Structural Analysis of the Bow Gantry of Dredge No.4

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1 Structural Analysis of the Bow Gantry of Dredge No.4 van Rijn, Gerard 1 ABSTRACT Dredge No.4 is a National Historical Site administered by Parks Canada. This paper was based on a report on the structural analysis of the bow gantry which is part of the efforts directed at conserving this important cultural resource. The Dredge is located on a site beside Bonanza Creek where it last operated and is near Dawson City, Yukon. The analysis was conducted on a three dimensional analysis of a model of bow gantry with full dead loading and live loading up to the original design loads. The results offer options for the reconstruction configuration and procedure in order to ensure long-term performance of the structure. BACKGROUND Following the gold rush in the Yukon after 1898, placer mining by individuals was gradually replaced by an industrial phase where large corporations invested in complex infrastructure to systematically dredge the gold fields. The original Dredge No. 4 and a twin No.3 were built in 1912 by Marion Steam Shovel Co. of Ohio. They were designed, like many other gold mining dredges to be assembled where they would begin work, in this case, near Dawson City. Dredge No.4 is a 1990 t. (2200 ton), 0.42m 3 (15 cu ft) bucket-chain gold-mining dredge. It has structural frame of wood compression elements and steel tension rods. Many of the horizontal wooden elements are laminated lumber using bolts and split rings while the vertical elements are large dimension timber. Dredge No. 4 worked Bonanza Creek until 1924 when it sunk near Bear Creek and was abandoned to the Klondike silt for several years. In 1927 it was raised and continued dredging Hunker Creek but in poor condition. As part of campaign to refurbish the dredge fleet, it was dismantled in August 1940 for salvageable material, especially the machinery and steel structural components. The major change in design for the rebuilt dredge was the extension of the digging ladder to 33.5m (110 feet) and lengthening the stacker arm but struts were also introduced at this time. The struts are the wooden braces between the upper part of the gantry and the main structural frame. Figure 1 shows the 1912 model while Figure 2 shows the 1940 model. In the 1940 model the struts were framed at a sharp angle almost parallel to the standing rigging. The bow gantry was also changed to not include the lower steel diagonals and a pin mechanism was added on deck whereas the previous edition of No 4 appears to have the posts continue through the deck, much like the aft gantry is today. The new dredge worked Bonanza Creek until the summer of 1959 when it was abandoned. It sank in the following spring when the creek flooded. By 1992 silt covered its main deck to a depth of 2.4m (8 ft). Concerns about excessive distortions led to a decision to refloat and reposition the dredge to higher ground. This was done in June of The following summer the digging ladder was placed on its own crib foundation which released tension in the running rigging. The struts, by then decayed and weak, especially the starboard one, showed signs of lateral torsional buckling failure and failures at the joints. There was an obvious rotational deformation about the starboard knuckle joint in the gantry post. The situation was monitored and by 1997 it became critical as more distortion was noticed. Based in part on a structural analysis, a decision was made to dismantle the gantry. In May, the bow gantry was dismantled by cutting it in two sections and lowering it to the ground. 1 Conservation Structural Engineer, Heritage Conservation Program, Real Property Services for Parks Canada, PWGSC 25 Eddy St, 5 th Floor, Hull, Quebec, K1A 0M5. Gerard_Van_Rijn@pch.gc.ca

2 SCOPE OF WORK To conduct a structural analysis of the bow gantry and recommend the degree of tension to be maintained in the rigging and factors to consider in re-construction and repair work in order to ensure reliability in service. CONSERVATION APPROACH This report deals largely with the principle of understanding the cultural resource which is the object of our conservation efforts. The principles of respect and integrity also play a part in the analysis but play a larger role when the options are created and considered. And, of course, this work is being done because of its high cultural value and for the benefit of the public. Dredge #4 is a level 1 cultural resource as defined by Parks Canada in accordance with their Cultural Resource Management policy. The Dredge is associated with the Acorporate industrial phase@ of gold mining in the Klondike following some 10 years after the initial gold rush of >98. Dredges themselves were only part of the massive infrastructure required to conduct placer gold mining but represent the working face of the mining operation. Today, Dredge No.4 is the only survivor of the largest dredges built in the Klondike, of which only three were built. The draft statement of Commemorative Intent says that Dredge #4 has been identified as the vehicle of the corporate industrial phase of the dredging operation. That Dredge #4 is valued as a very good physical specimen of a gold dredge. The physical values of the historic place will be Asafeguarded when the structure of the dredge is protected, maintained and monitored and when all appropriate professional inputs are enlisted in any activities that affect the heritage value of the site.@ Finally that, Athe heritage character defining features of the dredge are respected and revealed, that is its appearance defined by its mass, surface materials and colour; and its structural framing, as determined by its function.@. This report also deals only with one component of the dredge, the bow gantry which with the digging ladder which it supported is the working face of the dredge. Since, the dismantling of the bow gantry the appearance and the heritage character of the dredge has certainly lost an important component. In order to understand the bow gantry behaviour, it is necessary to understand the original design intent of the bow gantry / digging ladder assembly, to understand how the dredge operations effected the forces in the bow gantry, struts and rigging, and to understand what changes in the balance of forces needs to be made for its present use as a cultural resource, ensure its long-term durability and the safety of its users. STRUCTURAL ANALYSIS It was decided to model the bow gantry as a half-frame since the applied loading is symmetrical as is the resisting frame. The model includes the full length of half the standing rigging and a single strut. A commercial structural analysis computer program was used to analyze, first a two dimensional model and subsequently a three dimensional model. The model was loaded with all dead loads and then a live load at the tip representing the weight of the digging ladder and then increments of load up to full design load. These models also require estimates for material and section properties, dimensions and decisions on joint restraint. Dead and Live Loads Previous work provided an estimate of the dead load of many of the components of the bow gantry. The weight of the digging ladder was found in a search of 1940 drawings. Also the full design live loads for the running rigging were found and compared to breaking load of the running rigging wire rope and factor of safety of 4. There was close agreement. The density of wooden elements was increased to account for an increase in moisture content. A value of 30% was assumed. Many parts are laminated with split ring connectors and bolts thus increasing the apparent density further. A density of 6.4 kn/m 3 was used. Extra dead loads as concentrated loads at joints or uniformly distributed loads were applied to account for the weight of such things as scuffing plate, work platforms, gusset plates and ladders.

3 The running rigging maximum design load in 1940 was 1620 kn at 30 degrees to the vertical towards the bow. The force in the running rigging to lift the digging ladder was estimated to be 200 kn. This is, of course, without ice loading and without the buckets. Therefore it was decided to use increments of 200 kn in live load up to 1800 kn. The weight of the lower rigging block and the links were included in the dead load. Modelling the Standing Rigging The standing rigging consists of a pair of lines each consisting of 4-parts of galvanized 51mm-diameter wire rope. It was the most difficult to model because its stiffness depends upon the tension in it. A cable or rope supported at two roughly horizontal points is in pure tension only with no moment or bending capability. The tension is a function of the weight of the cable, the span and the sag in the cable. For the Dredge, the model considered has one support point fixed and the other moves with the deformation in the gantry frame under load. A cable system with too much sag in it is relatively easy to move at the support point and thus offers little resistance to movement. The axial stiffness of the rigging also varies dramatically with tension below a certain threshhold value. Above this value it is constant with tension so the rigging acts like a bar but made of 4 parts of wire rope. This is the range we chose for this model. If the tension in the rigging is to low then its stiffness is greatly reduced and most of the resistance would then have too be offered by the nearly parallel strut. And this resistance would result in large tensile forces in the strut. Clearly something to be avoided in a laminated wooden element. The theory was advanced that the standing rigging was intended to precompress the strut in order to do a number of things. First, it would increase the stiffness of the standing rigging attracting more of the load, Second it would place the strut in compression over most of the load range. Third, in any shock loading event the strut would dampen the vibrations and the strut could even buckle safely. It was further speculated that the amount of precompression in the strut was such that the strut was placed in the maximum compression allowed for the member. Computer Model The first model was a simple two dimensional model of half of the bow gantry with one strut and a four-part standing rigging but there were concerns that not enough was known about the load distributions in the post framing to make reasonable assumptions, so that it was decided to do a three dimensional analysis. The model is shown in Figure 3. The pretension in the standing rigging and the precompression in the strut was modelled by successive choices of settlement of the end joint of the standing rigging element in two directions so that the slope of the standing rigging would be maintained and the strut would have a force close to maximum allowed by the CSA Standard O86. The factored compressive resistance of the strut parallel to the grain was found to be 580 kn. The standing rigging was modelled as a pin-connected truss member with a modulus of elasticity, E, adjusted to reflect the estimated EA of 4 parts of 51mm diameter wire rope where the true steel area was used. This is valid if the tension in the rigging is high enough to cause a small sag in the line. RESULTS AND DISCUSSION Figure 4 shows the results of the structural analysis as a plot of forces in key members against load combination. It can be seen that about 500 kn of tension was required in the standing rigging to get a compression of 580 kn in the strut. How this might be achieved in practice is that the running rigging would lift the complete and heavily loaded digging ladder before the strut was completely installed. In this configuration, the main post has its maximum bending force when it is at its minimum axial force while at design load the reverse is true. A member with combined bending and compression, according to O86 must conform to the following interaction equation: (P a / P r ) + (M a / M r ) < 1.0 Where, P a is the actual axial force and P r is the maximum axial resistance allowed and M a is the actual moment and M r is the maximum moment allowed.

4 These values were calculated for the main post at the strut to be.8 at load case 1 and 0.75 at a design load of 1620 kn. This value was a minimum at load case 7 with a value of.39. In the same way, the forces in the strut as well as the shear and flexural in the main post at the strut are all near zero when the running rigging load is about 1200 kn. The design load of 1620 kn also includes ice loading, so that 1200 kn level might well represent normal operating conditions for the dredge. The curious placement of the top most diagonal ties on only one side of the gantry frame can also be explained by this analysis. The ties provide a counteractive moment to the upper main post where the moment under no running rigging load is at a maximum. This component was not included in the model. It was also noted that one of these ties was bent, probably dating from the period when the dredge was buried in the silt. The other tie may have still been active and thus would have caused some twisting of the frame consistent with its failure mode. In a similar vein, the middle horizontal beam was found to experience a fair amount of tension and this could explain the use of steel ties at all four corners of this member which connected the gusset plates. CONCLUSIONS AND RECOMMENDATIONS The gantry failed after 1993 due to a combination of circumstances. The load in the strut increased when the digging ladder load was removed and the strut was already in a weakened state due to decay. The bending moment in the upper main post also increased. While the analysis here was based on the assumption that the outer top diagonal was pin connected and therefore could not transfer moments this may not be totally true. A small moment in the knuckle joint along with the presence of decay may have caused the failure here. A high level of prestress in the standing rigging and strut was an advantage during the operation of digging for gold but is no longer justified, in fact, becomes an unacceptable risk in the presentation of this resource. Following the original design approach the standing rigging should only have as much tension so that the strut is near zero stress. This was determined to be about 120 kn for the 4-part line. The sag in the line would be about 230 mm (9 in.) In order to rebuild the bow gantry and restore a key element of this important cultural resource the following options are recommended: Option #1: Increase the sag in the standing rigging by including a link or extension at the end of the rigging. Option #2: Decrease the length of the new struts and allow the gantry an angle of repose that is closer to vertical than it was when first constructed, probably close to what it was just before dismantling. Option # 3: A combination of the above. In addition, based on the results of this analysis the following are also highly recommended : 1. The running rigging should pick up the load of lower block, links and chain but not the weight of the ladder itself. 2. The bow gantry should be reconstructed in-place in a vertical position. The running rigging could then be attached with the lowest possible tension and then the gantry could be rotated to its resting position. Finally the struts should be cut to fit the distance and angle that results. 3. The aft gantry assembly should be analyzed in the similar way.

5 Figure 1. Dredge No Figure 2. Dredge No. 4 as rebuilt in 1940

6 Z Y X Figure 3. Computer model of half bow gantry, strut and standing rigging.

7 standing rigging 1000 Member Force [kn or kn.m] main post shear at strut main post flexural at strut strut 3 posts near deck virtually equal main post axial at strut LL# Figure 4. Member forces versus load combination where live load at tip is equal to (LL# - 1) x 200 kn