DESIGN AND CONSTRUCTION OF HEAVY INDUSTRIAL ANCHORAGE FOR POWER-PLANTS

Transcription

1 DESIGN AND CONSTRUCTION OF HEAVY INDUSTRIAL ANCHORAGE FOR POWER-PLANTS Peter J. Carrato, William F. Brittle Bechtel Power Corporation, USA Abstract Fossil fueled power-plant projects provide many design and construction challenges for connection of steel to concrete. Heavily loaded anchorages are required to support building structures and hold down machinery and equipment. Column bases for coal fired boiler support structures can transmit loads in the order of 4000 kn of tension and 2000 kn of shear. This magnitude of tension load requires groups of high strength bolts 100mm and larger in diameter. Shear loads are resisted using hot rolled structural shapes or heavy plate (up to 50mm thick) as shear lugs. Power-plant equipment such as turbo-generators, condensers, stacks, fans and pumps require precision placement of anchor bolts and shear lugs, often in the vicinity of congested reinforcing steel and embedded pipe and conduit. Construction consideration for column bases and equipment foundations often have a significant impact on the structural design. 1. Introduction Construction of a power generating facility involves a variety of applications of anchorage to concrete. Literally truckloads of anchor bolts are used in the construction of these facilities as can be seen in Figure 1. This paper describes some of the wide variety of connection to concrete found in a typical, natural gas fired power plant. Figure 1 - Truckload of bolts 491

2 Special emphasis is given to seismic resistant applications from projects being constructed in Taiwan, Turkey and California, as well as, high precision anchorage of turbine generator sets. In a fossil fuel fired power plant, most connections to concrete are made in cast in place foundations. These connections can be categorized into those that support structures and those that anchor equipment. Structural support include typical column base plates for turbine halls (Figure 2), pipe racks, water treatment buildings, etc, and more exotic bases, some of which allowing for thermal expansion at columns that support heat recovery steam generator (HRSG) boilers (Figure 3). Anchoring of tanks for fuel, water, and chemicals presents a unique structural application due to the large numbers of bolts required for a single foundation. Figure 4 shows a template used to set a stack. Figure 2 Turbine Hall Figure 3 HSRG Figure 4 Stack Equipment anchorage is characterized by the need to resist dynamic loads using precisely located fasteners. Rotating equipment such as turbine generators, pumps and fans may require pre-loading anchor bolts to meet requirements given by their 492

3 manufacturer. Non-rotating equipment like condensers and valves can present unique applications due to transient or operating conditions. 2. Machinery Foundation Turbine Generators The turbine generators (T/G) are those pieces of equipment, which turn mechanical energy into electricity. It is often the most expensive component part of a power plant. As such, the anchorage of this equipment is given special attention. Anchoring devices used for turbine generators include not only those to secure the equipment in its final locations, but also those required to precisely position the machine. It is common to see anchor bolts 100 mm in diameter that must be placed with tolerances as tight as ± 3 mm to ± 1 mm. Critical design considerations for these applications include a variety of static and dynamic loads. The most significant design loads are those due to postulated machine malfunctions. Specifically the loss of a turbine blade or the short circuit of a generator can produce anchor bolt loads as high as 500 kn. The structural design of T/G anchorage also requires careful consideration of the thermal growth experienced by the machine as it heats up during normal operation. It is common for the temperature of the turbine casing to increase by 200 C as the machine goes from cold shut down to full operation. Some of the larger loads transferred to a T/G foundation are those associated with aligning the machines. The shafts of the turbine and generator must be in perfect alignment. Embedded structural steel profiles (hot rolled shapes) called jacking posts are used as reaction points for hydraulic jacks used to position the machines in a horizontal plane as shown in Figure 5. Jacking post forces are often as high as 150 kn. Figure 5 Jacking Posts A number of different techniques are used for vertical alignment. These include grout pads and shims, jacking bolts and shims, and patented positioning devices. The choice of vertical alignment method depends on weight of the machine and the experience of the contractor performing Figure 6 Grout Pad 493

4 the installation. In the grout pad method (Figure 6), small pillars of grout are formed on which shims may be set, if needed, to achieve the final vertical position of the machine. The base plate is then set. Jacking bolts are threaded into the base plate and thus allow the plated Figure 7 Jacking Bolt position to be adjusted by turning the bolt. This is illustrated in Figure 7. Once the final location is achieved, the plate is shimmed and then grouted. Patented positioning devices consist of small screw or hydraulic jacks that are placed in a pocket in the concrete foundation. After the jacks have positioned the machine the device is grouted in place. Figure 8 shows a Fixator brand of positioning devices being used to set a generator. To position and anchor a T/G set while allowing for thermal growth, requires a wide variety of anchoring devices. Jacking posts, positioning devices and anchor bolts have already been mentioned. In addition to these fasteners, sole plates, center line guides, and stop blocks are also used. A sole plate is an embedded bearing plate on which the machine rests and/or slides during thermal growth. Center line guides are positioned along the shaft of the machine and control the direction of thermal expansion. Stop blocks are Figure 8 Positioning Device employed to limit the extent of thermal growth. These steel blocks are precisely positioned at the end of sole plates and restrain the machine after a predetermined amount of movement. Holding down a turbine or generator often requires 20 or more anchor bolts. To accurately cast this number of bolts into a concrete foundation typically requires one of two possible construction methods. Either the bolts must be designed so that their position may be adjusted after the concrete has set or the entire bolt group must be placed using a template that assures their precise location. Use of through bolts or an adjustable sleeve, will allow bolt positions to be adjusted after placing concrete. For 494

5 elevated concrete decks, through bolts in oversized holes are recommended. When access to the underside of the slab is not available then an adjustable sleeve and pocket arrangement may be used. Both of these applications, shown in Figure 9; allow the bolts to be tensioned after setting the machine. When the anchor bolts are not to be preloaded after setting the machine a template may be used to position the bolt group. Templates are typically made from structural steel. The template should be fully welded or bolted together before drilling holes for the anchor bolts. The holes should only be 2 mm larger than the diameter of the bolts. Templates should be firmly supported preferably from existing concrete. It is not advisable to support a template from concrete formwork or rebar as the forms or rebar may shift during placement and consolidation of the concrete. Templates are intended to fix the location of the top of the anchor Figure 9 Bolt Sleeves bolts. For long bolts the lower portion of the bolt should be tied to the reinforcing steel thus assuring that the bolts will plumb. 3. Structural Anchors Structural anchors in power generating facilities present many challenges ranging from fastening a one half horse power pump to a 200 mm thick concrete pad to anchoring a column carrying 5000 kn of force to a 2 meter thick slab. Each of these various applications has its own unique concerns with designing the anchorage, positioning the bolts, installing the base plate and grouting the final assembly. By far, the most interesting designs are those associated with a combination large shear and uplift forces. In high seismic zones such as those found in Turkey, Taiwan and California large lateral loads must be transmitted from the superstructure to the foundation. Often these high lateral seismic loads can produce overturning moments on the structure that create uplift 495

6 at these anchorage points. Connections such as these that must resist both uplift and large lateral forces are the most challenging to design. In these cases, an arrangement of anchor bolts designed to resist tension only and shear lugs designed to resist lateral loads is required. Figure 10 Small Shear Lug Figures 10 and 11 show column base plates designed for such loads. Figure 10 shows a shear lug consisting a single 18 mm thick plate that is designed to resist moderate levels of lateral load (75 to 10 kn). Figure 11 shows an H shaped shear lug made of 50 mm thick plates. This lug is designed to resist significant levels of shear, up to 400 kn Figure 11 Large Shear Lug 496

7 Another practical way to resist large lateral loads is to embed the lower portion of the column shaft into a pocket in the foundation. This method, shown in Figure 12, works well in thick mat foundations such as those often used in power plant structures. There is little reference material that can be used to design a shear lug such as that shown in Figure 11. The design process should specifically consider the following 1) the strength of the lug, 2) the stiffness of the lug relative to the surrounding concrete and 3) the connection of the lug to the base Figure 12 Column Pocket plate. Of these three items connection of the lug to the base plate is the least understood and has the greatest impact on the cost of the anchorage. There are two design approaches used for this welded connection. The simplest and most straight forward considers the weld to take only the shear load transferred from the base plate to the lug. This results in very economical connection typically using a fillet weld. The other design approach is to assume that the bearing pressure applied to the face of the lug results in a bending moment on the weld of the lug to the base plate. This assumption invariably leads to a full penetration weld of the lug to the base plate. Such a weld applied to a 50 mm thick lug may increase the cost of the column-base plate assembly by as much as 50%. A well documented, definitive design method for large capacity shear lugs would be beneficial to design engineers. 4. Conclusion Design and construction of power plant facilities provide a wide variety of connections between steel and concrete. This paper has discussed a number of applications associated with anchoring structures and equipment. It is important to recognize that construction issues greatly influence the final design in many cases. Precision placement of anchor bolts and shear lugs, as well as provisions for grouting the final installation often require more engineering activity than the structural design of the connection. 497