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Paper: DaviedLawson

Paper

Stressed

skin

action of modern steel roof

systems

Professor

J

M.

Davies

DSc PhD FEng FIStructE FICE

University of Manchester

R M.

Lawson BSc PhD CEng MIStructE MICE

Steel Construction Institute

Synopsis

‘Stressed skin ’design has generally been applied primarily to

rooJ7 o r walls w ith a sin gle skin

o

sheeting directly fixe d to the

purlins, side-rails or beams by, fo r example, selfd rilling, self-

tapping screws or ired pins.Howevel; many modern roof systems

involve two metal skins with insulation positioned between them,

and this af ec ts their in-plane resistance. This pap er reviews the

potential fo r the stressed skin design

of

these modern rooji on the

basis o eight full- sca le diaphragm tests of diflerent, but ‘generic’,

roof systems. The tests showed that built-up roofs comprising a

liner tray,

Z

spacers, and roof sheeting perform ed well and that

significant ‘composite action between the roof sheeting and the

liner tray took place. The pe ormance of all liner trays could be

enhanced significantly if their local shear buckling resistance s

improved, leading to enhanced potential

for

use in ‘stressed skin’

design.

Introduction

‘Stressed skin’ design takes advantage of the in-plane strength and stiffness

of the cladding of a structure in order to enhance the performance of the pri-

mary framing. It is often used to eliminate wind bracing and may also be

used to reduce the sizes of the primary members or even to produce ‘frame-

less’ construction.

The stressed skin action of profiled metal roof sheeting is a well-known

phenomenon, and the design principles are now embodied in BS 595 :

Part

9’;a detailed account of the subject is provided in the

Manual of stressed

skin diaphragm design

y Davies Bryan’. Detailed design guidance is also

given in the recently updated European recommendations for the stressed

skin design of steel structures3. n these publications, the primary provisions

envisage that the main application would be when trapezoidally profiled

metal roof sheeting or decking is directly fixed to steel purlins or beams by

screws or pins passing through the troughs of the corrugations. Eurocode 3:

art 1 34

ncludes enabling clauses permitting stressed skin design and also

includes rudimentary provisions for structural liner trays.

Since the

1980s,

a number of alternative forms of roof construction have

been developed which are intended to offer a high degree of thermal insu-

lation and also to reflect new architectural features. In some cases, hey also

seek to minimise the number of through fixings. It has not been clear how

these new roof systems affect the general principles of stressed skin design.

Even if no formal account is taken of the in-plane shear resistance of these

roof systems, designers often take notional account of their influence on the

stiffness of the bare steel frame, which can be significant in reducing deflec-

tions under wind and vertical load. This mplied use of the available

diaphragm action without detailed calculations may be termed ‘serendipi-

ty’ stressed skin design.

In order to gain some information on the relative stressed skin perfor-

mance of modern roof systems, a series of eight tests has een carried out

on an approximately 6m square roof panel subject to an in-plane shear

force. The tests were devised so that the component stiffnesses could be

established. In most cases, a standard arrangement was used, with purlins

at

l

.9m spacing. The details of the test frame are shown in Fig

l

and Fig

shows the test frame as set up for test no.

3

with only the liner panel in

place. The dimension in the direction of the load was kept constant at 6m

but the dimension at right-angles 5780mm in Fig

1

varied between

5

and

6m

depending on the cover width of the panels used.

Qpes of modern roof system

The following generic types of modem roof system may be recognised:

30

1) Built-up roofs consisting of short-span double-skin systems incorporat-

ing a relatively thin metal liner tray, which serves primarily to support the

insulation, and a watertight outer skin which is generally a conventional roof

sheeting profile. This outer skin is likely to be both stronger and stiffer than

the liner tray,

so

that the connection between the two skins is a crucial fac-

tor that will strongly influence the stressed skin performance.

Thin liner trays are often used without seam fasteners, and this signifi-

cantly reduces their contribution to stressed skin action. In such cases, tests

were first conducted on the liner tray alone, as fixed in practice. Nominal

seam fasteners were then added, and the liner tray assembly was retested.

It was assumed that, if this type of built-up roof were to be used in formal

stressed skin design, seam fasteners would be used. The seam fasteners

were, therefore, kept in place for the final test on the complete system.

The liner trays forming the inner skin are usually one of the two types

shown in Fig 3.

A

typical profile pitch is 250-300mm, with a nominal

thickness of 0.4mm. Thus, they often have wide, flat regions which buckle

in shear at low loads. Their height is usually less than 20mm. Under stressed

skin action, these thin liner trays often operate in the post-buckled condi-

Zed spacer fastened with

5.5 dia. screws Q 75mm centres

1 132

top sheet

Zed

spacer

Section through fixed rafter

Fig 1 Details of the test ram e as set

up or

test

no

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Engineer Volume 77lNo 2 1 2 November

1999

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Paper: DaviedLawson

Fig

2.

Roof system at the end of test no. showing the failure mechanism

tion. Fig

4

shows a typical built-up roof with the upper skinupported on

Z

spacers and plastic ferrules. In test nos. 1 4 , he test programme pays ar-

ticular attention to the behaviour of a liner tray acting compositely with roof

sheeting.

2)

Long-span double-skin systems incorporating atructural liner tray and

a watertight metal outer skin. Structural liner trays are also termed cas-

settes’ and, as well as supporting the insulation, hey also serve to eplace

conventional purlins and sheeting railsby spanning between the primary

frames.

Recent research has investigated the performance f structural liner trays

and, as aonsequence, Eurocode 3:

Part 1.34

ncludes design rules for these

elements in bending with the wide flangen either tension or compression.

It also includes some rudimentary procedures for the stressed skin design

of these members. These ules are based on the resultsf tests carried out

in Germanys,

so

the geometrical constraints are essentially the limits f the

German test series. Fig

5

shows the cross-section of a typical cassette,

together with the nomenclatureof the Eurocode. Typical dimensions are:

b

400 600mm

h 50 17Omm

t

0 .75 1 .5m

The stressed skin design is based n the facthat the fundamental principles

remain applicable but the resistance will tend to be dominatedby the ten-

dency of the wide lower flange touckle in shear. Because of the geomet-

ric restrictions and the absence of the profile distortion term, which

is so

significant with trapezoidal profiles, the deflection calculation can be con-

siderably simplified. Test no.5 is a test on this type of system.

_ r \ \

Type profile

Fig

3

Typical liner tray projiles

Type

2

profile

Fig 4 . Typical built up roof system withhe upper skin supported

n

Z spacers

and plastic ferrules

bf

Narrow flange

Wide f lange7

1 1

ig 5. Cross section of a typical structural liner tray

Fig

6.

Typical standing seam roof systemith clippedjixing

3)

Either of the above types of inner skin may be usedwith a standing seam

or clip-fix outer profile. The reason for using a clipped outer profile is to

minimise the number of fasteners penetrating the waterproof skin. Clip fix-

ings also give the outer skin greater freedom of movement relative to the

remainder of the structure and, in so doing, they greatly reduce the poten-

tial for tressed skin action. This type f system may therefore bexpected

to rely heavily on the stressed skin performanceof the inner kin. Test no.

6

is a test of a standing seam roof system supported off a shallow liner tray.

4) Standing seam and lip-fix profiles are also

sed

alone without an inner

skin, and Fig 6 shows a typical arrangement. This type of system is dis-

counted in the present paper, as it ffers insufficient stressed skin action for

practical usage. Designers need to be cautious hen specifying this type of

system because some supporting structures make implicit appeal to the

additional stiffness given by ‘serendipity’ stressed skin action, and this will

not be available with single-skin standing seam andclip-fix profiles.

5) Composite panels sandwich panels) consisting of two thin metal faces

bonded to, and cting compositely with, a lightweight insulating core. The

faces may be flat,quasi-flat or profiled, and the coremay be oneof sever-

al materials such as polyurethane, polystyrene or mineral wool. However,

the precise make-up of the panel is unlikely to be mportant in the context

of stressed skin design. The panel as a whole is ikely to be morehan ade-

quate to resist the in-plane shear forces hat it attracts. The problems f shear

transfer are ntirely in theconnections.

Under normal circumstances, sandwich panels do not require a seam

connection between adjacent panels and, indeed, it is ifficult to provide this

connection.This immediately reduces their potential for stressed skin action.

The connections to the supporting members purlins or sheeting rails) are

also problematic. Through-fixing screws passing through both metal faces

and the core are notsually very effective in shear because

the

bottom face

is usually very thin and theconnection to the top face is eakened by the

tendency of the screw to bend. Proprietary concealed) connections usual-

ly have imilar problems and, because they are relatively few in number and

concentrate on the clamping force ecessary to resist wind suction, tend to

generate relatively little shear strength. Test nos. and 8 illustrate these

points.

Test series

The test series was esigned to cover a range of modem roof systems, and

materials were supplied by a number of manufacturers. The roof systems

were selected to be ‘generic’ and typical of a range of alternative products.

No

manufacturer’s trade names

are

given, but the systems areescribed

as

follows:

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I )

Double skin, dense insulation system

iner tray (0.4mm thick with a 19mm-high type 1 profile)

unbonded

PUR

insulation of 40mm thickness

oof sheeting (0.7mm thick and 32mm deep trapezoidal profile)

sheet-purlin fasteners (5.5mm dia.) in alternate corrugations at 320mm

sheet-seam fasteners (4.8mm dia.) at 475mm centres

centres

(2)Built-up roof system withZ spacers

iner tray (0.4mm thick with a 19mm-deep type 2 profile)

Z

spacers of 70mm depth and

l

.6mm thickness

oof sheeting (0.55mm thick and 32mm-deep trapezoidal profile)

sheet-purlin fasteners (5.5mm dia.) at 250mm centres

sheet-seam fasteners (4.8mm dia.) at 475mm centres

(3)Built-up roof system withZ spacers and plastic ferules

iner tray (0.4mm thick with an 18mm-deep type 1 profile)

Z

spacers of 45mm depth and 1Smm thickness

plastic ferrules of 42mm depth


Z

spacers fixed at 450mm centres to the purlins

sheet fasteners at 450mm centres throughout

4 ) Built-up roof system with rigid spacers

iner tray (0.4mm thick with a 19mm-deep type

1

profile)

spacers comprising 80mm-deep rigid brackets attached to the


sheet fasteners at 475mm centres throughout

purlins at 960mm spacings

(5) tructural l iner tray (cassette) withoof sheeting, but no purlins

structural liner tray (0.75mm thick and 80mm deep)

oof sheeting (0.55mm thick and 32mm deep)

iner trays fixed by three (5.5mm dia.) fixings/600 mm-wide panel

oof sheet fixings at 450mm centres

(6) tanding seam roof system

iner tray (0.4mm thick with a 17mm-deep type profile)

spacer stools of 70mm depth and l Smm thickness supporting a

oof sheeting (0.7mm thick and 70mm deep)

iner tray panel fasteners at 450mm centres

no seam fixings in the roof sheeting (clip fixings used)

channel of 1.6mm thickness

(7)Composite panels with through f ixings

composite panels with 0.5mm-thick external and 0.4mm-thick internal

hrough fixings (5.5mm dia.) at 475mm centres along the ends of the

skins and a 80mm-thick rigid polyurethane foam core

panel

(8)

Com posite panels with s ecret f ixing s

composite panels with 0.5mm-thick external and 0.4mm-thick internal

skins and 60mm-thick polyurethane foam

ixings (5.5mm diameter) at 280mm centres along the purlins

(note that the clip-fix detail offered no restraint to longitudinal

movement)

Where possible, initial stiffness tests were carried out on the liner trays alone,

and these were followed by resistance and stiffness tests on the approxi-

mately 6m square complete roof panel assemblies.

A

full set of test results is

given in

SCI

Report KT 325 , which is based on a test report prepared by

Professor J

M.

Davies and W. H. Deakin of the University of Salford.

Theoretical analysis

The individual diaphragms were analysed according to the well-established

principles which have been detailed in a number of publications, including

refs 1, 2, and 3. In the case of two-skin systems, the individual skins were

analysed as separate diaphragms, s that their strength and stiffness could

be considered both individually and in combination. Certain of the quanti-

ties used in these calculations require more detailed comment as follows:

Strength andflexibil i tyof individual fasteners

These quantities feature prominently in the design expressions. A wide vari-

ety of fasteners were used in the tests, as specified by the manufacturers of

TABLE

l

Fastener propertiesused in the analysis

Fastener type

Sheet-to-purlin fasteners

5.5mm dia. self-drilling, self-tapping

screws with steel washers


screws with steel washers and plastic seal

Seam fasteners

4.8mm dia. aluminium blind rivets

4.8mm dia. monel metal blind rivets


screws

Strength

(kN/mm sheet

thickness)

4.6

4.

2.8

2.8

2.4

Flexibility

( m W

0.06

0.45

0.13

0.35

0.35

NOTE:

With steel washers, a reduced sheet-to-purlin fastener strength f 3.8kNImm sheet

thickness was used

for

thin sheets of less than 0.55mm nominal) thickness.

the different systems. For the purpose of the analysis, they were grouped

according to the basic type

of

fastener and washer, andypical (conservative)

properties were used as shown in Table I

Sheet distortion

constant

K

This constant features significantly

in

the first component of diaphragm

flexibility, and refs

I

2 and 3 all include tabulated values. The calculation

of this constant was improved significantly in ref. 7, and earlier texts include

the less accurate values. The values used here have beencalculated using the

most recent theory and have been obtained exactly for each profile rather

than by interpolation from published tables.

Local shear buckling

It is a feature of some of the two-skin systems that the lower skin incorpo-

rates a wide, thin lower flange which is particularly susceptible to local

shear buckling. This flange can advantageously incorporate rolled-in longi-

tudinal stiffeners which significantly improve its shear strength, but these are

not always present. When this element is unstiffened, the critical shear stress

may be determined using the well-known formula:

5.35Z Et

cr =

12 1-v2)b2

However, in most cases, the lower flange includes longitudinal stiffeners and,

for such cases, the above formula is excessively conservative. Baehre has

reported a total

of

24 tests on structural liner panels (cassettes) subject to

diaphragm action and, in analysing his test results, he found that

it

was suf-

ficient and conservative to use the simplified version of the Easley formula,

i.e.

where

D , = EZ,/bk

L ,=

Et Il0.92

I is the second moment of area of the stiffened flat about its horizontal

bk is the width

of

the flat element

h is the width of diaphragm in the direction of the profile

t is the net thickness of the diaphragm after deducting for coatings, etc.

axis

It then follows that the shear strength (kN/m) is given by:

This is the equation in clause 10.3.5 of Eurocode 3: Part 1 3 It should be

noted that this equation is incorrectly printed in both Baehre s paper and in

the original printing of the Eurocode; it is corrected in the corrigenda to ENV

1993-1-3 dated 1997-02-25.

Test results and comparison with theory

The following is a brief description of the results of each of the tests, togeth-

er with a comparison with an analysis according to BS 5950: Part 9. Because

32

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35

Deflection mm)

ig

7. Load-deflection curve or test no, l showing the theoretical

peflormunce

of

the roof system

of early shear buckling, it is difficult to come to any conclusions regarding

the components of flexibility. More detail is given for the built-up systems

that have the greatest practical interest. One of the tests in progress is shown

in Fig 2.

In each case, the liner tray alone performed much as expected, so that the

comparisons are directed primarily towards the performance of the complete

roof system.

l ) ense insulation system

The liner panel alone reached a shear force of 7.5kN (1.3kN/m length), with

a flexibility similar to that predicted theoretically, before local shear buck-

les formed in the flat elements between the ribs.

The complete roof system, with the insulation in place, reached a shear

force of 35.3kN (5.9kN/m) before failure of the roof sheet seam fasteners.

The theoretical analyses gave the following values (the calculations for

the complete system assume full interaction between the liner tray and the

roof sheeting):

Lineranel: resistance 6.6kN (local shear buckling)

flexibility 0.42mmkN

Roof sheet:

resistance 21.4kN (end failure)

27.7kN (seam failure)

flexibility OSOmm/kN

Completesystem: resistance 6.6 21.4 = 28.0kN

(34.3kN for seam failure)

flexibility Os 0’50 = O.23mm

/ kN

0.42

+0.50

. . E )

The load-deflection curves for the complete system are shown in Fig

7.

Evidently, at a load of about 17kN, the liner tray buckled, thereby transfer-

ring more load into the roof sheeting. This test showed that the fixings

through the dense insulation were able to transfer in-plane forces into the

roof sheeting in order to achieve the full combined resistance, though with

some loss of stiffness.

( 2 )Built-up roof system with

Z

spacers

When tested on its own, the liner panel sustained a shear force of 20kN

(3.5kN/m), with a flexibility of about 0.4mm/kN, at which load it was evi-

dently close to failure in shear buckling.

The completed roof panel reached a shear force of 46kN (8. IkN/m)

before failure occurred by buckling of the liner panel and tearing at the roof

sheet seam fasteners.


the complete system assume full interaction):

Lineranel: resistance 13.9kN (shear buckling)

flexibility 0.3 1mm/kN

Both skins together

0

5

10 15

2 25

Deflection mm)

Fig

8.

Load-deflection curve or test no. 2

Roof sheet: resistance 13.0kN (end failure)

20.9N (sheet to Z-spacer fasteners)

28.5kN (seam failure)

flexibility 0.95mm/kN

Completesystem: resistance 13.9 13.0 = 26.9kN



The load-deflection curves for the complete roof system are shown in Fig

8.

This test showed that the Z spacers were able to transfer in-plane forces

into the roof sheeting with very little loss of stiffness. Indeed, it was appar-

ent that the liner tray was able to act ‘compositely’ with the roof sheeting

so that, at failure, both were fully active in resisting the applied shear force.

(3)Built-up roof system with ferrules

The liner panel alone was loaded up to 9kN and then unloaded. Although

the failure load was not reached, the deflections were evidently enhanced

by local shear buckling at an early stage.

The completed roof panel reached a shear force of 38kN (6.5kN/m)

before failure occurred. This was dominated by buckling

of

the liner tray,

but with failure of the seam fasteners in both layers

of

construction.



Lineranel: resistance 7.OkN (shear buckling)



23.2kN (sheet to Z-spacer fasteners)

27.8 kN (seam failure)


Completesystem: resistance

7.0

13.7 = 20.7kN



The load-deflection curves for the complete system are shown in Fig 9.

As in the previous tests, there was significant ‘composite’action between

the liner panels and the roof sheeting, although the deflections were

enhanced by the early buckling of the liner panel.

4 )

Built-up roof system with ‘rigid’ spacers

The liner panel alone was again loaded up to a shear force of 9kN and then

unloaded, with similar results to those described for test 3 above.

The complete roof panel reached a shear force of 13.2kN (2.3kN/m)

before buckling of the liner tray and bending of the spacer brackets pre-

vented further load from being applied.

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4

Both skins together

3

5

J

5 10 15

2

25

3

Deflection (mm)

Fig 9. Load-deflection curve for test no

3



Lineranel: resistance 6.6kN (shear buckling)



36.2 kN (seam failure)


Complete system: resistance 6.6 21.4

=

28.0kN


flexibility O.l8mm/kN

The load-deflection curves for the complete system are shown in Fig

10.

The results for this test should be compared with those for test nos. 2 and

3, which performed better because of the more frequent attachments pro-

vided by the

Z

spacers. The behaviour here could probably be made equiv-

alent to that of test nos.

2

and 3 by reducing the spacing of the brackets to

about 600mm.

(5) tructural line r tray with roof sheeting

The structural liner tray assembly differed from the previous roof systems

by requiring no purlins. The liner tray alone reached a shear force of 12kN

(2kN/m) before buckles formed in the flat plate elements. Its shear flexibility

was about 1.4mm/kN. The shear force at failure was close to that given by

theoretical analysis. However, there is no definitive theory for the flexibil-

ity of these sections, and tentatively applying the flexibility calculations of

BS

5950: Part 9 with no shear distortion term gave a rather optimistic pre-

diction of 0.64mmkN.

Eurocode 3: Part

2.34

ncludes an empirical estimate of the shear stiff-

ness of structural liner trays, although it is not made clear whether this

allows for some interaction with the outer skin of sheeting. Thus, in clause

10.3.5(6), it is stated that the shear stiffnesshnit length may be obtained

from:

where

L is the overall length of the shear diaphragm in the direction of span of

b

is the width of an individual liner tray (500mm)

b is the overall width of the shear diaphragm (5000mm)

a is a stiffness factor which may be conservatively taken as 2000N/mm

the liner trays (6000mm)

The above equation is taken directly from Baehre’spaper5 where

S,

is a shear

stiffness rather than a stiffnesdunit length. This appears to be a further error

in @e Eurocode, which is not covered by the recent corrigenda. Furthermore,

Baehre’s tests were carried out on the structural liner tray alone,

so

that it

34

12

10

8

6

4

2

0

Both skins together

skin only

1

10

15

20

25

3

35

Deflection (mm)

Fig

IO.

Loud-deflection curve

for

test

no.

4

may be deduced that the above equation is not intended to

include any allowance for the outer skin. Applying the above numerical val-

ues for test no.

5

gives a stiffness of 1.14kN/mm or a flexibility of

0.88mdkN . This is rather stiffer than the measured value without the outer

skin.

The completed roof panel reached a shear force of 32kN (5.5kN/m)

before local failure of the liner tray occurred at one comer. On the basis of

full interaction, the theoretical analysis gave a shear resistance of 39kN for

the complete roof. The measured flexibility of the complete roof panel was

l.Omdk N, which is similar to the value

of

0.88mm/kN predicted by

Eurocode 3. The final deflection was 6Omm.This test showed that the liner

trays are able to transfer significant in-plane force through their vertical ribs

to the roof sheeting. Improvement of the design of the structural liner tray

by rolling small, longitudinal stiffening ribs into the flat plate elements

should enhance its capability for ‘stressed skin’ design.

(6)Standing seam roof system

The liner panel reached a shear force of l OkN (1.7kN/m) before buckling

commenced, and the flexibility was approximately OSmmkN. The com-

plete roof panel reached a shear force of 18kN (3.0kN/m) before buckling

of the liner panel again occurred. The measured flexibility in the service-

ability range was again OSmmkN, and the final deflection was 32mm.

The test showed that there was only a small force transfer to the roof

sheeting and that he flexibility of the standing seam roof was relatively high.

The shear resistance derived mainly from the liner tray.

(7)

Composite panel with edge ixings

The fasteners were installed initially only at the edges of the panels, but the

test was terminated because of excessive distortion of the fixings at a small

shear force of 4kN. Additional edge fasteners were installed, but the shear

resistance increased to only 6kN ( lkN/m).

(8)Composite panel with ‘secret’fixings

The fasteners were installed initially only at the edges of the panels, but the

test was terminated at a shear force of 6kN. Additional edge fasteners were

installed at 475mm spacing across the panel, and the test failed owing to dis-

tortion of these fixings at a shear force of I5kN (2.5kN/m). The measured

flexibility was about

1

m m , nd the final deflection was 38mm.

Test nos. 7 and

8

showed that the stressed skin action of composite pan-

els depends largely on the distortion of the through fixings owing to the

thickness of the panel and on the thickness of the inner skin of the panel.

Implications for design

The tests showed that the built-up roof systems using Z spacers performed

well and could resist an in-plane shear force of 5-8kN/m length of the roof

panel, with a shear flexibility of 0.2-0.5mmkN (for a 6m square panel) in

the serviceability range. The good strength and stiffness performance sug-

gested that significant ‘composite’action between the roof sheeting and liner

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tray had occurred. The shear resistance of the single-skin roof sheet was

exceeded in test nos. 1, 2, 3, and 4. These stiffnesses are also commensu-

rate with the use of single-skin roof sheeting which was calculated to be of

the order of OSmm/kN.

The structural liner tray also performed well, indicating that the vertical

ribs of the liner tray were able to transfer the required forces to the roof

sheeting. Furthermore, a thicker liner tray, with more closely spaced ribs or

additional rolled-in stiffeners, could possess excellent properties for

‘stressed skin’ design.

Standing seam and clip-fix roof sheets have little capability for stressed

skin action, and such systems should be designed

on

the basis of the liner

tray acting alone.

The stressed skin action of the composite panels was adversely affected

by the absence of seam fasteners and by rotation of the through fixings or

by distortion of the fixings at the ends of the panels. This rotation occurred

owing to the eccentricity of force on the fixings, causing tearing of the thin-

ner lower sheets. The panel itself possesses considerable shear stiffness and

strength. In principle, therefore, it should be possible to enhance signifi-

cantly the in-plane stiffness and strength of composite panels. In theabsence

of specific measures designed

to

promote stressed skin action, composite

panels are not recommended for stressed skin action.

Conclusions

The in-plane shear resistance and flexibility of eight different modern roof

systems was investigated by full-scale tests on 6m square panels. The tests

showed that built-up roof systems making use ofZ spacers possess adequate

properties for their use in stressed skin design. All these systems depend on

the liner tray for some of their shear resistance and stiffness, which suggests

that there may be a benefit in improving their in-plane properties by care-

ful design in order to avoid local shear buckling.

The other roof systems showed less satisfactory performance and would

not generally be recommended for stressed skin design. However, their

stiffness may still have an important effect in reducing the deflections of

steel frames.

Acknowledgement

This work was carried out at the University of Salford and SCI, and was

funded by British Steel (Strip Products). The manufacturers; Precision Metal

Forming, Ward Building Components, Cape Building Products, European

Profiles, Huuraal, and Ash Lacy Ltd, are thanked for their supply of

materials for this project.

References

1.

2.

3.

4.

5.

6.

7.

BS 5950

The structural use

of

steelwork in building: Part

9:

Code

of

practice

or stressed skin design,

London, British Standards Institution,

1994

Davies,J M., Bryan, E. R.:

Manual

of

stressed skin diaphragm design,

Granada, 1982

European Convention for Constructional Steelwork: ‘European rec-

ommendations for the application of metal sheeting acting as a dia-

phragm’,

ECCS Document No.

88,1995 (available from SCI or BCSA)

Eurocode 3

Design

of

steel structures,

CEN

ENV

1993-1-3:

Supple-

mentary rules or cold-formed thin gauge mem bers nd sheeting, I996

Baehre,

R.:

Zur Schubfeldwirkung und-bemessung von Kassett-

enkonstruction’ (On the diaphragm action and diaphragm design in

cassette construct),

Stuhlbau, 7

1987, pp1 97-202

Davies,

J

M.

Deakin, W. H.: ‘An investigation of the stressed skin

action of modern roofing systems’, SCZ-RT-325,1993 available o SCI

members)

Davies, J

M.:

‘A general solution for the shear flexibility of profiled

sheets: I: Development and verification of the method; 11:Applications

of the method’,

Thin Walled Structure s,

4,1986, pp41-68 and 151-161

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