Foundations are commonly used to support heavy loads from multiple columns. Mats may
bear on competent soil, on soil with a low bearing capacity, or be supported on piles or drilled shaft foundations (caissons). Depending on the total load applied to the mat and underlying foundation system, the thickness of mat foundations can vary from 1 ft (0.3 m) to more than 20 ft (7 m).
The reinforcing system in the mat can be quite substantial, with heavy reinforcing bar mats in the bottom, top, or both locations within the mat depth. Improper detailing of the reinforcement can result in constructability issues impacting other trades, the schedule, and costs. This Detailing Corner describes practices that can be used to simplify the design, detailing, and placement of mat reinforcement.
DETAILING CORNER | Foundations
Joint ACI-CRSI Committee 315-B, Details of Concrete ReinforcementConstructibility, has developed forums dealing with constructibility issues for reinforced concrete. To assist the Committee with disseminating this information, staff at the Concrete Reinforcing Steel Institute (CRSI) are presenting these topics in a regular series of articles. If you have a detailing question you would like to see covered in a future article, please send an e-mail to Neal Anderson, CRSI’s Vice President of Engineering, at firstname.lastname@example.org with the subject line “Detailing Corner.”
Setting the Reinforcement Minimum requirements
The mat depth is normally set by shear strength requirements. The amount of reinforcement As for the top and bottom reinforcing layers is set by meeting ACI 318 Code1 requirements for flexural strength, minimum flexural reinforcement (Sections 10.5.1 through 10.5.4), and shrinkage and temperature reinforcement (Sections 18.104.22.168 through 22.214.171.124). Generally, As will be governed by flexural considerations, either through analysis or satisfying the minimum requirements. However, as the thickness of the mat increases, the minimum amount of shrinkage and temperature reinforcement will increase—it could control for very thick mats.
Once this reinforcement quantity is calculated, a suitable bar size and spacing can be selected. Depending on the layout configuration, the reinforcing bars can be placed in two layers (one mat) or four layers (two mats) at both the top and bottom. Per Code Section 126.96.36.199, the bar spacing is limited to five times the slab thickness or 18 in. (450 mm). Code Section 15.10.4 also sets the maximum spacing of mat reinforcement at 18 in.
Bars that are placed in the interior layers should follow the same spacing patterns as the main, outer reinforcement so that all bars in different layers are aligned (Fig. 1). This provides clear passage for concrete placement, which helps to reduce voids. It’s considered good practice to select the
size of the bars in the interior layers equal to or smaller than the outer layer reinforcing bars. Some designers prefer to specify bars in the interior layers with diameters different than the bars in the outer layer of reinforcement so they can be more easily identified and checked in the field. By a note or a section on the design drawing, the engineer should specify those bars that will be placed in the outer
layer and the ones in the inner layer.
It’s recommended that a clear spacing of at least 3 in. (75 mm) (more for deeper mats) be provided between the bars to facilitate concrete placement, as shown in Fig. 1. For deep foundation mats requiring
worker access inside the cage, it’s also good practice to provide openings in the top reinforcement. This can be accomplished by bundling the bars and providing additional steel around the resulting opening, as shown in Fig. 2.
When the column spacing is not laid out on a regular, symmetric grid, consider locating the bars on an orthogonal grid rather than skewing them with the actual column locations. Additional reinforcement can then be placed wherever it’s required.
It’s common practice not to use shear reinforcement in a mat. This ensures that the depth and stiffness is maximized and flexural reinforcement is minimized (ACI 336.2R, Section 6.1.2, Item 2). However, when shear reinforcement is required, it’s recommended that the selected vertical bars are larger than the main reinforcement and are placed at larger spacing—easing identification and inspection.
It’s preferable to extend column and wall dowels all the way down to the bottom mat of reinforcement.
The dowels should incorporate a 90-degree hook at the bottom end, so the tail of the hook can be used
for support and elevation control. This also allows the dowels to be tied to both the top and bottom
mats of reinforcement for stability, as the two tie points will properly secure the dowel bars from displacing (Fig. 3).
If lap splices in the foundation mat reinforcement are to be staggered, they need to be carefully detailed
on the design drawings. Otherwise, the staggered splices for different layers of reinforcing bars may
become quite confusing to place and subsequently inspect. If it’s possible to avoid staggering splices, this
should be the preferred placement for ease of constructability.
The common mill stock length of straight reinforcing bars is 60 ft (18.3 m). However, a local fabricator
may have limitations (such as storage space, crane capacity, and bend table size), requiring stocked
straight lengths less than 60 ft. It is thus advisable to verify with the local fabricator the maximum
available stock length. Because a mat foundation requires long runs of straight bars, it’s recommended
that the maximum straight bar length be used as much as possible. This minimizes the quantity of
potential lap splices. If an actual bar length shorter than the typical stock length is needed to complete
the reinforcing bar run, this “short bar” should be located at either end of the mat foundation. Alternately,
stock length bars could be provided throughout the mat, with the lap lengths increased along the run. Although the lap lengths will be greater than Code minimums, material waste and fabrication costs
could be reduced because a long bar will not have to be sheared to a shorter length. It will also aid in
constructability, as a separate bar length bundle will not have to be inventoried at the construction site.
Please Download PDF File Full (Reinforcing Bar Details for Mat Foundations)