Crsi Reinforcing Bar Detailing Pdf
Lise Henton
Once the size of the cross-section of a beam has been determined based on serviceability and strength requirements, the required area of flexural reinforcement, As, is calculated by setting the required flexural strength, Mu, equal to the design flexural strength, Mn. The size and number of reinforcing bars must be chosen to (1) provide an area of reinforcement equal to or greater than the amount that is required, and (2) satisfy the minimum and maximum spacing requirements in ACI 318-14, Building Code Requirements for Structural Concrete and Commentary.
Reinforcing bars that are spaced too far apart could result in relatively large flexural crack widths. Thus, the maximum center-to-center spacing, s, of the deformed longitudinal bars to limit crack widths is given by the following equation (see ACI Table 24.3.2):
where fs is the calculated stress in the flexural reinforcement closest to the tension face of the section due to service loads and cc is the least distance from the surface of the reinforcement to the tension face of the member. It is permitted to assume that fs = 2fy/3 where fy is the specified yield strength of the reinforcement. Table 1 contains values of the minimum number of bars required in a single layer for various beam widths based on Grade 60 reinforcement (fs = 40,000 psi), cc = 2 inches (1.5-inch cover plus the diameter of a #4 stirrup), and the overall longitudinal reinforcing bar diameter (approximate diameter to the outside deformations of the bar), which is given in Table 2.
Selecting the number of longitudinal bars within the limits of Tables 1 and 3 provides automatic compliance with the ACI 318 requirements for cover and spacing, given the assumptions noted above. The minimum clear spacing requirements of ACI 318-14, Section 25.2.1, are also applicable to contact lap splices and adjacent splices or bars. Using the largest practical bar sizes that satisfy these requirements usually results in overall cost savings.
ACI 318-14, Section 9.7.3, contains the requirements for the development of reinforcing bars in beams. For beams subjected to uniformly distributed gravity loads where the shape of the moment diagram is known, the development lengths in Figure 1 can be used. These recommended details include the requirements for structural integrity reinforcement in ACI 318-14, Section 9.7.7, and can be used for beams that have been designed using the approximate bending moment coefficients in ACI Table 6.5.2. The Notes in Figure 1 are as follows:
Lapping of continuous bottom bars at supports often presents congestion and installation problems. For example, it is common to splice all the bottom bars over the columns away from the section of maximum positive reinforcement, as shown in Figure 2a. This arrangement is the simplest to detail and is most suitable where the beams are wider than the columns. However, it can result in congestion in the beam-column joints. One way to circumvent this issue is to use the detail in Figure 2b: splice bars are provided in the joint, which are spliced to the bottom bars on both sides of the joint. This arrangement works very well with preassembled beam cages because no bottom bars pass through the column during installation. Even though this arrangement increases the amount of reinforcing steel that is required, the cost of the additional material may be more than offset by the savings in labor and other costs; it may be the most cost-effective arrangement in certain situations.
To avoid potential congestion issues at beam-column joints, it is good practice to specify beams that are at least 4 inches wider than the columns into which they frame. As floor systems become shallower (which also leads to overall economy), beams generally need to become wider. Proper stirrup detailing in wide beams is essential to ensure that the longitudinal flexural reinforcement and the stirrups are fully effective.
Research has shown that locating stirrups solely around the perimeter of a wide beam is not fully effective. Thus, stirrup legs are required in the interior of a wide beam. A common stirrup configuration is illustrated in Figure 3a, where three closed stirrups are provided. One problem with this configuration is that none of the stirrups traverse the full net width (that is, the full beam width minus the total side cover) of the beam. Thus, the overall width of the stirrup arrangement needs to be measured and verified in the field before installation, which translates to extra time and cost. During installation, it is possible for the net width to change when the preassembled cage is hoisted into position by crane; this increases the possibility that the provided cover will be less than that which is required. Another problem may occur where the stirrups are built in place instead of preassembled: one-piece closed stirrups make it challenging to place all the required longitudinal reinforcement in the beam, especially where large, long longitudinal bars must be threaded through the stirrups.
In the configuration illustrated in Figure 3b, a single, open stirrup is provided that extends the full net width of the beam. A stirrup cap consisting of a horizontal bar with a 135-degree hook at one end and a 90-degree hook at the other end is provided at the top of the configuration, which also extends the full net width of the beam. Providing a full-width stirrup helps in maintaining the correct concrete cover and facilitates installation of the beam reinforcement: longitudinal bars can be placed easily within the beam from the top before installation of the stirrup cap. Two sets of identical U-stirrups with 135-hooks are shown symmetrically placed within the interior of the beam. This configuration provides a cost-effective way of providing shear reinforcement for wide beams.
A drip groove or edge in a beam often presents a problem in maintaining the required cover to the reinforcement in the beam (Figure 4a). It is frequently not feasible to increase the concrete cover after the bars in the beam have been detailed. Raising the stirrups from the bottom to achieve the required bottom cover decreases the top cover (Figure 4b). A practical solution is to measure the concrete cover to the drip groove and detail the stirrups accordingly, as shown in Figure 4c. This impacts the overall effective depth to the flexural reinforcement and needs to be accounted for in the design.
Maintaining the proper concrete cover can also be challenging at beam intersections (Figure 5). In particular, layering the top steel in the slab at such intersections can create constructability issues. The sequencing and layering of beam and slab top reinforcement can also create congestion issues. The following sequence for bar placement is one way of avoiding problems associated with these intersections:
The Concrete Reinforcing Steel Institute (CRSI) recently published the 2000, 4th edition of "Reinforcing Bar Detailing." Developed as a textbook for reinforcing steel detailers, it is an integral part of the CRSI Reinforcing Bar Detailer Training Program, which is a members-only training program for CRSI fabricator members. The new detailing manual is also intended for use in junior colleges, technical schools, and vocational schools, as well as companies involved in rebar detailing for reinforced concrete construction projects. Detailing is the preparation of bills of material and placing drawings, sufficient for the rebar fabricator to cut and bend rebar for a specific project, and for the rebar placer to correctly install the rebar at the jobsite. The first 15 chapters of the book cover fundamentals, while the last eight cover applications.
The 28th edition of the Manual of Standard Practice is now available on CD-ROM in combination with the hardcover edition. This resource contains information on recommended industry practices for estimating, detailing, fabricating, and placing reinforcing steel for reinforced concrete construction. The Manual of Standard Practice has provided the industry standards for reinforced concrete construction since 1927.
This resource includes suggested specifications for reinforcing steel in Chapter 4. Chapter 3 on bar supports is commonly referenced in project specifications. New material in the manual includes a list of specific information on structural drawings that is required by the ACI 318 Building Code and updated illustrations of the markings on Grade 60 and Grade 75 reinforcing bars.
Kelly is heavily involved with the Concrete Reinforcing Steel Institute (CRSI) and many of its committees such as the Detailing committee, the Anchorages and Lap Splices committee and the Manual of Standard Practice committee. He is head of the CRSI Building Information & Modeling (BIM) task group, is active in the Detailer Certification Task group and the Detailing Standards Task group. He is also a current member of the Independent Steel Alliance (ISA) and the Post-Tensioning Institute (PTI) where he holds a Level 1 Certification from the PTI.
Education and training of new rebar detailers are important goals to Kelly, and since 2019 he has started working with various local Utah Technical Colleges and Universities such as Weber State University and its Strategic Workforce Initiative, Bridgerland Technical College, and notably Davis Technical college where they have included rebar detailing and modeling into their curriculum for the 2021/2022 Architectural and Engineering Design program. He also sits on the Occupational Advisory Boards for both Davis Tech and WSU.
This career has taken him on quite a journey supervising safe & successful projects in Vancouver, Seattle, Phoenix, Idaho, and now to Utah. In each location, he has been instrumental in completing such projects as the Intel Fab 42 Center and the NHL Phoenix Coyote Hockey Arena. He has gained valuable experience in contributing to all kinds of projects from bridges to water treatment plants to high rises. Regardless of the project, Brian takes great pride in the reinforcing industry & all the dedicated people who make it happen on a daily basis.
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