Examples of On-Building Pier Designs
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For most IN-roof or ON-roof installations, you will want a pier that is at least partly isolated from the observing floor and dome structural support. You can accomplish this in several ways.

The most straightforward isolated pier for an on-house installation is a vertical pier constructed independent of the floor, with the pier reaching from the base of the telescope into the ground to bedrock or a firm base as shown in Figure 6.6. Such a pier is sometimes very easy to fit into a building (e.g., down between the cars in a two car garage), but in other cases, requires either creative thinking or a changed set of values: Can your family accept a steel pipe down through the middle of the living room? A tall pier reaching to an upper floor may require cables or rods to stiffen it and help prevent sway or vibration.

If you do have a tall pier, such as needed to reach to a second floor, it will have a low frequency of vibration (often slower than once per second), a relatively high Q (perhaps ten or more), and a relatively unstable direction (small earth movements or temperature shifts tilt or bend the pipe). These characteristics can result in a long duration of oscillation lasting many seconds while the pier swings back and forth after you touch the scope. However, even with a low frequency of vibration, you may be able to achieve success if you provide appropriate damping, i.e., lower the Q. For example, one astronomer having a tall pier uses a circle of carpet surrounding the pipe. The carpet dampens the pier oscillation, yet does not transmit vertical movement of the floor to the pier. This is a very simple and elegant solution even though the whole installation violates most pier mythology.

Figure 6.6 DOME/PIER FOUNDATION SUPPORT DESIGNS

If you use a pipe for the pier, you can easily increase the rigidity and increase the frequency of vibration by filling the pipe with concrete. The stiffness of concrete more than offsets the larger weight which would cause slower vibration. A different approach is to fill the pipe with sand to increase the damping and to stiffen the pipe. In this solution, the sand grains rub together to dissipate the energy of vibration, thus reducing the Q of the pier. Both approaches seem to work well.

Rather than construct a pier completely separate from the foundation structure, another approach for on-house installations is to construct a pier that is only partially isolated from the house. A pier tied into the wall structure, but isolated from the floor holding the dome and astronomer, will be isolated from virtually all the personal movement (which is usually the major problem), while still providing a stiff support. Properly designed, it would be immune from all but actual tilts of the house and very slow weather related changes in dimensions of the house structure.

Let's assume your house has an observatory resting on a wood floor built of 2x10 joists. You can build a semi-isolated pier on a pair of 2x6s (or steel beams) supported at the walls of the room (i.e., tied into the house structural members forming the base of the wall). The 2x6s supporting your pier would extend across the middle of the room, but would be raised off the floor. If you used two pairs of 2x6s at right angles, then the pier would have stiffness against tilting in both East-west or North-south directions. Of course, 2x6s above the floor will introduce some real impediments for your astronomy! However, such a structure is entirely feasible if built below the ceiling in the room below the observatory, with the pier projecting up through a hole in the floor.

A more elegant solution is to take advantage of the floor joists being on 16 in. centers, giving space between them for a separate pier support as shown in Figure 6.7. This will let you work between the floor and the ceiling below. You can remove a portion of the floor to expose the lengths of two floor joists. Then insert two or more 2x8s or other members between the joists, allowing vertical clearance from both the observatory floor and the ceiling below. The pier support joists must be strongly cross braced, so that the pier cannot flex or twist easily side to side. For a stronger design, use steel structural members, instead of wood. Support the ends of the new beams at the walls. You can then replace all the flooring, except for a hole for the base of the pier. The result is an independent support for the pier, not directly affected by movements of the observer on the floor in the immediate area.

Figure 6.7 PIER WITHIN FLOOR STRUCTURE

When using these techniques, it may be desirable not to connect the pier beams rigidly into the wall structure. Rather, you might simply rest the beams on supports at each wall as shown in Figure 6.8, so that twisting of the house wall structural members will not cause the pier support beams to twist or tilt. Thus, even as the house "bends over" in the wind, the twisting of the wall will not (to first order) tilt or twist the pier beams.

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