30
Note: Grade 60 steel. b = 1.0, l = 1.0, ldh not less than 8db nor 6 in.
Note: Grade 60 steel. b = 1.0, l = 1.0, ldh not less than 8db nor 6 in.
reinforcement spacing is less than three times the diameter of the hooked bar. Note that whether the standard hook is detailed to engage over a longitudinal bar has no influence on the required hook length.
When insufficient hook length is available or in regions of heavy bar congestion, mechanical anchors may be used. There are a number of proprietary devices that have been tested and prequalified. These generally consist of an anchor plate attached to the bar end.
7.17.4 Splices
There are three choices for joining bars together: (1) mechanical device, (2) welding, and (3) lap splices. The mechanical and welded splices must be tested to show the development in tension or compression of at least 125% of the specified yield strength f of the bar. Welded splices must conform to ANSI/AWS D1.4, "Structural Welding Code — Reinforcing Steel.'' Since splices introduce weak leaks into the structure, they should be located as much as possible away from points of maximum force and critical locations.
7.17.4.1 Tension Lap Splices
Generally, bars in tension need to be lapped over a distance of 1.3ld (Class B splice, see Section 7.17.1 for 1d), unless laps are staggered or more than twice the required steel is provided (Class A splice = 1.01d).
7.17.4.2 Compression Lap Splices and Column Splices
Compression lap splice lengths shall be 0.0005fydb, but not less than 12 in. If any of the load demand combinations is expected to introduce tension in the column reinforcement, column bars should be lapped as tension splices. Class A splices (1.0 id) are allowed if half or fewer of the bars are spliced at any section and alternate lap splices are staggered by id. Column lap lengths may be multiplied by 0.83 if the ties provided through the lap splice length have an effective area not less than 0.0015hs. Lap lengths within spiral reinforcement may be multiplied by 0.75.
7.18 Deflections
The estimation of deflections for reinforced concrete structures is complicated by the cracking of the concrete and the effects of creep and shrinkage. In lieu of carrying out a refined nonlinear analysis involving the moment curvature analysis of member sections, an elastic analysis may be used to incorporate a reduced or effective moment of inertia for the members. For beam elements an effective moment of inertia may be taken as
where the cracking moment of the section
The cracking stress or modulus of rupture of normal weight concrete is fr = 7 . 5p7 (7 . 73)
For alllightweight concrete f should be multiplied by 0.75, for sandlightweight concrete, by 0.85.
For estimating the deflection of prismatic beams, it is generally satisfactory to take Ie at the section at midspan to represent the average stiffness for the whole member. For cantilevers, the Ie at the support should be taken. For nonprismatic beams, an average Ie of the positive and negative moment sections should be used.
Longterm deflections may be estimated by multiplying the immediate deflections of sustained loads (e.g., selfweight, permanent loads) by
The timedependent factor X is plotted in Figure 7.29. More refined creep and shrinkage deflection models are provided by ACI Committee 209 and the CEPFIP Model Code (1990).
Deflections of beams and oneway slab systems must not exceed the limits in Table 7.16. Deflection control of twoway floor systems is generally satisfactory by following the minimum slab thickness
Duration of load, months
FIGURE 7.29 Timedependent factor X.

Post a comment