Design of Industrial Buildings
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Transcript of 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
Items to be considered while selecting
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Items to be considered while selecting
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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Selection of bay width
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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Selection of bay width
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.