0 for 900 µm thicknessand £27.0 for 1300 µm thickness.

Nullifire S605 Coating thickness µm for concrete filled CHS

Typical Column Previous Revised

168.3 x 8.0 2,250 900

193.7 x 10.0 1,550 600

244.3 x 6.3 3,450 750

323.9 x 8.0 2,250 400

Table indicating reductions in thickness of intumescentpaint for fire protection that have been made possible by recent tests.


Above Tables: comparing relative costs of typical internal column after fire protection; the table compares costs of UC, CHS and RHS columns for various methods of fire protection.

CHS Fire Protection Options

External board Intumescent Paint Internal Concrete filling

Column UC Circular Hollow SectionsOptions

Unfilled Composite Plain concrete Bar Reinforcedconcrete

Option 1 100 111 88 134 88

Option 2 100 159 113 186 107

Option 3 100 111 97 186 107

RHS Fire Protection Options

External board External board Intumescent Internal Concrete fillingPaint

Column UC Rectangular Hollow SectionsOptions

Unfilled Composite Composite Plain concrete Bar Reinforcedconcrete

Option 1 100 108 92 90 163 113

Option 2 100 111 121 115 231 123

Option 3 100 111 100 102 231 123


Bibliography

1. British Steel Tubes & Pipes: ‘Design of SHS Welded Joints: BS5950 and ENV1993-1-1-Annex K’ TD393

2. British Steel Tubes & Pipes: ‘SHS Welding’ TD394

3. British Steel Tubes & Pipes: ‘Fire resisant design: A guide to evaluation of structuralhollow sections using BS5950 Part8’ TD408 & TD409

4. British Steel Tubes & Pipes: ‘Intumescent coatings & SHS concrete filled columns’ TD410

5. British Steel Tubes & Pipes: ‘Design manual for SHS concrete filled columns: Part 1 Structural Design; Part 2 Fire Resistant Design’ TD296

6. British Steel Tubes & Pipes: ‘SHS Design to BS5950 Part 1’ TD365

7. British Steel Sections, Plates and Commercial Steels: ‘Fire resistant design of Structural Steelwork ‘- Information sheets M01-M09

*8. CIDECT: ‘Structural stability of hollow sections’ Verlag TUV 1996

*9. CIDECT: ‘Design guide for structural hollow section columns exposed to fire’ Verlag TUV 1996

*10. CIDECT: ‘Design guide for concrete filled hollow section columns under static and seismic loading’ Verlag TUV 1995

*11. CIDECT: ‘Design guide for fabrication, assembly and erection of hollow sectionstructures’ Verlag TUV 1998

12. Steel Construction Institute: Composite column design to Eurocode 4 SCI publication142 1994

13. Steel Construction Institute: Joints in steel construction - Composite connections SCI publication 213 1998

14. Steel Construction Institute: Joints in simple construction - Volumes 1 & 2 SCI publications P205 & P206 1993

15. ‘Tubular Structures VII’: Proceedings of the sixth international symposium on tubular structures A A Balkema 1996

16. ‘Tubular Structures VIII’: Proceedings of the seventh international symposium on tubular structures A A Balkema 1998

17. Ove Arup & Partners: ‘Composite hollow steel tubular columns filled using high strength concrete’ Dept. of the Environment 1995

18. Association for International Cooperation and Research in Steel-Concrete CompositeStructures: ‘Concrete filled columns - A comparison of international codes andpractices’: International conference on composite construction - September 1997

19. L H Lu: ‘The static strength of I-beam to rectangular hollow section columnconnections’ Delft University Press 1997

* CIDECT design publications are available from the Steel Construction Institute

Note:Further informationon Structural HollowSections is availableon Tubes & PipesTechnical Data CD-Rom or contactus on our freephonetechnical helpline0500 123 133

Tubes & Pipes

PO Box 101Weldon RoadCorbyNorthants NN17 5UATel +44 (0)1536 402121Fax +44 (0)1536 404111Freephone Technical Helpline 0500 123133

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