The Future and Known Concerns
As pointed out in the preface, this1st editi
As pointed out in the preface, this1st edition of the guidebook is to indicate the currenton of the guidebook is to indicate the current state of the industry. That is reflect the design of those sheds that currently form the state of the industry. That is reflect the design of those sheds that currently form the
market place. As such there are many factors that have never been adequately covered by market place. As such there are many factors that have never been adequately covered by the engineering
the engineering of such coldof such cold-formed structures, and in like manner t-formed structures, and in like manner they have equally wellhey have equally well been ignored in the production of these charts.
been ignored in the production of these charts. {NB: It should be noted that these charts {NB: It should be noted that these charts are based on the current wind loading code, that means that the sheds have a 10%
are based on the current wind loading code, that means that the sheds have a 10%
probability of failure under wind l
probability of failure under wind loading in a 50 year period, where as previous industry oading in a 50 year period, where as previous industry designs were based on 5% probability of fail
designs were based on 5% probability of failure. Thus heights and spans determined from ure. Thus heights and spans determined from these charts may exceed those in previous specifications.}
these charts may exceed those in previous specifications.}
Since these frame charts can be used as
Since these frame charts can be used as the basis for designing other the basis for designing other components of acomponents of a shed
shed, the , the following list identifies areas of design tfollowing list identifies areas of design that have traditionally not hat have traditionally not been given duebeen given due consideration.
consideration.
1)
1) Internal pressure coefficients tend to Internal pressure coefficients tend to be less than be less than recommended in the recommended in the loadingloading code, but no scientifically verifiable j
code, but no scientifically verifiable justifications provided. Then again ustifications provided. Then again a 0.7 internala 0.7 internal pressure coefficient would decrease the windward wall loading to zero, thi
pressure coefficient would decrease the windward wall loading to zero, this mays may actually be more beneficial than the pressure c
actually be more beneficial than the pressure coefficients adopted. Either way oefficients adopted. Either way nono evaluation of the effects of different int
evaluation of the effects of different internal pressure coefficients is considered. Ifernal pressure coefficients is considered. If there is internal partitioning of the building then it is possible for internal pressure there is internal partitioning of the building then it is possible for internal pressure coefficients to vary throughout a building, and therefore frames are not subject to a coefficients to vary throughout a building, and therefore frames are not subject to a single internal pressure coefficient.
single internal pressure coefficient.
2)
2) Sheds are Sheds are categorised on the categorised on the basis of wind Tebasis of wind Terrain Category, but rrain Category, but wind shieldingwind shielding factors vary from one supplier to another. As does the
factors vary from one supplier to another. As does the use of the 0.95 multiplier foruse of the 0.95 multiplier for lack of directional
lack of directional wind data. (Now wind data. (Now made partly redundant by AS1170.2:2002)made partly redundant by AS1170.2:2002) 3)
3) External pressure coefficients used for External pressure coefficients used for the 90 the 90 degree, longitudindegree, longitudinal wind loadingal wind loading case also vary. As does the
case also vary. As does the minimum number of bays.minimum number of bays.
4)
4) Explicit consideration Explicit consideration of of drag is drag is missing.missing.
5)
5) AllowanAllowance for sece for self-weight and dead-loads vary. lf-weight and dead-loads vary. But then again But then again shed suppliersshed suppliers change the claddings as they please.
change the claddings as they please.
6)
6) Load case Load case combinations vary. Some combinations vary. Some combine occasional pcombine occasional point loads with uniformlyoint loads with uniformly distributed live loads, others take them
distributed live loads, others take them separately. Others also combine windseparately. Others also combine wind loading with these live loads.
loading with these live loads.
7)
7) Little consideration for combined stresses. The only Little consideration for combined stresses. The only situation considered is colsituation considered is columnumn bending and axial compression. This however is seldom a crit
bending and axial compression. This however is seldom a critical design case,ical design case, whilst wind uplift producing maximum bending and tensio
whilst wind uplift producing maximum bending and tension is. The original coldn is. The original cold --formed structures code AS1538 did not gi
formed structures code AS1538 did not give guidance on this condition, but thatve guidance on this condition, but that does n
does not mean ot mean that it shothat it should have been ignored. Admittedly AS1538 was somewhatuld have been ignored. Admittedly AS1538 was somewhat restrictive in requiring the limiting str
restrictive in requiring the limiting stress for bending, based on flexural lateraless for bending, based on flexural lateral
instabilities, to be used for both compression and tension. The current cold-formed instabilities, to be used for both compression and tension. The current cold-formed steel structures code AS4600, now allows full section capacity to
steel structures code AS4600, now allows full section capacity to be used whenbe used when checking combined bending and tension. When combined stresses are being checking combined bending and tension. When combined stresses are being checke
checked both rafters and columns should be checked, d both rafters and columns should be checked, for bending with axial tensionfor bending with axial tension and compression. Bending and
and compression. Bending and shear should also be checked but likewise as nevershear should also be checked but likewise as never been exp
been explicitly considerlicitly considered.ed.
8)
8) Deflections and Deflections and other serviceability other serviceability criteria not criteria not considered.considered.
9)
9) Girts and Purlins Girts and Purlins are assumed are assumed to provide lateral restraint, buto provide lateral restraint, but not checked t not checked toto confirm. Als
confirm. Also if they do only provide lateral restraint, checks may identify o if they do only provide lateral restraint, checks may identify the needthe need to provide fly bracing to also provide torsional restraint.
to provide fly bracing to also provide torsional restraint.
10)
10) The industry bolts both The industry bolts both Girts and Purlins Girts and Purlins flange to flange flange to flange with columns and rafters.with columns and rafters.
No explicit consideration for the ef
No explicit consideration for the effects of such holes on any of the strfects of such holes on any of the structuraluctural members. No consideration for the loss of torsional r
members. No consideration for the loss of torsional restraint, that was provided byestraint, that was provided by the cleat to the girts and purlins.
the cleat to the girts and purlins.
11) Bridging used is generally not
11) Bridging used is generally not that recommended by suppliers of girts and purlins.that recommended by suppliers of girts and purlins.
And being connected to bottom flanges on
And being connected to bottom flanges only provily provides lateral restraint, whilstdes lateral restraint, whilst proprietary bridging provides both torsional and lateral restraint. Yet purlin/girt proprietary bridging provides both torsional and lateral restraint. Yet purlin/girt suppliers design tables are still used for purlin and girt selection. Most of these suppliers design tables are still used for purlin and girt selection. Most of these tables are based on detailed flexural torsional analysis of a specific structural tables are based on detailed flexural torsional analysis of a specific structural system; in consequence they achieve higher capacities than direct
system; in consequence they achieve higher capacities than direct application ofapplication of the simple formulae given in AS4600.
the simple formulae given in AS4600.
12) Door framing may or may not be designed.
12) Door framing may or may not be designed.
13) Connection design is relatively simplist
13) Connection design is relatively simplistic, and some connections are detailedic, and some connections are detailed upside down for the dominant wind loading case. Detail lacking in connection upside down for the dominant wind loading case. Detail lacking in connection design:
design:
a.
a. Compression Compression Stiffener Stiffener DesignDesign b.
b. Tension Tension Stiffener Stiffener DesignDesign c.
c. Bearing Bearing Stiffener Stiffener DesignDesign d.
d. Flange Flange Doubler Doubler Plate Plate DesignDesign e.
e. Web Web Shear Stiffener Shear Stiffener Design or Design or web web doubler plate.doubler plate.
f.
f. AlloAllowance wance for prying for prying forces. Which forces. Which if they if they exist would exist would also require also require checkingchecking the need for a compression stiffener near the edge of the end plate.
the need for a compression stiffener near the edge of the end plate.
g.
g. Consideration of Consideration of partial fixity. Neither partial fixity. Neither endplates nor endplates nor bolts are dbolts are designed withesigned with consideration for deflection. If the bolts stretch then there is rotation of the consideration for deflection. If the bolts stretch then there is rotation of the joint, and the full fi
joint, and the full fixity assumed in the frame analysis is not valid. xity assumed in the frame analysis is not valid. Likewise ifLikewise if column flanges or endplates bend. In other words the serviceability
column flanges or endplates bend. In other words the serviceability of theof the connection affects the strength of the whole structure.
connection affects the strength of the whole structure.
h.
h. Design of weldDesign of welds to end platess to end plates. The achieve. The achievement of a T-ment of a T-joint, completejoint, complete penetration butt weld (CPBW) with thin
penetration butt weld (CPBW) with thin materials is highly questionable.materials is highly questionable.
Such a weld would have little practical difference from fillet
Such a weld would have little practical difference from fillet welds. welds. In maIn manyny instances 3mm or smaller fillet welds equal to the thickness of the material, instances 3mm or smaller fillet welds equal to the thickness of the material, are inadequate. Hence the engineer’s automatic adoption of
are inadequate. Hence the engineer’s automatic adoption of CPBW is aCPBW is a potentially hazardous practice.
potentially hazardous practice.
i.
i. No No consideration consideration for for the the parent parent material material losing losing strength strength in in the the heat heat affectedaffected zone (HAZ) adjacent to the
zone (HAZ) adjacent to the weld. Noting that the critical design sections areweld. Noting that the critical design sections are at the joints, and therefore the full strength of the material
at the joints, and therefore the full strength of the material is required at theseis required at these locations.
locations.
Whilst stiffeners and doublers may be specified in the design documents, no Whilst stiffeners and doublers may be specified in the design documents, no design calculations have been shown to demonstrate structural adequacy of the design calculations have been shown to demonstrate structural adequacy of the components concerned. Additionally welding such stiffening plates into place i components concerned. Additionally welding such stiffening plates into place iss all pointless if the welding reduces the strength of the
all pointless if the welding reduces the strength of the main member.main member.
Of course some of these points may have been considered by the aut
Of course some of these points may have been considered by the aut hors of the designhors of the design codes, with allowance hidden away in obscure design factors. But if t
codes, with allowance hidden away in obscure design factors. But if the allowance ishe allowance is hidden then for all intents and purposes it does not exist.
hidden then for all intents and purposes it does not exist.
Likewise some of the points may have been considered by experienced engineers who Likewise some of the points may have been considered by experienced engineers who dete
determinrmined that ed that the effects were insignifthe effects were insignificant. But if thericant. But if there is no explicit declaration of e is no explicit declaration of suchsuch
consideration in the calculations, then the calculations cannot be considered as truly consideration in the calculations, then the calculations cannot be considered as truly demonstrating structural adequacy of the proposed structure.
demonstrating structural adequacy of the proposed structure.
Clearly if increased attention to detail, increases member size, then suppliers will
Clearly if increased attention to detail, increases member size, then suppliers will not benot be happy
happy and will consider such design uncompetitive and will consider such design uncompetitive with other suppliers. Iwith other suppliers. In addition buyersn addition buyers will consider such sheds too expensive and go elsewhere. More importantly
will consider such sheds too expensive and go elsewhere. More importantly improving theimproving the design of future
design of future sheds, does not resolve the problem of all the sheds that have alreadysheds, does not resolve the problem of all the sheds that have already been built and which have the potential to fail
been built and which have the potential to fail at a load below their intended designat a load below their intended design
loading. However, the intended design load does not have to be the mandated design load loading. However, the intended design load does not have to be the mandated design load if the communities expectations of design life and risk of
if the communities expectations of design life and risk of failure during that period, isfailure during that period, is incompa
incompatible with the mandated loadintible with the mandated loadingg . For example do such . For example do such buildings need tbuildings need to have o have anan importance level of 2?
importance level of 2?
Since the market exists, and there have been few recorded failures, it is the design life and Since the market exists, and there have been few recorded failures, it is the design life and the risk of failure of these types of structures that really needs to be addressed and
the risk of failure of these types of structures that really needs to be addressed and understood
understood by the coby the community. mmunity. In other words it In other words it is not entirely a is not entirely a problem of increasedproblem of increased knowledge and research into engineering mechanics and strength of materials, but a knowledge and research into engineering mechanics and strength of materials, but a problem of community value and perception.
problem of community value and perception.
Hopefully these guidebooks are a step in the right direction towards identifying community Hopefully these guidebooks are a step in the right direction towards identifying community expectations and values regarding specific building structures.
expectations and values regarding specific building structures.