LABORATORIUM VOOR
SCHEEPSCONSTRUCTIES
TECHNISCHE HOGESCHOOL
- DELFT
RAPPORT Nr.
SSL 118
BETREFFENDE:
4
Bijdragen voor het Comniissie rapport van
Commissie 3b - Stiffened Plating in
3-Dimensional Structures. I.S.S.C.
1967.
Corruated - and swedgedlatj.
A review of published work since the 196k I.S.S.C.
Since the 196k Congress several reports have been published covering various kinds of loading and different types of corrugations. In the following
para-graphs the reports will be discussed briefly.
Corrugated bulkhead, hydrostatically loaded
in the
elastiø region.The experiments on a full-scale ahallow-troughed corrugated bulkhead in the Ship Structures Laboratory in Deift culminated
in a
paper (i» and a report (2).The bulkhead ia part of
a testing tank for
watertight bulkheads which has been fully described in the Proceedings of the I.S.8.C.1961.
Briefly it may be stated that the experimente are part of a series with the
following general objectives.
Investigating the behaviour of bulkheads under distributed loading; first in the elastic region, finally in the plastic region.
Verificating simplified theoretical methods of calculation.
Comparing whenever possible of laboratory experiments and measurements on board of ship.
k) Investigating the application of computer-assisted methods of calculation. One of the methods mentioned under sub 2) has been published by Jaeger et al (3).
The main conc1usion in connection with the calculation of maximum stresses, obtained so far are summarized below.
The bulkhead Can be considered as a group of identical and identically loaded continuous beams on rigid supports.
The rigid supports mentioned above are realised by the stringere. As long as nothing more definite about the degree of fixation of the
beam's ends are known these ends should be considered as simply supported. a) Transverse bending stresses can be assessed by considering a strip of
unit height across the bulkhead as a continuous beam on rigid supports (the folds of the corrugation profile).
e) Contraction causes no additional transverse stresses.
The tiret of the conclusions is also supported by an experiment of Skjeggestad and Bakke (k).
SSL 118
O Io C
- -.--.----.--- r
ste o TESTING TANKï
i i i
i
iLi
i
I I I
&Q38 d 220 d-238 3 ' L- 4k
940 1100 940 BULKHEAD MODEL PL 3mmFIG1A
o o o 6 3't FIG. 1.Copy taken from S.F.I. Rapport nr. R57. Jan.
1965.
"Corrugated bulkhead laterally loaded toultimate failure".
Skjeggestad, B. and Bakke, E. model scale
1:3.7
L 20 ¿00
r-A-A
_a -S-. -
--
__-Buihead model and testing tank.
Section through bulkhead
(units in mm )
at a corner.( units in mm)
$ D BULKHEAD MODEL
Corrugated bulkhead, under distributed load, in the plastic region.
The teat arrangement is shown in figure 1 which has been taken from (k). The end-conn.ctione of
the
corrugations have been supposed as simply supported. The scale of the model was 13.7, the scantlingsbeing in
accordance withthe
rules of
Det
Noreke Ventas,. The latter does not alterthe
fact that thestringers are so heavily dimen8ioned that the test piece may as well be
con-sidered as three test pieces in one. Strain values were obtained by means of
electric wire resistance strain gauges with a filament length of
i6 mm.
The loads that were to produce plastic hinges in the corrugated beams have
been calculated for either bending only or shear only. From the two values thus obtained limit loads were calculated for the case of combined bending and shear. Ph. xperimenta]. results agreed better with the load value for the case of
ben-ding only (this was the higheSt) a fact which the authors attribute to membrane stresses.
After severe plastic deformation caused by buckling the first cracks appeared
at
a
load some 60% higher than the one at which extensive yielding had taken place. After repairs had been effected serious cracking occurred at a somewhathigher load and the exeniments were discontinued.
Cracking began in the tension flange of the corrugation profile in way of the
intersection with a frame, later on cracks appeared in the centerpanel near to
a stringer. This is dust what can be expected.
During experiments within the plastic region the model was at times completely unloaded and then reloaded. The load-deflection curves show a practically lineair behaviour of the structure.
Local buckling of the individual plate panela did not oociu' in th. elastic
zone owing to the dimension. Of these panaIs.
It must be stated finally that the stress concentration values can be but
indicative, what with a scale factor of 3.7 and a gauge length of 16 mm on the model.
Corrugated and swedged ulatin
in
deckhouse sides walls.The Society of Naval Architects of Japan has published in
(5)
the results oftheoretical and experimental studies concerning the bending of ships'
deck-houeø
and
superstructures including those with corrugated or ewedged platingas aide walls.
The writers make use of the following definitions.
i) A vertically corrugated plate is one where the corrugated profile is placed
vertically. This i contrary to common practice where this type of plate is called horizontally corrugated because the corrugations proper or fold
HORIZONTALLY SWEDGED OR
CORRUGATED.
-y
VERTICALLY S WEDGED OR
lines are positioned horizontally.
Mutatis mutandis the same reversion applies to the writers description of a horizontally corrugated plate.
2) No distinction is made between corrugated and swedgid plating.
In order to avoid confusion the more caminan definitions will be used in what
foll9wB.
The use of corrugated or swedged plating as sides of deckhouses or superstruc-tures has some very attractive aspects because of the orthotropic characteris-tics of these structural elements (see figure 2) and because of their relatively light weight.
In the case of figure 2-A the theory is not developed on the basis of the ortho-tropic plate but on that of the flat plate element considered as a beam loaded in its own plane and imposing only shear forces on the adjoining elements. From the theory it can be deduced that longitudinal stresses from the hull are not transmitted to the deck forming the top of the deckhouse.
It may be necessary to support these sido walls with vertical stiffeners; if so, care must b. taken that the advantage mentioned above will not be wholly lost. This is realised by not connecting the lowest corrugation trough (or swedge + part panel) to the stiffener.
The Case of figure 2-B is divided into two elementary problems viz.: The deckhouse is subject to tension only.
The deckhouse is subject to bending only.
As regards sub a. ten.ion in this case means shear
and
although the effect of shear lag is not considered it appears to be taken care of in the use of aneffective width.
Concerning sub b. this type of plating does not have very much resistance against the bending moments shown in the figure and neither has the deck, 80 that the total structural resistance against bending is taken as nil.
In all cases expressions for the effective area and moment of inertia of the deckhouse are given. (These relate of course to bending of hull and deckhouse).
Ixoerimenta with models.
Although in the theoretical part deckhouses are mentioned practically to the exclusion of superstructures, the experiments deal solely with superstructures. However the latter may be considered as a special case of the former, to wit a deckhouse extending from side to side is a superstructure.
14
Three testpiecos had su'oerstructures with horizontally corrugated side walls,
one whore the walls were not stiffened, one with vertical stiffeners without a connection to the lowest corrugation trough and one were the stiffeners were attached over their whole length. For all models the superstructures were
1000 mm long, 1400 mm wide and 110 mm high. The corrugation angle was 900 and
all panels were 10 mm in breadth. Plate thickness was .14 mm while the stiffeners were .8 mm thick.
Correspondence between model experimental results and theoretical calculation was very good and as long as the stiffeners (if any) are not attached over their whole length the stresses in the superstructure deck are effectively reduced.
One model was tested with vertically corrugated plating, the dimensions of the superstructure being 1000 x 1400 x 205 mm with a plate thickness of 1.6 mm. The load was one of pure bending applied to the hull.
The originally calculated values heavily overestimated the stresses in the superstructure deck. In the theory the magnitude of these stresses is mainly governed by one parameter which in turn is dependent for the most part on the
type of endconnection assumed for the corrugated beams or their plate panels. A recalculation of the parameter based on a more realistic assumption of the endconneotions resulted in much improved values which were however somewhat lower than the observed ones.
A final adjustment of the relevant parameter led to satisfactory agreement. It would be interesting to know if the endconditions which resulted in the correct parameter values will be correct in other cases.
Shipboerd measurements.
After the model experiments discussed above measurements were carried out on the deckhouse of a submarine chaser 3ubjected to a sagging moment when
dry-docked. The sidewalla in this case were vertically swedped, the swedges being of a trapezoidal shape.
The calculated stress values in the deckhouse top did not agree very well with the measured values because the deckhouse was not situated symmetrically
about
.5
L and because part of the wall consisted of flat platee. Also the deckhouse was lese effective than was assumed.An analysis of stresses measured in the swedged plating confirmed the baaic assumptions of the theory namely shear loading at top and deck and behaviour
as a group of columns. (see figure 2-B).
If a proper effective shear modulus could be established the theoretical method will give satisfactory results.
LIST OF REFERENCES.
(i) Jaeger, li.E. and Van Katwijk, P.A.
"Vrification Lxp&rimentale d'une M$thode de Calcul de Cloisons en Thies Ondul,es Plie Verticaux". Association Techn. et Mar. et Aêro. Session
1965.
Jaeger, H.E. and Van Katwijk, P.A.
"Stress and Strain Distribution in a Vertically Corrugated Bulkhead". Netherlands' Research Centre T.N.O. for Shipbuilding and Navigation,
Shipbuilding Department. Report No. 738.
(Intern. Shipb. Progr. 133 (1965) pp 353 and following.)
Jaeger, }I.E., Burgbgraef, B., and Van der Ham, I.,
"Investigation of the Stress Distribution in Corrugated Bulkheads with Vertical Troughs". Netherlands' Research Centre T.N.O. for Shipbuilding and Navigation. Report No. 155.
(Intern, Shipb. Progr. 2 (1955) pp 3-29.)
(1+) Skjeggestad, B., and Bakke, E.
"Corrugated Bulkhead Laterally Loaded to Ultimate Failure". S.F.I. rapport nr.
R57. Jan. 1965.
(5)
60th Anniversary
series. Vol. 9.Published by the Society of Naval Architects of Japan.
Tokyo, 1964, pp 51-53, 107-.119, 131-137, 14k-150.
Unfairness and Distortions of Stiffened
Platee.
In previous proceedinge this committee paid attention to initia]. unfairness
and distortion of stiffened plates and made it clear how the problem of ob-taining useful practical knowledge about the order of magnitude of these de-viations is complicated by the large number of variables involved and the
enormous amount
of data that is required before any statistical study can bemade.
So it may be some time before expressions with which to calculate the probable unfairness or distortions of a
given
structure will be a reality. The Shipyardsthemasives can assist in dealing with this problem if they were to gather as much data as possible from their own structures. In this way such imponderables as production methode, workmanship, welding techniques etc. might be neglected
so that the problems regarding the relevant parameters could be somewhat
clari-fled.
As it is not possible to eliminate the deviations altogether it is only sensible to learn how to minimise them so that they may be neglected and'how to work with them.
By now the importance of initial distortions is generally recognised but in most papers on experimental work and its resulte they are not mentioned uniese it is to explain discrepancies with the theory that Can not be accounted for otherwise
and
again in most oases they hav not been measured beforehand.As long as no definite facts or figures can be given concerning actual initial unfairness or distortion of orthogonally stiffened plates and yet if more is to be known about their influence on strength characteristics all laboratory experiments should start with establishing the distortions of the testpieces. Once they are known it may be possible to assess their influence in order to avoid unpleasant surprises especially when loading beyond the elastic limit. If no qualitative estimate is possible then danger spots may be put under
ob-servation either intermittently or continually for the duration of the
experi-ments. This way the behaviour under increasing load of the particular spot becomes known and Can be studied, while at the same time warning la given in case of a development which may endanger the whole experiment such as a pre
mature local collapse that may trigger an overall collapse. Papers on
experi-mental work abound with remarks concerni, ng unsatisfactory results due to
ini-tial deformation, but quite often no more pertinent information is given.
I
Furthermore the inclusion of measuring the initial unfairness whenever
practi-cable and possible will provide
factual experience concerning questione as to if and when distortion may be disregarded or may be used to advantage or must be reckoned with. This may seem s bit far-fetched bus well known for in-stance that in ceso of a laterally loaded orthogonally stiffened platethe
overall elastic bending moment and shearforce distribution is not noticeably influenced bythe normally existing
unfairnea or distortions. (Here difficul-ties are almost always caused by insufficient knowledge about the correct boun-dary conditions).However as soon as stresses must be derived from the&e moments and forces, that is when the structurel dimensions become comparable with those of their
devia-tions, the influence of distortion or unfairness will become manifest.
Quite another type of initial deformation Is caused by the assembling of pre-fabricated elements, either because of simply welding the sub-assemblies to-gether or because of making them fit toto-gether. The latter cause may lead to distortion of whole sub-structures, especially if these consist of stiffened plates or corrugated plates (seo figure 1). In most cases this results in a built-in predispoeition towards s certain mode of collapse because these initial deformations can be of considerable magnitude.
In practice one will have to deal with the finished structural assembly and therefore with a combination of the two kinds of deviations discussed above. Ai stated before the beet that Cnfl be hoped for io that in time it will become possible to minimis all tyee of ais, or shape deviations co that their
influ-ence my be neglected, Until auch stime a solution maybe found in establishing a practical upper limit value or an overage value for the distortion based on
data made available by the Shipyards (and) or Classification socIeties. If the number of data obtained per Shipyard is sufficiently large for a statistical
analysis then it
might
be possible to establish separate limit or norm-average values, otherwisethis may
be tried for a group of yards in the individualcountries as it is thought that workmanship, production
methods etc. (see
above)will not differ much among the large yards In one country.
As to the number of relevant parameters methods must be found by which they can be grouped together to form new ones, for instance an area number might be
defined as A
and a
thickness number as T the meaning of (sect. area)p
the symbols is given in figure 2. Separate studies could be devoted to struc..
'y- L
ture, with symmetric or a-symmetric atiffener.,alternativeljï the welding
cur-rent could be used to distinguish groups.
euggea-tioris that my contribute to solving the probler of unfiirness and distortion in stiffened plates. The assistance of the shipyards in this matter is essen-tial, in the first place because ultimately
they will
profit the moat when the problem is solved and secondly because no structural research institution can afford to nplay a team of technicians for the sole purpose of gathering theenormous nwnbr of
data
that is needed. Measurements can b. carried out with the help of relatively simple instruments and the accuracy need not be betterthan 0,5 mm.
Finally it should be realized that the refineients of stress calculation
theories need not go beyond the accuracy of the combined production and strain measuring techniques, since the latter define the only possibility of checking
the 0a3culationa.
h
FIG. 1.
DISTORTION OF A CORRUGATEDSUL KHEAD CAUSED BY MAKING /7 FIT BETWEEN THE COLUMNS.
