Article No: 163

2006-05-03 09:10:27
Tech Talk
By: Ed Sauter

Figure 1

Codes are developed to provide assurance that structures are designed and constructed to withstand the test of time, not only for life safety but also for long-term durability. Yet codes can also be so complex that implementation causes struggles that delay or, worse yet, halt work.

Residential construction is no different than any other construction market today. Recognizing this, the American Concrete Institute created a committee specific to the residential industry (332-Residential Concrete) to create guides and standards that help shape the quality and efficiency of this market. Nearly 20 years after the formation of this committee, the first ACI Residential Concrete Code has been published. This article continues to discuss key elements found in this new standard that alter or improve the requirements for residential concrete as the industry prepares to shift from design and construction by ACI 318, chapter 22 to ACI 332.

Everyone knows that successful real estate is based on … connections, connections, connections.
This article has been discussing the merits of the provisions in ACI 332 for the footing and foundation walls. Significant effort has been placed into the standards to which these two principal components are designed and constructed to assure that the foundation remains "the rock" on which the house is built. We all agree that most people assume you need location, location, location to develop good real estate potential. But what this doesn't calculate is that if the framing is not securely anchored to the foundation wall, the location will only be as good as the next wind that comes its way to help the developer re-develop the site.

This month's feature section of the ACI 332 Standard looks at the requirements for connections to the foundation wall, specifically the anchor bolt. Although there are other methods of securing the sill plate, the anchor bolt remains the most common method or at least the most recognizable (figure 1). Section of ACI 332-04 states:

A positive connection by means of steel anchors shall be required between the top of the wall and the lateral bracing system.

The top of the (foundation) wall is relatively easy to understand; in fact, most steel anchor systems--if not all--are placed soon after the concrete is placed in the wall forms while it is still in a fluid state along the top of the wall. ACI 332 makes requirements for the spacing and size of these anchors:

(a) The minimum diameter of anchors shall be 1/2 inch; (b) The minimum embedment depth of anchors shall be 6 inches; (c) The maximum spacing of the anchors shall be 6 feet; (d) A minimum of one anchor shall be located within 12 inches of each change of wall direction, height, or termination; and (e) A minimum of one anchor shall be located within 12 inches of each side of each door or window opening.

These anchors are specifically required to transmit the lateral forces applied to the foundation wall from earth and hydrostatic pressures to the lateral bracing system. The lateral bracing system is considered the framing members of the floor diaphragm that span from foundation wall to foundation wall. Not only do these members support the finished floor above, they also serve to maintain the position of the top of wall so that the foundation design can perform as intended, spanning from this top connection to the connection at the footing.

Figure 2

Today's basements continue to grow in popularity as prime lower-level living spaces. In response to this trend, basement heights have increased on average from the 7-foot to 8-foot heights of the basement you may have grown up with to basements that now begin at 9 feet and extend up to 15 feet or more. The taller the height of the basement, the greater the pressure applied to the foundation wall and the more concentrated the force grows on the connection to the lateral bracing system. The minimums provided by ACI 332 are not intended to adequately resolve the requirements for all foundation designs, but rather serve, as stated, as a minimum. The commentary for this same section goes on to further define this issue, stating:

When appropriate, the connection to the lateral support system should be reviewed by a registered design professional (for example, conditions with high soil pressures, or tall walls, such as 60 pounds/feet 2 soil pressure or a 9-foot wall height).

The conditions cited in this commentary section are relatively common in today's residential market due to the significant benefits of lower-level living. The response to the increased loads applied to foundations and heights of foundations must then be to ensure that the restraint provided by the lateral bracing system is adequate to handle the loads transferred to it. Often, the weakness is not in the specific connection of the sill plate to the foundation, but rather in the anchoring of the floor joists to that same sill plate. This is the connection detail that should also be considered during the design of the foundation system. For the purposes of ACI 332 as a concrete standard, however, the requirements of this connection are outside the defined scope.

Figure 3

Although ACI 332 discusses the connection requirements in relation to steel anchor or bolt-type design requirements, many areas of the country now see anchor straps as a primary solution to securing the sill plate to the top of the foundation wall. Anchor straps create a high capacity for handling a variety of load forces efficiently, but are only effective if the user follows proper installation procedures. As stated by the manufacturers of these products, there are two acceptable methods for installation and connection of the straps. These are shown in figures 2 and 3 as referenced from the web site for one of these manufacturers. In figure 2, the strap is properly spread prior to inserting into the wet concrete so that when the sill plate is set between the strap ends, it can be secured to the foundation without the concern for rotation. Similarly, figure 3 shows proper installation if the spread process is not advantageous. In this design, the strap extending through the sill plate also protects from rotation and securely anchors the plate to the foundation wall.

Figure 4

Figure 4 shows what happens when the foundation contractor intends one use of the strap while the framing contractor follows another, resulting in a gap that may allow the sill plate to slip from the foundation connection. Depending on the communication between the two contractors, figures 5 and 6 can both result in successful development of the strap anchor design. Figure 6 shows what could become a weak connection under moderate to severe loading if the framing contractor is not prepared to complete the installation as intended.

Figure 5

Regardless of the method used, the connection of the foundation to the floor framing is an essential step in the construction process of the home--one that should not be systematically dismissed for sake of economy or speed during construction or design. By concentrating on key aspects of the designed structure such as this, permanence of homes can be assured and both homeowner and realtor can once again base their excitement on location, location, location.

Figure 6