design of anchor reinforcement in concrete pedestals
CSA Today, Vol. IIl, No. 12
1. INTRODUCTION
The Appendix D of the ACI 318-05 provides design requirements for anchors in un-reinforced concrete. It addresses only the anchor strength and the un-reinforced concrete
strength:
1. Breakout strength,
2. Pullout strength,
3. Side-face blowout strength
4. Pryout strength.
Even though the Appendix D of the ACI 318-05 permits the use of supplementary reinforcement to restrain the concrete breakout (Section D.4.2. 1), it does not provide specific guidelines in designing such reinforcement. Commentary of Section D.4.2.1 indicates that the designer has to rely on other test data and design theories in order to include the effects of supplementary reinforcement.
In petrochemical industry, concrete pedestals commonly support static equipment (ie. horizontal vesscls and heat exchangers) and pipe-rack or compressor building columr In order to fully-develop the strength of anchor in un-reinforced concrete, the Appendix D of the ACI 318-05 requires the use of significantly large concrete pedestals/octagons. It is generally not economical to provide such large concrete pedestals/octagons. Therefore, the anchorage design in petrochemical industry almost always includes designing supplementary reinforcement. When supplementary reinforcement is used to transfer the full design load from the anchors, it is generally referred as anchor reinforcement. Figure 1 shows anchors of a compressor building column on a reinforced concrete pedestal.
This paper presents a method for designing anchorage in concrete pedestals with anchor reinforcement to anchor static equipment or columns in petrochemical facilities.
The anchor tension and shear forces are assumed to be resisted by the vertical reinforcing bars and ties, respectively. The calculation for determining the required amount of vertical reinforcing bars and ties is presented.
A design example of column anchorage in a reinforced concrete pedestal is given to illustrate the proposed design method.
2. DESIGN PHILOSOPHY
The following general design philosophy is used when the anchor forces are assumed to be resisted by the steel reinforcement:
1. Concrete contribution is neglected in proportioning the steel reinforcement.
2. When a non-ductile design is permitted, the reinforcement should be designed to resist the factored design load.
3. When a ductile design is required, the reinforcement should be proportioned to
develop the strength of the anchor. If the anchor is sized for more than 2.5 times factored tension design loads, it is permitted to design the reinforcement to carry 2.5 times the factored design load, where 2.5 is an overstrength factor.
4. When reinforcement is used to restraint concrete breakout, the overall anchorage design should ensure that there is sufficient strength corresponding to the three other failure modes described in the Introduction (pullout failure, side-face blowout failure, and pryout failure).
The three failure modes will be addressed as follows:
a. The pullout strength
b. The side-face blowout failure
c. The pryout failure is only critical for short and stiff anchors. It is reasonable to assume that for general cast-in place headed anchor with h_ef,min=12d_o, the pryout failure will not govern.
All rebars that are < d_max from the anchor can be effective for resisting anchor tension.
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