Design of Industrial Buildings

8/10/2019 Design of Industrial Buildings

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Design of industrial buildings


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Major components of industrial

building Roof trusses

Gantry girder

Side rails or girts with claddings

Gable rafter

Gable columns Rafter bracings

Vertical bracing in longitudinal side

Gable wind girder at eave level

Eaves girder Main columns

Column brackets


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Roof truss

Roof truss is a frame work which supports the

roofing and ceiling material.

It is supported on either end on either walls or

lines of columns

When a roof truss is attached to and

supported on steel columns , at the ends, it

gives rise to a bent.


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Nomenclature of members of trusses

1. rafters or top chord membersthey directly supportpurlins, mainly subjected to axial compression due to LLand DL

2. Main tie or bottom chord membersmainly subjected

to tensile forces due to DL and LL3. Struts- members which do not belong to top or bottom

chord and are subjected to compressive forces

4. Slingsmembers which do not belong to top or bottomchord but are mainly subjected to tensile forces

5. Sag ties- members that are not subjected to any loadsbecause no load is acting at that joint. even then thismember is provided to reduce the sag in other members


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Sag rods and purlins

The upper chord of the roof truss is a sloping member,the weaker axis of the purlin member (which may beeither an angle section, channel section or I section ) isnormal to the slope, and consequently , the purlins are

subjected to biaxial bending due to gravity loads.

Since the z value or rigidity of an I beam or channel isquite small about its weak axis, sag rods are ofteninstalled in the plane of the slopes which reduce the

span for bending about the weak axis Generally 2 lines of sag rods are provided in each bay ,

which are connected to the ridge purlins.


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Gantry girders or crane girders

Gantry girders carry hand operated or electricoverhead cranes in industrial buildings, to lift heavymaterials , equipments etc and to carry them from 1location to the other within the building.

The essential componets of crane system are:

1. crane bridge or cross girder

2. trolly or crab mounted on crane bridge

3. gantry girder or crane girder4. crane runway (rail)

5. column brackets


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The crane bridge spans the bay of the shop.

The trolly or crab mounted on the crane bridgecan travel transversely along the bridge

The bridge has wheels at the ends, and is capableof moving longitudinally on rails

The rails are mounted on gantry girders

The gantry girders span between brackets

attached to columns which may either be steel orof RCC. Thus the span of gantry girder is equal tothe centre to centre spacing of the columns.


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Gable rafters, Gable wind girder and

Gable columns At the gable end, no truss is provided , but instead gable rafters are provided .

These gable rafter may either be supported on gable walls(if no openings arerequired) or on gable columns

The gable columns when provided , permit wider openings at the gable end, andat the same time support the DL and WL acting on the gable end claddings.

For a span of truss 8 to 10 m , one can avoid gable columns, but for larger spans,

intermediate columns or gable columns can be provided at a spacing of 4 to 6 m. The configuration so provided is known as gable frame.

The configuration of the gable frame should be chosen so that it can resist thewind load acting on the gable face in addition to the DL and WL coming from theroof sheeting.

Thus the gable rafters are subjected to both bending as well as axial forces, incontrast to the rafters of the truss which are subjected to only axial forces if thepurlins are provided at the nodal points.


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Gable wind girder is provided at the eaves

level, at the end panel of the building , to

resist the wind loads on the gable end.

It is thus a horizontal girder formed by bracing

together the lower node points of the end

truss and the gable columns


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Eaves girder

Eave is the edge of the leaning roof.

Eaves girder is a girder or stiffener beam taken roundthe building , at the eaves level, to serve severalfunctions :

1. It acts as a stiff binder beam2. Side cladding may be hung from the eaves girder, in

some cases

3. The wind bracing along with the eaves girder acts astruss in plan view, in which eaves girder is a compression

chord4. It supports drain gutters and other secondary members

Two channels face to face (or I beams) are used as eavesgirder


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Side rails or girts

The height of the industrial building may range from 6 m to 12m.

It is not advisable to build the side walls of such height, since these willbend as cantilever

Such walls will be very thick and will require heavier foundations.

Alternatively , one may construct the side walls up to 3 to 4 m height , and

then provide sheet cladding. These side sheets are supported on side rails or girts.

These side rails are spaced 1 to 1.5 m apart.

These rails are in turn , supported directly or indirectly on columns.

The side rails are subjected to vertical bending due to DL and horizontalbending due to WL .

If the spacing of the columns is more (say greater than 6 m) the size of therails or girts becomes uneconomical .

In that case, gird framing , consisting of horizontal beams and verticalrunners are to be provided.


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Structural framing

For purposes of structural analysis and design, industrial buildingsare classified as

Braced frames and

Unbraced frames

In braced buildings, the trusses rest on columns with hinge type

connections and the stability is provided by bracings in the 3 mutuallyperpendicular planes.

The bracings are identified as follows :

(a) Bracings in the vertical plane in the end bays in the longitudinaldirection

(b) Bracings in the horizontal plane at the bottom chord level of the

roof truss(c) Bracings in the plane of upper chords of the roof truss

(d) Bracings in the vertical plane in the end c/s usually at the gable ends


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Function of bracing

Function of bracing is to transfer horizontal loadsfrom the frames (such as WL or earthquake orhorizontal surge due to acceleration and breakingof travelling cranes) to the foundation

The longitudinal bracing on each longitudinal endprovides stability in the longitudinal direction

The gable bracings provide stability in the lateraldirection

The tie bracing at the bottom chord level transferlateral loads (due to wind and earthquake) oftrusses to the end gable bracings


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Bracings

The bents, consisting of truss and columns can resist vertical loadsand all horizontal loads acting in their own planes. However , theyoffer very little resistance to horizontal loads on acting normal totheir planes

The trusses and columns of an industrial building must be

thoroughly braced to preclude collapse of structure due to wind orearthquake or the effects of moving loads such as cranes.

The function of bracing is to transfer the horizontal forces from theframes to the foundations of the building.

Bracings are provided in following 3 planes :

(1) inclined plane of the upper chords of the truss(2) horizontal plane of the lower chord of the trusses

(3) vertical planes of the columns


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Braces are provided in the form of X,K or knee bracing.

Out of these, X-bracing is quite common.

In a long building, every fourth or fifth bay should bebraced.

Even in shorter building, a min of 2 bays should bebraced

When wind blows in the longitudinal direction (i.enormal to the plane of the truss) a horizontal truss will

be required to transmit the wind load on the gable endto the column, and a cross frame (or cross bracing) inthe longitudinal vertical planes of the columns will berequired to transmit the loads to the foundations.


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Bracing of industrial bents in

transverse direction

An industrial bent, consisting of 2 end

columns and trusses top is braced against

transverse forces independent of others.

Due to this, each industrial bent remains

stable transversely , immediately after

construction.

This can be achieved by 4 methods shown


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When the column load is heavy( due to large

span of truss) , and consequently the size of

footing is large, the bent can be braced by

fixing the base and providing mechanicalhinges at the top.

The method is suitable when the height of the

building is small so that the overturningmoment is also small.


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When the span of the truss is small, bent can

be braced by providing knee braces.

The column base may be hinged, resulting in

zero BM on the foundation, and consequent

reduction in the cost of the foundation

The reduced moment is transferred to the

column at the junction of knee-brace with the

column.


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In the case of a bent with knee-braces and fixedcolumns, the moments are further reduced , thoughthe foundation becomes costlier these are to resist BM.

Though the braces resist the overturning momentcaused by lateral loads, they produce additionalstresses in the whole of the truss.

Knee braces also reduce the headroom . Where theheadroom requirements are severe , knee braces are

avoided and the bent is braced by fixing the columns attheir base , and providing rigid connections betweenthe column and truss.


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Unbraced frames

Unbraced frames in tehe form of portal frames is the mostcommon form of construction for industrail buildingd

The frames can provide large column free areas, offeringmax adaptibility of the space inside the building. Such largespans require less foundation , and eleimante internalcolumns, valleu guttersand internal drainage.

Advantagesmore effective use of steel than in simplebeams

- easy extension at any time in the future

- ability to support heavy concentrated loadsDisdvantagesrelatively high material unit cost

- susceptibility to differential settlement andtemperature stresses


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Items to be considered while planning

and designing an industrial building

Selection of roofing material and wall material

Selection of bay width

Selection of structural framing system

Roof trusses

Purlins, girts and sag rods

Bracing systems to resist lateral loads

Gantry girders , columns ,base plates andfoundations


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Items to be considered while selecting

Roofing material

Type of roof deck

Type of purlin used

Purlin spacing Deflections of secondary structural members

Roof pitch

Drainage requirements



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a cladding/wall system

Cost

Interior surface requirement

Aesthetic appearance (including color)

Acoustics and dust control

Maintenance

Ease and speed of erection

Insulating properties

Fire resistance


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Cladding/decking

Cladding or wall system carries only its ownweight and weight of the loads imposed bywind

Cladding will have an impact on the design ofgirts, wall bracing, eave members andfoundation

In Roof decking, the sheeting supportsinsulation and water proofing, self weight andloads due to wind and/or snow


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Cladding/Decking material used in

practice

Corrugated galvanized iron (GI) sheets

Light-gauge cold-formed ribbed steel or

aluminum

Asbestos cement (AC) sheets


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Galvanized iron (GI) sheets

Corrugated iron sheets are galvanized for protection against corrosion .

Most common sizes of corrugated GI sheets are

(a) 8 corrugations (75 mm wide and 19 mm deep) per sheet

(b) 10 corrugations ( 75 mm wide and 19 mm deep) per sheet

The weights of the sheets vary from 50-156 N/mm2.

When the sheets are installed , side laps and end laps should be provided tomake the joint water proof.

The sheets should be used with following overlaps:For roof: Side overlap1 to 2 corrugations

End lap150 mm

For side cladding : Side overlap1 corrugation

End lap100 mm

The sheets are fastened to purlins or side girts by 8 mm diameter J type or L-type bolts at a max pitch of 350 mm

The spacing of purlins depends upon the applied loading, thickness of sheetsand length of sheets


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AC sheets

Asbestos sheets are better insulators for suns heat compared to GI sheets

They are used commonly the factories and godowns .

They are available in 2 common shapes viz. Corrugated and Trafford.

They are available in the lengths of 1.75 ,2 ,2.5 and 3 m.

They are available in the thickness of 6 mm and 7mm

The max permissible spacing of purlinsfor 6 mm sheets - 1.4 m

and for 7mm sheets1.6 m

The weight of the asbestos sheets varies from 160- 170 N/mm2

They are t be used wit a longitudinal overlap of 150 mm and a side overlap of1 corrugation.

Spacing of purlins are to be adjusted such that as far as possible the cuttingof sheets is avoided


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A bay is defined as the space between 2

adjacent bents.

The roof truss along with columns constitutes

a bent.

The space between 2 rows of columns of an

industrial building is called aisle or span.


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In most cases , the bay width may be dictated by the owner requirements. Gravity loads generally control the bay size.

The choice of the wall system dictates whether or not girts are provided for the structure .

If girts are required , light gauge C or Zgirts may be chosen , which are most cost effective.

Based on both strength and stiffness( L/180 ) requirements , the max economical span of such girtsis approximately 9 m.

Hence, for buildings w/o cranes , a 9m bay is the most economical choice.

A 12 m bay may prove economical for large square buildings For crane buildings (for light and medium cranes) , bays of approximately 4-8 m may be economical

because of the cost of the crane gantry girders.

Large bays may increase the cost of the tension flange bracing of the gantry girders

Soil conditions may not have major impact on bay width in the range of 4-8m, when shallowfoundations are used.

However when piles are used in poor soils , larger bays may be economical , because they reducethe number of foundations

Though the bay widths in the range of 4-8 m provide economy , truss spans may range from 10-25m or more.

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Design of Industrial Buildings