analysis of clifton springs long jetty construction...

Individual Project A Mansfield

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Analysis of Clifton Springs Long Jetty Construction Methods

Flinders University Maritime Archaeology Field School

Portarlington 2008

Author

Anthony Mansfield

Acknowledgements

The author would like to thank his team supervisor, Jun Kimura, for his encouragement during the field school, and Dr Sam Turner of the St Augustine Lighthouse Museum for his questions regarding the structural detail of the jetty

which led to this particular project.


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Introduction This project is submitted as part of the assessment for the Flinders University

Maritime Archaeology Field School of February 2008 held at Portarlington, Victoria.

The Clifton Springs Long Jetty is an old jetty which has mostly collapsed. The

seabed around the jetty is littered with debris from the structure of the jetty.

Determining which part of the jetty the debris is from can be difficult if the original

structure is unknown. This project aims to assemble, from the observed jetty

remnants, the method of the jetty’s construction and thus provide a guide to the

original configuration of debris from the jetty.

History The jetty was constructed sometime between 1850 and 1888 to allow bay steamers

to transfer passengers to the Clifton Springs Baths. The jetty was used until 1920

and by 1925 had fallen into disrepair. After this time there is no records found of its

use by any commercial or government body. The jetty is today little more than a

long row of piles, some of which have completely fallen, and some remaining span

structure.

Aims This project has two main aims.

The first is to identify the method of construction of the jetty deck. This will allow

the correct identification of debris on the seabed around the jetty. A future

extension of this project would be to compare the construction of this jetty with

other jetty’s of the era to identify local variations.

The second aim is to identify non conformances and failures in the debris. Non

conformances would be items of debris which do not conform to the identified deck


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construction. Failures are the areas of the jetty structure which have failed in a

similar method and may have a common cause related to the method of

construction. The non conformances and failures will be briefly discussed.

A possible extension of this project would be to compare the construction methods of

this jetty with other jetties of the era which may shed light on questions such as

when the jetty was actually built and what the builders anticipated its service life

would be.

Significance The construction methods used demonstrate both the technology and materials

available at the time and the expected life of the jetty. These two aspects provide

information on the actual and expected activity at the site.


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Method The method adopted was to

measure the remnant jetty

structure still standing and

partially displaying

construction methods. By

assembling information from

along the remnant jetty a

composite frame system

could be developed.

The jetty was judged, upon closer inspection, to be too unsafe to climb on to get the

required measurements. A series of photographs were therefore taken with scale

bars. These photographs were then manipulated in Photoworks to produce the

dimensions of the remaining structure.

The seaward end of the jetty was photographed on the 10th Feb and the shore end

on the 15th Feb.

Dimensions and Descriptors While the scale bar and the initial measurements were SI units, the jetty was

originally designed and built in the era of imperial measurement. Thus the

measurements taken have been generally presented in imperial units as this provides

a more logical interpretation of the data.

In order to differentiate between the various different structural elements a naming

convention has been used. Lateral beams (across the jetty) are referred to as

bearers and longitudinal beams (along the jetty) as joists.

Figure 1 - Scale Bar Method


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Results

Part 1 – General Construction Method The jetty is a simple wooden construction. It is 700m long and 4m wide on the

deck. For the majority of its length is uses sets of three 12” diameter wooden piles

with wooden cross beams for each support span. The combination of piles and

beams in these support spans are the bulk of the recognisable debris on the seabed

and are thus the main focus of this project.

Figure 2 - General Construction

Piles The centre pile is vertical and the outer two piles lean inwards at an angle of

between 6 and 9 degrees. The piles are wooden (species not identified, anticipate a

locally sourced supply) and generally at least 12” diameter. Majority of wastage in

the remaining piles has occurred at the water line.


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Cross Beams (Bearers) Each set of three piles is sandwiched between two horizontal bearers of 12”x4” by

14’2” (min). Each bearer is rebated into the piles to a maximum depth of 2” and

secured by 1” iron through-bolts.

Figure 3 - Support Span Plan View

Longitudinal Beams (Joists) Resting on top of the bearers are four joists of 12”x4” by at least 14’6” (distance

over adjacent support spans) although these joists usually cover more than one

span. These joists are oriented with the long edge (12”) vertical. This provides the

strongest support for vertical deck loading.


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Figure 4 - Support Span Side Elevation

Joist Connections Two of the joists are secured outboard of the outer piles and two are secured on the

bearers part way between the mid and outer piles.

The two outermost joists are secured to the outer piles by a single horizontal

through-bolt and to each of the bearers by a vertical through-bolt. Where two outer

joists connect end to end at the outer pile they use a stepped butt join with two

horizontal bolts through the pile and the normal vertical bolts through the bearers.

Note that the step in the joist does not begin until the inner edge of the bearer.


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Figure 5 - Joist End Connection Detail - Plan View

The two inner joists do not run along side any piles so these are simply secured by a

vertical bolt through each bearer. Where two inner joists connect end to end the

join has been placed between the bearers and the same bolting arrangement used.

In this instance there is no direct connection between the ends of the joists.


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Figure 6 - Outer Joist End Connection Detail

Bolts The bolts are 1” diameter and lengths varying from 16” to 26”. Due to the minor

dimensional variations in the pile spacings and beams it is likely that the holes for

these bolts were bored in situ. An interesting technology question is how these

holes were bored, considering that some are up to 24” long through two beams laid

edge to edge and still bored accurately enough that they did not break out of the

beams’ sides.


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Figure 7 - Through-bolt nut detail

The bolts use square heads on one end and a thick square washer secured with a

square nut on the other end. Square nuts were used as they were simple to

manufacture and provided a large bearing surface for wrenches. This was important

if the nut material was not particularly strong, eg iron rather than steel.

These bolts may have been painted or tarred after installation to slow corrosion

however there was no evidence of this detected during this project.

Decking It is surmised that fairly substantial decking was laid over the joists however there is

no evidence remaining on the jetty of the sizes of this decking. The decking would

have had to support groups of people (loading of 250kg/m2 plus) and possibly

general cargo (loading unknown) over a span between joists of 4’4” so whatever was

used had to be quite strong.

The decking would have also assisted in tying together the joists and provided a

great deal of horizontal shear strength for the jetty. The loss of the decking, either

by storm damage or salvage, would have seriously weakened the remaining jetty

structure.

Deck Fixing An artefact recovered during the jetty excavation was an encrusted square section

iron spike. It is unknown where this spike originally came from but it may have been

a deck fixing spike. Without closer inspection of the remaining jetty structure this is

purely conjecture.


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Figure 8 - Spike encrustation showing square cross section.

Overall Strength Design The jetty had to resist a large number of directional forces. Some of these are:

Lateral wave loadings on the piles

Lateral ship loadings on the piles

Vertical (down) passenger and cargo loadings on the decking and support

structure

Vertical (up) wave slap loadings on the decking

The worst of these would have been the lateral loadings which would have been

resisted by the seating of the piles in the seabed and, to a lessor extent, the bolted

connections. The usual span design did not provide much lateral resistance,

generally acting as a pin jointed structure. The designers of the jetty did recognise

this however and addressed it with special structure, discussed in Part 2.


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Part 2 – Non conformances and failures

Cross Braced Spans Several support spans have been observed on the seabed which do not follow the

structure described in Part 1. These spans have, in addition to the two bearers, at

least one additional horizontal bearer around 4’10” (1.5 metres) lower on the piles

and a cross beam running diagonally across the span between these upper and lower

bearers. It could be reasonably assumed, based upon the overall structural design,

that the lower bearers and diagonal beams were also doubled and sandwiched the

piles.


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Figure 9 - Cross Braced Span (taken from seabed site plan)

These spans with diagonal bracing would have provided extremely good lateral

strength for the jetty. As only two of these have been identified in the seabed debris

it may be that these extra strength spans were only placed either at high load areas

(perhaps where ships would come alongside the jetty) or at regular intervals along

the outer end of the jetty.


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Two Pile Spans The shore end of the jetty dispenses with the mid pile for at least 50 metres out

from the beach.

This end of the jetty did not have to be as strong as the outer end and so it is likely

that the designer dispensed with the mid pile for economy reasons.

Bearer and Joist Breakage In both the seabed debris and the remnant structure it has been observed that the

joists and bearers tend to break in regular locations [see Figure 9 - Cross Braced

Span (taken from seabed site plan)]. These locations correlate to the locations of

the through bolts which join the bearers and joists. These bolted connections, while

providing overall strength for the jetty, have produced a weak spot in the beams.

During the general structural failure of the jetty these weak spots have regularly

failed, producing the common beam and joist configurations seen on the seabed.


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Conclusions The basic arrangements of structure described in this project is sufficient to identify

and explain the use of most items of structural debris found on the seabed during

the site survey. The project therefore has achieved its basic aims.

analysis of clifton springs long jetty construction...

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