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.
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
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
5.12.9 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”:
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.
E.7.1.2 Mechanically-Anchored
See 5.12 and E.6.2.1.2 for requirements
for mechanically anchoring a tank.
E.7.4
CONNECTIONS
Connections and attachments for
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,
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
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