Analysis 2: Precast Panel Feasibility Introduction

The existing structural façade is comprised of site-cast and tilted concrete panels that presented aesthetic and logistical challenges for the design team. Difficulties with color consistency and honeycombing led to costly repair work and added coordination time to an already compressed schedule. At significant cost, a specialized concrete paint was eventually selected to coat each panel. Controlled manufacturing and quality control procedures are advantages of precast over comparable tilt-up construction. An alternative factory precast system will be considered for feasibility and to address these specific challenges.

Precast Module Selection

Figure 2.1 Current Tilt-Up Elevation

Precasting plants and facilities are most efficient when producing large quantities of similar units. The individual units or modules should be convenient for the manufacturer, taking into account the casting-bed dimensions, material handling requirements, transportation and onsite erection. Therefore, the original panel design (Figure 2.1) must be divided into smaller and more manageable modules to achieve economies of scale and meet transportation regulations for interstate highways and roads. Specific recommendations for panelizing structural façades are provided by the Tilt-Up Concrete Association. The High Concrete Group, a successful Pennsylvania precaster, also provided consultation and guidance in developing the modules for a feasible precast system.

• Each modular unit should utilize the economies of factory produced formwork by focusing on obtaining the maximum form reuses and keeping the number of shapes to a minimum. Wall panels are typically cast in fiberglass or concrete forms and average around 20 reuses with minimal rework between pours. In the final panel design, sharp edges or corners should be avoided and replaced with chamfered edges to prevent possible edge damage during handling.

• Structurally, modular units should avoid the positioning of window and door openings directly at the panel edges. Construction tolerances between adjacent frames are not always adequate in accommodating the opening and may lead to visible cracking or damage during

28

handling. Therefore, a minimum of 18-inches of solid panel is recommended between all openings and panel edges for structural stability during handling and erection.

• Transportation from the factory to the jobsite is another factor in deciding the shape and dimensions of modular units. According to the Code of Virginia, the maximum allowable gross weight for semi-trailers is 40-tons without the need for additional permitting or escort vehicles. If a moderately sized 45-foot tractor-trailer truck and flatbed are approximately 20-tons, the load from the precast panels cannot exceed 20-tons per shipment. Additionally, the code specifies a maximum height of 14-feet for the vehicle, as measured from the road to the tip of the load. The vehicle width is restricted to 8-feet, which allows safe transit within lanes on highways and local roadways. Special permits from the State of Virginia can be obtained for $12 per delivery to increase the allowable width up to 14-feet without requiring a front escort. Therefore, the ideal modular units should not exceed 12 x 14-feet in the width or height directions.

• The erection speed is primarily determined by the number of lifts and the geometry of the panels. In precast construction, lifting a 20-ton panel consumes only slightly more crane time than placing a 10-ton unit. For erection, precast will be most cost effective when the number of modules is at a minimum. However, as a general rule, the crane capacity should be roughly double the weight of the heaviest panel or module. Therefore, a balance must be established between the panels and required crane size. In terms of panel geometry and lifting speed, L-shaped modules should be avoided, since they usually require strongbacks and involve longer setting times.

Figure 2.2 Proposed Modular Elevation

After understanding the various manufacturing and erection limits, the original tilt-up panel design was divided into the modules shown in Figure 2.2. The diagram represents a typical elevation for one face of the hexagonally shaped building. Although each elevation has some variation, the formwork can easily be modified quickly and inexpensively between pours to accommodate the original aesthetic design. For reference, summaries of the maximum module dimensions and weights are summarized in Table 2.1. While Unit 2 exceeds the 12-foot width limit for transportation, consultants with the High Concrete Group recommended against dividing the panel. Therefore, deliveries for Unit 2 will require special permitting from the State of Virginia.

29

Geometry Modular

Designation

Maximum

Length

Maximum

Width

Area

(ft2) Weight

(Ton) (1) (2)

Unit 1 18’-0” 4’-8” 86 4.8

Unit 2 25’-5” 13’-2” 197 11.1

Unit 3 18’-0” 9’-6” 171 9.6

Unit 4 16’-1” 2’-0” 32 1.8

Notes: (1) Specific Weight of Reinforced Concrete = 150 lb/ft3 (2) Panel Depth = 8”

Table 2.1 Modular Schedule

Additional Design Considerations

Due to complex load paths caused by irregularly shaped panels, the final design will be complicated and relies on a close relationship between the structural engineer and the precast manufacturer. The precaster will develop detailed shop drawings and run calculations to assure that the panels will be properly handled during shipping and erection. Concrete Precast Systems, a precast plant operating 15-miles from the jobsite, specializes in custom architectural panels and would be a promising candidate for subcontracting. Before beginning the redesign, the precaster would require the anticipated service loading conditions, as summarized in Table 2.2. The load will be distributed across the top of Unit 2 and passed to Units 1 and 3 through a direct bearing connection. After reviewing the loading requirements and considering the unrestrained wall height, the High Concrete Group suggested a panel thickness of 8-inches.

30

Description Distributed Load (lb/ft2) Load (lb) (1) Long-Span Joists 10 5,600 Metal Decking 4 2,240 Roofing & Insulation 3 1,680 Sprinklers 8 4,480 Mechanical & Misc. 20 11,200 Live Load 20 11,200 Σ 65 36,400

Notes: (1) Tributary Area = 560 ft2

Table 2.2 Service Loads per Panel

The High Concrete Group recommended grouted and welded connections to join each modular unit. Direct load-bearing connections are managed by embedding dowels and corrugated metal tubes that become patched with high-strength grout in the field. The dowels must be designed to allow adequate development within the panels being joined, which creates a ductile connection and resists moments created from the service loading condition. Welded connections are used primarily for alignment purposes and to join panels that do not carry substantial loads. Steel rods and plates must be embedded into the panels during the precasting process, while field fillet welds are made to secure the final connection. Both connection types are relatively hidden and not visible from the exterior. Caulking joints are installed along each connection line to allow freeze-thaw expansion and to provide a watertight barrier. Figure 2.3 illustrates the locations and typical construction details for both types of connections.

31

Figure 2.3 Proposed Connection Details

Preliminary steel reinforcement layouts were derived from the original tilt-up panel design. Since the façade will not be designed as a shear wall, the minimum steel ratios are 0.0125 in the longitudinal direction and 0.002 in the lateral direction. Figure 2.4 shows a preliminary rebar layout, using the initial tilt-up design as a template. A single layer of reinforcement is specified 2-inches from all edges to control cracking from shrinkage and temperature changes. Horizontal ties are identified as typical in all units, which offer lateral support and maintain the alignment of vertical reinforcement. The final design and rebar selection will be performed by the structural engineer and precast manufacturer to maximize economy.

32

Figure 2.4 Preliminary Rebar Layout

33

Erection Plan

Figure 2.5 Erection Sequence

Based on the module weights and anticipated lifts, a 150-ton crawler crane provides adequate lifting capacity and minimal mobilization. The crane should move clockwise around the building perimeter, stopping at each corner to erect half of the panels for the adjacent walls. The corners are logical locations for the crane, since the 120º angle between the exterior walls allows for reduced swing times and faster erection. Once deployed at a corner, the crane will begin placing the wall panels, starting from the furthest lift and working backwards towards the crane operator. The first completed corner follows the sequence outlined in Figure 2.5. As shown in Figure 2.6, the crane follows a similar procedure at each subsequent corner, fully completing one building side and half of its adjacent wall. Based on this construction sequence, the critical crane lift is a panel module weighing 11-tons and placed at a distance of 75-feet. A typical 150-ton crawler crane has a maximum lifting capacity of 16-tons at 80-feet and is recommended for the erection of the precast modules.

1

4

5

3

2 7

6

8

34

Figure 2.6 Erection Sequence

System Comparison

Description Quantity Unit Unit Cost Total Concrete, 5000 psi, colored 312 CY $155.00 $48,360Reinforcing, #4 to #7 54,000 LB $0.50 $27,000Formwork, 20 uses 1,190 SF $80.00 $95,200Transportation to Site(1) 33 EA $850.00 $28,050TOTAL FABRICATION $198,610

Non-Union Erection(1) 96 EA $900.00 $86,400¾” Field Joints(1) 570 LF $2.00 $1,140Grout Connection(1) 60 EA $36.00 $2,160Weld Connection(1) 84 EA $15.00 $1,260Crawler Crane 2 WK $8,000.00 $16,000TOTAL INSTALLATION $106,960

TOTAL PRECAST $305,570Notes: (1) Cost Data Provided by High Concrete Group

Table 2.3 Estimated Cost of Precast System

16

15

14

13

12 11

10

9

35

Estimates of the proposed load-bearing precast system reveal slight cost advantages over the current tilt-up panels. From the cost data in Table 2.3, the precast system totaled approximately $305,000 for materials, fabrication and erection. Comparatively, the tilt-up system was previously estimated at $277,000, a difference of $28,000 in favor of the site cast panels. However, at the request of the architect, a specialized concrete paint (manufactured by the Tnemec Company) was specified to correct inconsistencies in color and appearance. By requiring the expensive coating and additional labor, the cost of the panels increased to $356,000, making the precast system a more attractive option.

While the cost of manufacturing and erecting precast is more expensive than comparable tilt-up construction, achieving the desired architectural appearance adds significant expense. Tilt panels represent an economic structural alternative that balance quick construction with low maintenance and exceptional durability. However, the addition of colors, textured form liners, and rustication strips exceeds the practical limits of onsite casting. Construction sites lack the strict quality management system and controlled environment found within precast manufacturing plants. If the architect or owner desires a high-profile façade with an emphasis on homogeneity, a precast system provides the best value. Tilt-up becomes most economical when small surface defects or slight discolorations are more acceptable, as in manufacturing or light commercial construction.

Description Cost

Tilt-Panel Materials & Erection $277,000Tnemec Paint $79,000TOTAL TILT-UP COST $356,000 Precast Materials & Erection $277,520Transportation $28,050TOTAL PRECAST COST $305,570

SAVINGS $50,430

Table 2.4 System Cost Comparison

In addition to slight cost savings over tilt-up, the precast system offers more significant schedule reduction. Based on consultations with the High Concrete Group, a non-union labor crew can erect 14-precast pieces in a single day. Assuming continuous panel deliveries, the entire building perimeter could be placed and installed within 7-days. After adding 1-week for mobilization and equipment setup, the schedule impact for the proposed precast system is only 12-days. This duration is precisely half of the 24-day schedule required for the tilt-up system, which requires the preparation of casting beds, pouring and curing of concrete, and erection of the panels. Figures 2.7 and 2.8 illustrate the differences in schedule for each system. While the precast panels save 12-days in construction, the overall impact to the project completion date is a reduction of only 7-days.

36

Figure 2.7 Tilt-Up Panel Schedule Impact

Figure 2.8 Precast Panel Schedule Impact

By tightening the overall schedule by 7-days, the owner experiences a savings of approximately $14,000 in reduced general conditions. Table 2.5 summarizes the cost reduction and separates project management from site condition fees. While not an overly impressive savings, the reduction in coordination and management for the panel corrective work adds an additional intangible value to the project.

37

Base Building Base Category/Activity Qty Unit Rate Cost

Project Management Vice President Senior Project Manager 1.4 Wks $1,900 $2,660Project Manager 1.4 Wks $1,250 $1,750Asst. Project Manager 1.4 Wks $475 $665Superintendent 1.4 Wks $1,800 $2,520Asst. Superintendent 1.4 Wks $550 $770 Total Project Management $8,365 Site Conditions Jobsite Office 0.35 Mo $1,150 $403Copier/Fax/PC/Office Equipment 0.35 Mo $2,300 $805Field Telephone 0.35 Mo $500 $175Temporary Power 0.35 Mo $375 $132Temporary Water 0.35 Mo $60 $21DSL Line Setup & Monthly 0.35 Mo $275 $97Cellular Phones 0.35 Mo $540 $189Small Tools, Equipment, Supplies 0.35 Mo $850 $298Local Travel 1.4 Wk $940 $1316Courier Service 1.4 Wk $330 $462Progress Photos 0.35 Mo $110 $39Sanitary Facilities 1.4 Wk $290 $406Trash Removal 1.4 Wk $570 $798Daily Clean-up 1.4 Wk $455 $637 Total Site Conditions $5,778

GRAND TOTAL $14,143

Table 2.5 General Conditions Savings from 7-day Schedule Reduction

Recommendation Based on preliminary design research and cost estimates, selecting a precast panel system over tilt-up is feasible and economical. While less expensive to construct, tilt-up panels do not offer the high-quality surface finishes desired by the owner and often require corrective work after erection. The installation time associated with precast is also roughly half that of the construction and erection of tilt-up, which reduces the overall schedule and general conditions costs. However, precast panels have longer lead times and require completed designs 8-weeks before onsite installation. Precast systems are not recommended for every project, but should be considered for buildings requiring fast-tracked construction or high-profile facades.

38