Structural Breadth Study

Transcription

1 Structural Breadth Study Connection Redesign

2 Structural Breadth Study Connection Redesign Existing EIFS Connection The expansion project is a steel frame structure with concrete slab on deck. There is a total of 2200 tons of steel to be erected on the building over a scheduled duration of 22 weeks. A tower crane was placed in the center of the building, inside the main atrium. This was the most logical location for the crane, as it is mostly open space. The budgeted amount for the steel contract is $6.03 million. One important aspect that had to be considered by the designer was how the Exterior Insulation Finish System (EIFS) was going to be attached to the structure of the building. EIFS is used for most of the exterior finish of the hospital. The majority of the EIFS panels are being installed in 27 or 28 sections. The panels are attached to 6 steel studs with mechanical fasteners. Figure 5 below shows a detail of the attachment of the EIFS panels to the structure of the building. The existing detail contains two sets of double channels connected to two 5/16 plates. These plates are bolted to the double channels with a total of six bolts and welded to the W14 beam and the tube steel. The entire assembly was created onsite by the steel erectors. Figure 5: Existing Connection Design There are several disadvantages to constructing this detail. The two sets of double channels are excessive and more difficult to install; two pieces take longer to erect than one. The two 5/16 plates must be welded on the top and the bottom of the channels, which takes a lot of time. The connection is located below the 25 of 97

3 concrete floor and must be accessed from the floor underneath. This would require ladders or lifts, which are inconvenient for the erectors and welders. This connection is inefficient in terms of erection time and labor costs. The connection typically occurs every 27 around the perimeter of the building on several floors. There are 200 occurrences of this detail. The steel erection of the building took 22 weeks to complete, per contract. However, an additional 16 weeks of work was required to complete miscellaneous steel bolt up, welding, and bracing. Proposed EIFS Connection In order to improve the efficiency of the steel erection process, an alternative to this detail will be examined. The proposed design aims to reduce erection time by reducing the number of pieces and the amount of welding. A detail of the proposed connection design is shown in figure 6. The main difference between the existing detail and the proposed detail is the number of pieces in the connection. While the existing included 6 pieces to be installed between the W shape and the tube steel, the proposed design only requires 4. This makes a significant difference in terms of erection time, given that there are 200 of these connections. Another significant difference between the designs is the connections of the pieces. The proposed detail requires less welding than the original and fewer field installed bolts. Normally, fastening bolts takes less time than welding these pieces together. Figure 6: Proposed Connection Design In addition to reducing the number of members, the proposed design will have Angle A, Angle B, and the 5/16 plate prefabricated in the shop prior to delivery. Prefabricating these pieces will significantly reduce the erection time of this process. It will also save money in labor costs for welders and steel erectors. There will be an additional cost for prefabrication and delivery, as they will be more difficult to deliver prefabricated pieces than the individual pieces. 26 of 97

4 The alternative design allows for the same amount of adjustment available in the original. In fact, the connection would be easier to adjust, as there are fewer pieces and fewer connections. The dimensions of the pieces do not change from one design to the other; the only aspect that has changed is the shapes of the pieces. There is no reduction in the structural integrity of the connection. The design calls for ASTM A 325 bolts. The bolts are to be used to hold the pieces in place and to allow for any adjustments to be made. Once the prefabricated pieces are in place, the pieces would be welded in their final position. Below is a comparison of the connections. Full images of the connection can be found in Appendix D. Analysis Results Figure 7: Existing and Proposed Steel Connection A cost analysis was performed to determine the advantages of the proposed design. The majority of the cost savings are in labor, as the proposed design takes much less time to erect. A total of $566 was saved off the budget, along with 6 days on the duration of this activity. The results of the analysis are in figure 8, below. A more detailed analysis can be found in Appendices E and F. 27 of 97

5 Cost Labor + Material + Delivery + Prefab Existing: $5,130 + $8,558 + $389 + N/A = $14,077 Proposed: $1,710 + $7,591 + $1,035 + $3,174 = $13,511 Schedule Cost Savings: $566 Existing: Proposed: 9 days 3 days Schedule Savings: Figure 8: Summary of Proposed Design Savings 6 days The proposed change does not yield significant cost savings. While the erection labor cost was reduced, the added prefabrication costs negate any cost savings. The cost for material was cheaper in the proposed design because fewer pieces of steel were used. The delivery cost of the alternative design was greater than the existing condition, as the pieces would have to be shipped prefabricated, reducing the amount of steel each truck could hold. A value of $75/ton was used to calculate the delivery cost, as opposed to the existing $25/ton. The main benefit of this change is the reduction in the schedule by 6 days. The prefabrication of the pieces reduced the required labor time in the original design. The prefabrication time should not affect the erection time, as they are performed independently. Another example of saved time occurs during the physical installation. The original design would require a considerable amount of layout and coordination, as there are more pieces to be installed. The prefabricated unit would be easier to install and requires less preparatory work. Connection Redesign Conclusion The analysis of the alternative design shows a slight cost savings and a reduction of 6 days from the steel erection schedule. Given the design s restraints, it would be beneficial for the contractor to install this design as opposed to the original design. Assuming the prefabricator can deliver the pieces when needed, it would be beneficial for both the management team and the steel erector. 28 of 97