IV. Analysis I Structural Truss Redesign

Transcription

1 IV. Analysis I Structural Truss Redesign Introduction The Stuckeman Family Building for Architecture and Landscape Architecture features many design aspects which distinguish it as a unique building. Included in these features is an entire façade composed entirely of copper cladding, long horizontal and vertical fins designed to block sunlight from entering the windows at peak times, and a four-story truss section that supports a thirty foot cantilevered section of the building. The three stories that are cantilevered contain studio spaces and an outdoor balcony area. The truss also supports a portion of the structure that features a missing column for the first two floors at the main building entrance. The loads for the cantilevered section and missing column are carried through cord members in the truss and distributed to a mini-pile foundation system. Problem Statement The current truss section utilizes over-sized cord members, columns, and diagonal truss members to support the associated loads. These members are costly to manufacture, ship to site, and difficult to erect. With the removal of the structural truss, the size of these members can be reduced, producing large savings in both manufacturing and erection costs. The connections associated with a truss of this magnitude also become overly expensive and labor intensive. The majority of the complex connections can be eliminated with the removal of the structural truss. The final driving factor in the elimination of the structural truss is the reduction of erection difficulty. Highly detailed connections and large members heighten the difficulty of erection. The complicated erection and use of temporary shoring towers add generously to the cost of the structural system. Large savings in cost and schedule can be experienced with the removal and redesign of the structural truss section for The Stuckeman Family Building. - Page 44 -

2 Solution/Methods The analysis for the elimination of the structural truss was achieved by starting the redesign at the roof of the building and working downward to the mini-pile foundations. The factored roof and floor loads were dictated in the drawings and were applied accordingly. Initially, columns and girders were added to the truss structure to create a simple, uniform structure. The structural members running perpendicular to the structural truss were left unchanged since they were assumed to only carry the roof and floor loads and not the truss loads. The floor/roof loads, façade, and existing beam weights were factored into the redesign of the girders, followed by the columns. This design protocol was followed for each level until the structure was complete to the ground level. Foundations and mini-piles were then added for the locations where columns had not previously existed. Assumptions Structural Cords The truss loads are assumed to be carried by only the girders and columns. All beams running perpendicular to the truss are assumed unchanged by the removal of the truss system. Connections The redesigned structure is assumed to be a simple, uniform structural system with simple, bolted connections. Columns The columns on the South side of the truss are assumed to be braced only at the second and fourth levels; whereas columns on the North side of the truss are assumed to be braced at the second, third, and fourth levels. Façade The weight of the façade is assumed to be distributed evenly through the height of the structure. This is a reasonable assumption since structural tubes that comprise the façade run the entire height of the building and tie back into the structure at each floor level. The weight of the façade is also assumed to be a proportional average weight of areas that have windows and areas that don t have windows. - Page 45 -

3 Member Sizes The member sizes chosen in the structural truss redesign are selected from other areas of the building that experience the exact situation in terms of loads, coverage area, and adjacent member sizes. The selected member sizes are then run through brief structural calculations to ensure they are suitable for the intended location. Also in consideration for selecting member sizes is the fabrication. Fabrication of the structural steel members is accelerated by the specification of common member sizes; therefore, girder and column sizes were selected to be similar to adjacent sizes where applicable. Estimate The redesign estimate is assumed to be a rough estimate of the cost and schedule impacts of the structural truss elimination. Structural steel costs, connection costs, and labor impacts are all estimated to give a general idea of the magnitude of cost and schedule changes incurred by this redesign. Calculations Since the spanning beams were assumed to remain unchanged, the redesign of the structural truss section began at the roof level with the removal and redesign of the structural cord girders. Roof loads, as well as, the copper façade and windows were factored into the load for these girders and designed accordingly. This process was then repeated for the fourth and second floors, using floor loads, copper façade and window loads, and imposed loads from nearby sections of the building. The drawings below indicate the girders that were revised in the redesign, as well as, their previous and new sizes. - Page 46 -

4 High Roof Plan Fourth Level Plan - Page 47 -

5 Second Level Plan After the girder sizes were established on all levels, columns were then redesigned. Using the imposed loads from the roof, floor levels, façade, and structural members, the columns were designed from the roof down to the foundations. The column lengths and splice points remained unchanged except for the areas where columns were added or extended to the ground level. Furthermore, column sizes were selected in the re-design to keep the most consistency of size to quicken the manufacturing process. All diagonal truss members, excluding the two end members, were deleted in the redesign. These members provided a substantial cost savings in manufacturing and erection. The following drawing shows the diagonal truss members that were removed from the structural truss section of the building, as well as the columns that were redesigned or added. - Page 48 -

6 Truss Elevation Results Steel Tonnage Reduction With the removal of the truss system in the structural redesign, the total tonnage for the project was reduced from tons to tons. The ton decrease represents a 7.5 percent reduction in the overall structural steel tonnage. In the truss area affected by the redesign the total steel tonnage decreased from tons to tons, which is a 34.2 percent steel reduction. Additionally, the elimination of detailed structural connections can produce a savings of approximately 10% of the overall steel tonnage in the affected area according to Mark Rossi of Ohio Steel Industries. This reduces an additional tons of structural steel to achieve the new building total of tons. Structural Members Original (tons) Re-Design (tons) Percent Decrease Entire Structure % Affected Area % Connections Original (tons) Re-Design (tons) Percent Decrease Entire Structure % Affected Area % - Page 49 -

7 Steel Erection Reduction Steel erection costs decreased significantly with the redesign of the structural system. Chuck McKee of Century Steel estimated that crane time would have been reduced by approximately 2 weeks with the removal of the truss from the structure. The crane and associated crew for this project were charged at a rate of $4500/day. This cost translated to a two week period produces a savings of approximately $45,000. In addition to the erection costs, additional savings would be experienced since temporary shoring equipment would not have needed to be used. The rental costs for the temporary shoring equipment was approximately $10,000 for the length of the steel erection. Foundation Additions The removal of the cantilever section of the building and the structural truss requires the addition of four extra columns, pile caps, and mini-pile clusters. The depths of the additional mini-pile clusters were estimated by taking the average of nearby clusters from mini-pile test reports. This depth of 38 feet was then used to estimate the added cost of $43,200 for the four extra columns, pile caps, and mini-pile clusters. Schedule Implications The elimination of highly-detailed and labor intensive connections, large structural members, and temporary shoring drastically affects the erection time for this section of the building. The reduction in labor was initially estimated as the elimination of two weeks of work for an erection crew and crane. This translates directly into an overall project schedule reduction of two weeks since the steel erection began at this area, and also, since the crane and erection crews are on the critical path. Two weeks of schedule reduction can save the general contractor thousands of dollars if the project was to fall behind; or if the schedule savings is passed on - Page 50 -

8 directly to the owner, can be worth thousands of dollars in incentives, occupancy schedule savings, and revenue costs. General Condition Implications Eliminating two weeks from the steel erection schedule eliminates the costs of two weeks of General Condition costs. In Section III, page 43 the General Conditions for the Stuckeman Family Building project were estimated to be approximately $3,270 per day. For the two weeks the project schedule was accelerated, the General Conditions savings is $32,700. This constitutes a General Conditions savings of approximately 3% compared to the General Conditions Estimate total on page 43 of this report. Cost Implications Cost Implications (Affected Area Only) $200, $150, $100, $50, $0.00 ($50,000.00) ($100,000.00) Base Condition General Conditions Connection Costs Temporary Shoring Erection Costs Added Foundations Steel Costs Cummulative Change - Page 51 -

9 From the subsections above, the cost reductions of the structural redesign can be totaled as follows: Steel Tonnage: tons X $2500/ton*= $96,550 Connections: tons X $650/ton* = $7,332 Steel Erection: $4,500/day X 10 days = $45,000 Temporary Shoring: (Lump Sum) = $10,000 General Conditions: $3,270/day X 10 days = $32,700 Additional Foundations: $10,800/fnd. X 4 fnds. = ($43,200) Total Cost Reduction: $148,382 * The structural steel cost of $2500 per ton was derived from the RS Means average as explained in the Detailed Systems Estimate on Page 41. This cost includes materials, equipment, and labor. The cost of $650 per ton was selected from current steel costs since the labor and equipment costs are broken out separately in the breakdown. This total can then be compared to the actual and estimated costs for the structural system in the previous sections of this report. The actual amount of the structural package for the Stuckeman Family Building was approximately $4,000,000, which is similar to the assemblies estimate total of $4,248,973. However, both of these estimates are for a complete structural package, including steel, concrete, foundations, mini-piles, and non-structural steel members. The D4 Building Estimate produced an amount of $1,596,757 for Metals which includes structural and nonstructural steel components, as well as, miscellaneous metal fabrications. Thus, the cost reduction of the structural truss redesign should be compared to the Detailed System Estimate for the structural steel system, which generated a total of $1,294,643. Comparing this estimate to the value engineering savings of $148,382 displays a total cost reduction of approximately 11.5 percent of the total structural steel costs. - Page 52 -

10 Visual Implications (renderings) - Before After - Page 53 -

11 Conclusions The cost analysis for the removal of the truss may have not produced immense cost reductions as expected; but it is believed that the reductions in budget would be greater than calculated in this analysis if actual costs were tracked in the reallife application. Erection labor costs and manufacturing reductions were estimated as closely as possible, but were by no means exact. The reduction in steel tonnage may also not have been as significant as expected. This is mainly due to the fact that four additional columns were added to allow the complete removal of the structural truss. The addition of four extra columns, foundations, and costly mini-pile clusters seems to offset the impact of the value engineering results. This analysis did prove, however, that the cantilevered section of the building could have been removed from the original design without drastically affecting the aesthetics or functionality of the building. Reductions in the budget, as displayed on the chart below, would be gladly accepted by any Owner for the minimal changes that the aesthetical appearance experiences; therefore, the final recommendation would be to have pursued the change in design prior to the commencement of construction. Budget Reduction from Re-Design Percent Change Structural Steel Only $1,294,643 $148, % Structural System $4,000,000 $148, % Overall Building (D4 Cost) $22,468,021 $148, % Overall Building (Actual) $22,500,000 $148, % - Page 54 -