Uses of Anchor Bolts in Structural Concrete

Anchor bolts are used commonly in structural concrete. In a general use of the term, `anchor' bolts attach things to the concrete. Or, more specifically, to anchor something may mean to provide some kind of attachment (say, via bolt or bolts), so that something doesn't slide off, or move with respect to, the concrete. Whether the term is used generally or specifically may need to be determined by context (the specific use of the bolt). Following are some common uses of anchor bolts in modern structural concrete construction.

1. Anchor bolts are used to attach wood framed (or other) walls to foundations.

In this sense the bolts keep the walls from sliding off the foundation, and from being pulled up off the foundation. Consider for example a wood framed wall with sheathing and siding, and on the top of the wall rests a roof with significant overhang. A big gust of wind could come along and hit the wall head on, and push up on the underside of the eave, and also cause a suction on the upper roof surface. Such gusts of wind may produce more uplift force than the weight of the construction itself, and unless everything is properly `tied' together, including the bottom (sill plate) of the wall to the foundation, the construction can come apart. Alas, building codes require the bottom of the wall to be bolted to the foundation. In this application the bolt is a `tie' and an anchor, where `tie' refers (in building design and construction) to something preventing uplift. And the anchor bolts keep the wall from pushing inward with this same blast of wind, or, on the opposite side of the house, keep the wall from being sucked outward. For residential construction, therefore, a typical code requirement is ½ in. diameter anchors at 6 ft on-center (o.c.) along the wall. And this is a minimum requirement. In areas of greater wind or seismic risk, the required anchor bolts may be bigger, or closer together, or both. And this is just to attach the wall. If the anchor bolts are serving other purposes (as described below), there may be more bolts required.

2. Anchor bolts are used to transfer horizontal loads from concrete walls to roof and floor systems (diaphragms).

Consider the top of a concrete basement wall, or the top of an above grade (main or upper story) concrete wall. In the case of a concrete basement wall, the top of the wall is supported laterally (from pushing `in') by the main floor. In the case of the top of an above grade concrete wall, the top is supported laterally (from pushing `in') by a roof (or another floor). In such cases the concrete wall is `connected' to the floor or roof system (or vice versa) with anchor bolts. Observing proper edge distances in concrete and wood sill plates is important to prevent splitting of wood and breaking out the side of the concrete in such cases, as the load is perpendicular to the wall, and plate, and thus `toward the (wood and concrete) edges'. If the forces from the walls are large, for example, walls retaining earth, the number and / or size of the bolts required may be much, much more than the ... " ½ in. diameter bolts @ 6 ft o.c." above.

3. Transferring Loads From Floor and Roof Systems (Diaphragms) to Concrete Walls or Foundations.

Once forces are delivered to floor or roof systems acting as diaphragms (acting as essentially very large, flat beams), as described above, they are generally `taken out' of the diaphragms via anchor bolts to the other concrete or foundation walls. In the analogy of the floor or roof acting as a big, deep beam, these other walls are the `beam' supports, or reactions. Thus, the forces in these anchor bolts are parallel to the walls. The force transferred is typically called `unit shear', and is generally the determining factor in the design of the diaphragm as well as in determining the size and spacing of the anchor bolts. And, especially if these walls are short, the amount of anchorage may (again) end up being (way) more robust than the ... " ½ in. anchor bolt at 6 ft. o.c., blah, blah." But not always. In prescriptive Residential construction, for example, the direct attachment of the floor or roof to concrete wall may be accomplished with the ... 6 ft (or in some cases 4 ft) spacing.

4. Anchorage of Shear Walls and Braced Wall Panels.

Where roof and floor diaphragms are attached to framed walls the unit shear loads are taken out by drag struts (also called `collectors') and into (the tops of) shear walls. In prescriptive light frame wood construction the `double top plates' serve as collectors and the braced wall panels act as the shear walls. The bottoms of the shear walls and braced wall panels must be anchored to the foundation (via anchor bolts).

5. Bolts for Hold-down and Tie-down Devices.

Shear walls in general also require hold-down or tie-down devices at their ends to prevent uplift (tipping) of the shear walls. These devices are either directly anchored to the foundation below or are tied to framing which are tied to the foundation. The actual connection to the foundation is accomplished through a bolt or other device `anchored' in the concrete.

6. Bolts attaching ledgers to concrete walls or foundations.

Ledgers can be used to carry the gravity (downward) or lateral loads from floors, decks, or roofs, into walls. In the case of roofs and wind or floors and decks in high seismic areas the ledgers may also resist uplift forces. Where these ledgers are attached to concrete walls, the bolts are considered anchor bolts. These bolts are generally subject to shearing (load parallel to concrete face), and in some cases tension if the ledger is being pulled away from the wall or by the prying effect of downward loads.

7. Bolts tying joists and beams to walls and foundations.

In regions of high seismic risk individual beams and joists need to be tied to the concrete walls or foundations. This is accomplished by anchoring to the concrete with bolts or other approved devices. (See Figures R611.8(2),(3), (4),(5), and similarly for blocking Figure R611.8(7).)

8. Bolts attaching pipes, cables, other things to concrete.

Anchor bolts may be used to attach all sorts of non-structural things to concrete, whether to concrete walls, foundations, or even concrete ceilings above.

Conclusion

The anchor bolt applications described above generally put the bolt in either tension (bolt trying to pull away) or shear (load parallel to face of concrete),or both. In some applications bolts may carry compressive forces, for example, a pole base to a concrete pier. Otherwise, most compressive forces in connections are transmitted through bearing, even though the connection may have anchor bolts (for other purposes). In any anchor bolt connection, both the bolt and the concrete must be `strong' enough. As such, things like minimum bolt size, embedment, edge distance, concrete strength, must be determined, and observed in construction. Depending on the application, different factors of safety are employed; and, depending on the application, bolt installation and / or connection to the bolt may require Special Inspection. In general, bolts installed in concrete may be either `cast in place' or `post installed'. These issues or items will be discussed in later conversations.

References

Building Code Requirements for Structural Concrete, ACI 318, American Concrete Institute, P.O. Box 9094, Farmington hills, Michigan, 48333.

International Building Code, International Code Council, 4051 West Flossmoor Road, Country Club Hills, IL 60478.

International Residential Code, International Code Council, 4051 West Flossmoor Road, Country Club Hills, IL 60478.