TANK ANCHORAGE - API 650

Source:                                           B. Mistry E-Mail of October 10, 2011

Title:                                                Consolidate and Improve Tank Anchor Design

Impact:                                           The business impact of this item is neutral.

TANK ANCHORAGE - API 650 5.12   TANK ANCHORAGE 5.12.1            When a tank is required to be anchored per 5.11 (Wind), Appendix E (Seismic), Appendix F (Internal Pressure), or when a tank is anchored for any other reason, the following minimum requirements shall be met. 5.12.2            Anchorage shall be provided to resist each of the applicable uplift load cases listed in Tables 5-21a and 5-21b. The load per anchor shall be:                                         tb = U/N where tb       = load per anchor, U        = net uplift load per Tables 5-21a and 5-21b, N        = number of equally spaced anchors (a minimum of 4 is required [6 min. if Seismic governs]) tb  shall be increased to account for unequal spacing. 5.12.3  The spacing between anchors shall not exceed 3 m (10 ft.). 5.12.4   Allowable stresses for anchor bolts shall be in accordance with Tables 5-21a and 5-21b for each load case. The allowable stress shall apply to the net (root) area of the anchor bolt.  Fy shall be taken at maximum design temperature for insulated anchors and at room temperature for exposed anchors. 5.12.5   The Purchaser shall specify any corrosion allowance that is to be added to the anchor dimensions. Unless otherwise specified, corrosion allowance for anchor bolts shall be applied to the nominal diameter and not to the threaded part of anchor bolt. The minimum anchor bolt diameter is 1 in. plus any specified corrosion allowance.  When anchor bolts are used, they shall have a corroded shank diameter of no less than 25 mm (1 in.).  Carbon steel anchor straps shall have a nominal thickness of not less than 6 mm (1/4 in.) and shall have a minimum corrosion allowance of 1.5 mm (1/16 in.) on each surface for a distance at least 75 mm (3 in.) but not more than 300 mm (12 in.) above the surface of the concrete. 5.12.6 Attachment of the anchor bolts to the shell shall be through stiffened chair-type assemblies or anchor rings of sufficient size and height. An acceptable procedure for anchor chair design is given in AISI Steel Plate Engineering Data, Volume 2, Part 5 E-1, Volume II, Part VII “Anchor Bolt Chairs” (Nov. 2011).  When acceptable to the Purchaser, hold down straps may be used. if the shell attachment is via chair-type assemblies or anchor rings of sufficient size and height.  5.12.7  Other evaluations of anchor attachments to the shell may be made to ensure that localized stresses in the shell will be adequately handled. An acceptable evaluation technique is given in ASME Section VIII Division 2, Appendix 4, using the allowable stresses given in this section for Sm. The method of attachment shall take into consideration the effect of deflection and rotation of the shell. 5.12.8          Allowable stresses for anchorage parts shall be in accordance with 5.10.3 the ANSI/AISC 360 using allowable strength design methodology (ASD). A 33% increase of the allowable stress may be used for wind or seismic loading conditions.  Wind loading need not be considered in combination with seismic loading. 5.12.9          The maximum allowable local stress in the shell at the anchor attachment shall be in accordance with Tables 5-21a and 5-21b unless an alternate evaluation is made in accordance with 5.12.7. 5.12.10 When specified by the Purchaser, the anchors shall be designed to allow for thermal expansion of the tank resulting from a temperature greater than 93°C (200°F). 5.12.11 Any anchor bolts shall be uniformly tightened to a snug fit, and any anchor straps shall be welded while the tank is filled with test water but before any pressure is applied on top of the water. Measures such as peening the threads or adding locking nuts, shall be taken to prevent the nuts from backing off the threads. 5.12.12 The embedment strength of the anchor in the foundation shall be sufficient to develop the specified minimum yield strength of the anchor. Hooked anchors or end plates may be used to resist pullout, except where seismic governs.  (See E.6.2.1.2 restrictions for hooked anchors for Appendix-E tanks).  When mechanical anchorage is required for seismic, the anchor embedment or attachment to the foundation, the anchor attachment assembly and the attachment to the shell shall be designed for anchor attachment design load PA. The anchor attachment design load, PA, shall be the lesser of the load equal to the minimum specified yield strength multiplied by the nominal root area of the anchor or three times design uplift load per anchor bolt, tb, defined in 5.12.2. 5.12.13 The foundation shall provide adequate counterbalancing weight to resist the design uplift loads in accordance with the following: 5.12.13.1 The counterbalancing weight, such as a concrete ring wall, shall be designed so that the resistance to net uplift is in accordance with Tables 5-21a and 5-21b. When considering uplift due to a wind or seismic moment, an evaluation shall be made to insure overturning stability of the foundation and to insure soil-bearing pressures are within allowable stress levels as determined using the recommendations of Appendix B. 5.12.13.2 When a footing is included in the ring wall design, the effective weight of the soil above the footing may be included in the counterbalancing weight. “Table 5-21a and Table 5-21b, Revise Column 3 Heading to include Anchor Strap”: Allowable Anchor Bolt Stress (MPa) Allowable Anchor Bolt or Anchor Strap Stress (MPa) Allowable Anchor Bolt Stress (lbf/in2) Allowable Anchor Bolt or Anchor Strap Stress (lbf/in2) E.6.2.1.2 Mechanically-Anchored If the tank configuration is such that the self-anchored requirements cannot be met, the tank must be anchored with mechanical devices such as anchor bolts or straps. The design of the anchorage and its attachment to the tank shall meet the minimum requirements of 5.12.  In addition, hooked anchor bolts (L- or J-shaped embedded bolts) or other anchorage systems based solely on bond or mechanical friction shall not be used when anchors are required for seismic load. Post-installed anchors may be used provided that testing validates their ability to develop yield load in the anchor under cyclic loads in cracked concrete and meet the requirements of ACI 355. When tanks are anchored, the resisting weight of the product shall not be used to reduce the calculated uplift load on the anchors. The anchors shall be sized to provide for at least the following minimum anchorage resistance: wAB     = {1.273*Mrw/D2– wt(1 – 0.4Av)}                                                          (E.6.2.1.2-1) plus the uplift, in N/m (lbf/ft) of shell circumference, due to design internal pressure. See Appendix R for load combinations. Wind loading need not be considered in combination with seismic loading. The anchor seismic design load, PAB, is defined in Equation E.6.2.1.2-2:       PAB = wAB (p*D/nA )                                                                                   (E.6.2.1.2-2) where, nA is the number of equally-spaced anchors around the tank circumference. PAB shall be increased to account for unequal spacing. When mechanical anchorage is required, the anchor embedment or attachment to the foundation, the anchor attachment assembly and the attachment to the shell shall be designed for PA. The anchor attachment design load, PA, shall be the lesser of the load equal to the minimum specified yield strength multiplied by the nominal root area of the anchor or three times PAB. The maximum allowable stress for the anchorage parts shall not exceed the following values for anchors designed for the seismic loading alone or in combination with other load cases: *          An allowable tensile stress for anchor bolts and straps equal to 80% of the published minimum yield stress. *          For other parts, 133% of the allowable stress in accordance with 5.12.8.   *          The maximum allowable design stress in the shell at the anchor attachment shall be limited to 170 MPa (25,000 lbf/in.2) with no increase for seismic loading. These stresses can be used in conjunction with other loads for seismic loading when the combined loading governs. E.7.1.2 Mechanically-Anchored See 5.12 and E.6.2.1.2 for requirements for mechanically anchoring a tank. When mechanical-anchorage is required, at least six anchors shall be provided. The spacing between anchors shall not exceed 3 m (10 ft). When anchor bolts are used, they shall have a corroded shank diameter of no less than 25 mm (1 in.). Carbon steel anchor straps shall have a nominal thickness of not less than 6 mm (1/4 in.) and shall have a minimum corrosion allowance of 1.5 mm (1/16 in.) on each surface for a distance at least 75 mm (3 in.) but not more than 300 mm (12 in.) above the surface of the concrete. Hooked anchor bolts (L- or J-shaped embedded bolts) or other anchorage systems based solely on bond or mechanical friction shall not be used when seismic design is required by this appendix. Post-installed anchors may be used provided that testing validates their ability to develop yield load in the anchor under cyclic loads in cracked concrete and meet the requirements of ACI 355. E.7.4 CONNECTIONS Connections and attachments for anchorage and other lateral force resisting components shall be designed to develop the strength of the component anchor (e.g., minimum published yield strength, Fy in direct tension, plastic bending moment), or 4 times the calculated element design load. Penetrations, manholes, and openings in shell components shall be designed to maintain the strength and stability of the shell to carry tensile and compressive membrane shell forces. The bottom connection on an unanchored flat-bottom tank shall be located inside the shell a sufficient distance to minimize damage by uplift.  As a minimum, the distance measured to the edge of the connection reinforcement shall be the width of the calculated unanchored bottom hold-down plus 300 mm (12 in.). J.3.9 ANCHORING Because of the proportions of shop-assembled storage tanks, overturning as a result of wind loading must be considered.  If necessary, adequate provisions for anchoring shall be provided.  See 5.12 for tank anchorage design guidance. “PAGE L-8” Table 2 Bolts and Anchors Complete all bolting and anchorage information (see 4.7, 5.11.3, 5.12, E.6.2.1.2, E.7, F.7.4, and J.3.9), including head and nut shape and material specifications.  Show units of measure for the corrosion allowance and see 5.3.2.  Corrosion allowance may be marked “NA” for galvanized, special corrosion-resistant coated, or stainless steel anchor bolts. M.3 Modifications in Stress and Thickness M.3.9 If the anchors are insulated, see 5.12.4 for modification to the allowable stresses specified in Tables 5-21a, and 5-21b. and 5-22a and 5-22b shall be multiplied by the ratio of the material’s yield strength at the maximum design temperature to 205 MPa (30,000 lbf/in.2) if the ratio is less than 1.0 (see Tables M-1a and M-1b for yield strength reduction factors).