Concrete Foundation Anchor Bolts Design & Selection:

 

An anchor bolt is used to attach objects or structures to concrete. There are many types of anchor bolts, consisting of designs that are mostly proprietary to the manufacturing companies. All consist of a threaded end, to which a nut and washer can be attached for the external load. Anchor bolts are extensively used on all types of projects, from standard buildings to dams and nuclear power plants. They can also be used to firmly affix embed plates to a concrete foundation when used with a structural steel element.

 

The simplest anchor bolt is a cast-in-place anchor. As seen in the figure 1, most designs consist of a standard bolt with a hexagonal head, which is cast in the wet concrete before it sets. There are other designs, some consisting of a bent bolt with a hook on the end, or some other sort of bending. Cast-in-place anchor bolts are the strongest type of fastener, but the casting is difficult, and they are usually only used for heavy machines mounted on poured concrete floors. Another use of this anchor bolt is to connect the concrete foundation of a building to its wall. With this, the building is more resistant to earthquakes. Currently there are several devices to assist in holding and in placing anchor bolt to set in the concrete. These devices mostly made from composite plastic. Once the concrete has been poured and set, the only other types of bolts that can be used are mechanical and epoxy bolts. Epoxy bolts are the strongest, but can be very tricky to install, since the epoxy has to be mixed to exact specifications, the hole must be very clean, and the set time has to be watched. As well, there must be a rigorous testing program. In Boston's Big Dig project, these procedures were not well carried out, which resulted in a large concrete slab crushing a motorist.

 

Anchor bolts are a vital link between equipment and the foundation. Unfortunately, designers often overlook important points concerning anchor bolts, such as how long and strong they should be and the amount of preload. Anchor bolts, as well as other parts of the support system, such as sole plates and chocks, can be one of the principle points of failure on new construction projects. Failure usually occurs during the first year of operation. While the number and size of the anchor bolts are set by the equipment manufacturer, their length, configuration, and material of construction are in the hands of the foundation designer. Figure 1 shows good and bad designs.

 

Figure 1 Evolution of anchorbolt designs. (Illustration courtesy of Robt. L. Rowan & Assoc., Inc.)

Length: Short anchor bolts have historically caused problems in compressor foundations. Horizontal cracks in the foundation often result. The best practice today is to make them as long as possible, terminating them in the concrete mat  under the concrete foundation. In this manner, they do not contribute to horizontal cracking and have the added benefit of adding a post-tensioning effect.

 

Material: Anchor bolts for any dynamic machine cannot be too strong. Today, anchor bolts made from steel, conforming to ASTM A-193 with a yield strength of 105,000 psi, are not much more expensive than steel half as strong. As the need for high clamping forces for compressors is being recognized, alloy steel bolts to ASTM A-193 provide the necessary capacity without going to a larger anchor bolt.

 

Preload: While some compressor manufacturers will specify an initial torque value for the initial installation, often field experience will show a much higher (maybe two to three times) clamping force will be required to lower frame movement/vibration. Unless the anchor bolts put into the foundation to start with have extra capacity, the machine will not perform as it should, or a costly retrofit will have to be done.

 

Support Systems:

 

Figure 2 shows a range of options on how to support a gas compressor from the older method of full bed grouting, to the latest technology of adjustable support systems.

 

Figure 2 Types of compressor frame support systems. (Illustrations courtesy of Robt. L. Rowan & Assoc., Inc.)

 

Adjustable supports are the system of choice today, because they eliminate a potential problem of poor initial alignment which happens from time to time with full bed grouting. Adjustable systems also allow the optimum hot running condition to be achieved as the frame can be re-aligned to correct for the alignment changes  that occur as the equipemt heats up during its first 100 hours of operation.

 

Grout:

 

Since the introduction of epoxy grouts for gas compressor grouting in 1957, the use of cementitious grouts mixed with water has virtually stopped. Epoxy grouts are stronger, resist oil and many chemicals, and perform well in dynamically loaded situations.

 

While grout need not be stronger in compressive strength than the concrete underneath, a good grout will be tough enough to take impact and cyclical loads from the dynamic machine it supports. For that reason, compressive strengths above 5,000 psi and tensile strength above 1,000 psi are all that are required. Higher compressive strengths are not necessarily better if the product is brittle and cracks excessively in service. Almost all good machinery grouts can crack, so expansion joints are required. The expansion joints should be strategically placed so cracks will not develop in the prime load transfer area adjacent to the anchor bolts.

 

Figure 3 Section view, looking from the flywheel end towards the oil pump end. (Illustration courtesy of Robt. L. Rowan & Assoc., Inc.)

 

Repair of Foundations

 

Almost every foundation 20 twenty years old and designed with only minimal steel reinforcing is a candidate for replacement or repair. Common repair techniques include removing the top 24 inches to 30 inches of grout and concrete, cutting off and up-grading the anchor bolts, adding a heavy rebar layout in the excavated area and post-tensioning the repair to old remaining concrete.

 

What to use for the post-tensioned repair described above is extremely important.  If the job schedule will allow 21 to 28 days, portland cement concrete is the best choice. If a 24-hour curing product is needed, then a polymer modified concrete should be used. Either product will have a modulus of elasticity of at least 4,000,000 psi, and will have negligible creep at typical compressor foundation temperatures.

 

What should not be used as a deep pour repair material to replace the removed concrete is the epoxy grout material that is used as the final cap on top of the foundation. Epoxy grouts are just that, a grout designed to be used in 2 to 4 inch thicknesses. Epoxy grouts, as a class of material, have a modulus of elasticity ranging from under 1,000,000 psi up to 2,500,000 psi, with the lower range being the most prevalent. This means epoxy grout will compress under load, two to five times more than concrete. Additionally, some epoxy grouts creep enough at typical foundation temperatures to cause equipment misalignment. There have been catastrophic machine failures as a result of deep pours of epoxy grout. A new compressor foundation should not be designed with a 14-inch thick upper pour of epoxy grout nor should an older concrete foundation be repaired that way.

 

Besides up-grading the anchor bolts, an adjustable support system is also added to allow easier realignment. Figure 3 shows a typical foundation repair design.

 

References:

 

1. Prakask, Shamsher, and Vijay K. Puri, Foundations for Machines: Analysis & Design, Wiley Series in Geotechnical Engineering.
2. Rowan, Robert L. & Associates, Inc., Re-Grouting Reciprocating Gas Compressors, 5 Year Repairs vs. 20 Year Reliability Criteria, 1:12 Grouting Technology Newsletter.