((a)) ** Determination of amount of unfunded vested benefits allocable to employer withdrawn from plan** The amount of the unfunded vested benefits allocable to an employer that withdraws from a plan shall be determined in accordance with subsection (b), (c), or (d) of this section.

((b)) ** Factors determining computation of amount of unfunded vested benefits allocable to employer withdrawn from plan**

((1)) Except as provided in subsections (c) and (d), the amount of unfunded vested benefits allocable to an employer that withdraws is the sum of—

((A)) the employer’s proportional share of the unamortized amount of the change in the plan’s unfunded vested benefits for plan years ending after , as determined under paragraph (2),

((B)) the employer’s proportional share, if any, of the unamortized amount of the plan’s unfunded vested benefits at the end of the plan year ending before , as determined under paragraph (3); and

((C)) the employer’s proportional share of the unamortized amounts of the reallocated unfunded vested benefits (if any) as determined under paragraph (4).

((2))

((A)) An employer’s proportional share of the unamortized amount of the change in the plan’s unfunded vested benefits for plan years ending after , is the sum of the employer’s proportional shares of the unamortized amount of the change in unfunded vested benefits for each plan year in which the employer has an obligation to contribute under the plan ending—

((i)) after such date, and

((ii)) before the plan year in which the withdrawal of the employer occurs.

((B)) The change in a plan’s unfunded vested benefits for a plan year is the amount by which—

((i)) the unfunded vested benefits at the end of the plan year; exceeds

((ii)) the sum of—

((I)) the unamortized amount of the unfunded vested benefits for the last plan year ending before , and

((II)) the sum of the unamortized amounts of the change in unfunded vested benefits for each plan year ending after , and preceding the plan year for which the change is determined.

((C)) The unamortized amount of the change in a plan’s unfunded vested benefits with respect to a plan year is the change in unfunded vested benefits for the plan year, reduced by 5 percent of such change for each succeeding plan year.

((D)) The unamortized amount of the unfunded vested benefits for the last plan year ending before , is the amount of the unfunded vested benefits as of the end of that plan year reduced by 5 percent of such amount for each succeeding plan year.

((E)) An employer’s proportional share of the unamortized amount of a change in unfunded vested benefits is the product of—

((i)) the unamortized amount of such change (as of the end of the plan year preceding the plan year in which the employer withdraws); multiplied by

((ii)) a fraction—

((I)) the numerator of which is the sum of the contributions required to be made under the plan by the employer for the year in which such change arose and for the 4 preceding plan years, and

((II)) the denominator of which is the sum for the plan year in which such change arose and the 4 preceding plan years of all contributions made by employers who had an obligation to contribute under the plan for the plan year in which such change arose reduced by the contributions made in such years by employers who had withdrawn from the plan in the year in which the change arose.

((3)) An employer’s proportional share of the unamortized amount of the plan’s unfunded vested benefits for the last plan year ending before , is the product of—

((A)) such unamortized amount; multiplied by—

((B)) a fraction—

((i)) the numerator of which is the sum of all contributions required to be made by the employer under the plan for the most recent 5 plan years ending before , and

((ii)) the denominator of which is the sum of all contributions made for the most recent 5 plan years ending before , by all employers—

((I)) who had an obligation to contribute under the plan for the first plan year ending on or after such date, and

((II)) who had not withdrawn from the plan before such date.

((4))

((A)) An employer’s proportional share of the unamortized amount of the reallocated unfunded vested benefits is the sum of the employer’s proportional share of the unamortized amount of the reallocated unfunded vested benefits for each plan year ending before the plan year in which the employer withdrew from the plan.

((B)) Except as otherwise provided in regulations prescribed by the corporation, the reallocated unfunded vested benefits for a plan year is the sum of—

((i)) any amount which the plan sponsor determines in that plan year to be uncollectible for reasons arising out of cases or proceedings under title 11, or similar proceedings.

((ii)) any amount which the plan sponsor determines in that plan year will not be assessed as a result of the operation of section 1389, 1399(c)(1)(B), or 1405 of this title against an employer to whom a notice described in has been sent, and

((iii)) any amount which the plan sponsor determines to be uncollectible or unassessable in that plan year for other reasons under standards not inconsistent with regulations prescribed by the corporation.

((C)) The unamortized amount of the reallocated unfunded vested benefits with respect to a plan year is the reallocated unfunded vested benefits for the plan year, reduced by 5 percent of such reallocated unfunded vested benefits for each succeeding plan year.

((D)) An employer’s proportional share of the unamortized amount of the reallocated unfunded vested benefits with respect to a plan year is the product of—

((i)) the unamortized amount of the reallocated unfunded vested benefits (as of the end of the plan year preceding the plan year in which the employer withdraws); multiplied by

((ii)) the fraction defined in paragraph (2)(E)(ii).

((c)) ** Amendment of multiemployer plan for determination respecting amount of unfunded vested benefits allocable to employer withdrawn from plan; factors determining computation of amount**

((1)) A multiemployer plan, other than a plan which primarily covers employees in the building and construction industry, may be amended to provide that the amount of unfunded vested benefits allocable to an employer that withdraws from the plan is an amount determined under paragraph (2), (3), (4), or (5) of this subsection, rather than under subsection (b) or (d). A plan described in (relating to the building and construction industry) may be amended, to the extent provided in regulations prescribed by the corporation, to provide that the amount of the unfunded vested benefits allocable to an employer not described in shall be determined in a manner different from that provided in subsection (b).

((2))

((A)) The amount of the unfunded vested benefits allocable to any employer under this paragraph is the sum of the amounts determined under subparagraphs (B) and (C).

((B)) The amount determined under this subparagraph is the product of—

((i)) the plan’s unfunded vested benefits as of the end of the last plan year ending before , reduced as if those obligations were being fully amortized in level annual installments over 15 years beginning with the first plan year ending on or after such date; multiplied by

((ii)) a fraction—

((I)) the numerator of which is the sum of all contributions required to be made by the employer under the plan for the last 5 plan years ending before , and

((II)) the denominator of which is the sum of all contributions made for the last 5 plan years ending before , by all employers who had an obligation to contribute under the plan for the first plan year ending after , and who had not withdrawn from the plan before such date.

((C)) The amount determined under this subparagraph is the product of—

((i)) an amount equal to—

((I)) the plan’s unfunded vested benefits as of the end of the plan year preceding the plan year in which the employer withdraws, less

((II)) the sum of the value as of such date of all outstanding claims for withdrawal liability which can reasonably be expected to be collected, with respect to employers withdrawing before such plan year, and that portion of the amount determined under subparagraph (B)(i) which is allocable to employers who have an obligation to contribute under the plan in the plan year preceding the plan year in which the employer withdraws and who also had an obligation to contribute under the plan for the first plan year ending after ; multiplied by

((ii)) a fraction—

((I)) the numerator of which is the total amount required to be contributed under the plan by the employer for the last 5 plan years ending before the date on which the employer withdraws, and

((II)) the denominator of which is the total amount contributed under the plan by all employers for the last 5 plan years ending before the date on which the employer withdraws, increased by the amount of any employer contributions owed with respect to earlier periods which were collected in those plan years, and decreased by any amount contributed by an employer who withdrew from the plan under this part during those plan years.

((D)) The corporation may by regulation permit adjustments in any denominator under this section, consistent with the purposes of this subchapter, where such adjustment would be appropriate to ease administrative burdens of plan sponsors in calculating such denominators.

((3)) The amount of the unfunded vested benefits allocable to an employer under this paragraph is the product of—

((A)) the plan’s unfunded vested benefits as of the end of the plan year preceding the plan year in which the employer withdraws, less the value as of the end of such year of all outstanding claims for withdrawal liability which can reasonably be expected to be collected from employers withdrawing before such year; multiplied by

((B)) a fraction—

((i)) the numerator of which is the total amount required to be contributed by the employer under the plan for the last 5 plan years ending before the withdrawal, and

((ii)) the denominator of which is the total amount contributed under the plan by all employers for the last 5 plan years ending before the withdrawal, increased by any employer contributions owed with respect to earlier periods which were collected in those plan years, and decreased by any amount contributed to the plan during those plan years by employers who withdrew from the plan under this section during those plan years.

((4))

((A)) The amount of the unfunded vested benefits allocable to an employer under this paragraph is equal to the sum of—

((i)) the plan’s unfunded vested benefits which are attributable to participants’ service with the employer (determined as of the end of the plan year preceding the plan year in which the employer withdraws), and

((ii)) the employer’s proportional share of any unfunded vested benefits which are not attributable to service with the employer or other employers who are obligated to contribute under the plan in the plan year preceding the plan year in which the employer withdraws (determined as of the end of the plan year preceding the plan year in which the employer withdraws).

((B)) The plan’s unfunded vested benefits which are attributable to participants’ service with the employer is the amount equal to the value of nonforfeitable benefits under the plan which are attributable to participants’ service with such employer (determined under plan rules not inconsistent with regulations of the corporation) decreased by the share of plan assets determined under subparagraph (C) which is allocated to the employer as provided under subparagraph (D).

((C)) The value of plan assets determined under this subparagraph is the value of plan assets allocated to nonforfeitable benefits which are attributable to service with the employers who have an obligation to contribute under the plan in the plan year preceding the plan year in which the employer withdraws, which is determined by multiplying—

((i)) the value of the plan assets as of the end of the plan year preceding the plan year in which the employer withdraws, by

((ii)) a fraction—

((I)) the numerator of which is the value of nonforfeitable benefits which are attributable to service with such employers, and

((II)) the denominator of which is the value of all nonforfeitable benefits under the plan

((D)) The share of plan assets, determined under subparagraph (C), which is allocated to the employer shall be determined in accordance with one of the following methods which shall be adopted by the plan by amendment:

((i)) by multiplying the value of plan assets determined under subparagraph (C) by a fraction—

((I)) the numerator of which is the value of the nonforfeitable benefits which are attributable to service with the employer, and

((II)) the denominator of which is the value of the nonforfeitable benefits which are attributable to service with all employers who have an obligation to contribute under the plan in the plan year preceding the plan year in which the employer withdraws;

((ii)) by multiplying the value of plan assets determined under subparagraph (C) by a fraction—

((I)) the numerator of which is the sum of all contributions (accumulated with interest) which have been made to the plan by the employer for the plan year preceding the plan year in which the employer withdraws and all preceding plan years; and

((II)) the denominator of which is the sum of all contributions (accumulated with interest) which have been made to the plan (for the plan year preceding the plan year in which the employer withdraws and all preceding plan years) by all employers who have an obligation to contribute to the plan for the plan year preceding the plan year in which the employer withdraws; or

((iii)) by multiplying the value of plan assets under subparagraph (C) by a fraction—

((I)) the numerator of which is the amount determined under clause (ii)(I) of this subparagraph, less the sum of benefit payments (accumulated with interest) made to participants (and their beneficiaries) for the plan years described in such clause (ii)(I) which are attributable to service with the employer; and

((II)) the denominator of which is the amount determined under clause (ii)(II) of this subparagraph, reduced by the sum of benefit payments (accumulated with interest) made to participants (and their beneficiaries) for the plan years described in such clause (ii)(II) which are attributable to service with respect to the employers described in such clause (ii)(II).

((E)) The amount of the plan’s unfunded vested benefits for a plan year preceding the plan year in which an employer withdraws, which is not attributable to service with employers who have an obligation to contribute under the plan in the plan year preceding the plan year in which such employer withdraws, is equal to—

((i)) an amount equal to—

((I)) the value of all nonforfeitable benefits under the plan at the end of such plan year, reduced by

((II)) the value of nonforfeitable benefits under the plan at the end of such plan year which are attributable to participants’ service with employers who have an obligation to contribute under the plan for such plan year; reduced by

((ii)) an amount equal to—

((I)) the value of the plan assets as of the end of such plan year, reduced by

((II)) the value of plan assets as of the end of such plan year as determined under subparagraph (C); reduced by

((iii)) the value of all outstanding claims for withdrawal liability which can reasonably be expected to be collected with respect to employers withdrawing before the year preceding the plan year in which the employer withdraws.

((F)) The employer’s proportional share described in subparagraph (A)(ii) for a plan year is the amount determined under subparagraph (E) for the employer, but not in excess of an amount which bears the same ratio to the sum of the amounts determined under subparagraph (E) for all employers under the plan as the amount determined under subparagraph (C) for the employer bears to the sum of the amounts determined under subparagraph (C) for all employers under the plan.

((G)) The corporation may prescribe by regulation other methods which a plan may adopt for allocating assets to determine the amount of the unfunded vested benefits attributable to service with the employer and to determine the employer’s share of unfunded vested benefits not attributable to service with employers who have an obligation to contribute under the plan in the plan year in which the employer withdraws.

((5))

((A)) The corporation shall prescribe by regulation a procedure by which a plan may, by amendment, adopt any other alternative method for determining an employer’s allocable share of unfunded vested benefits under this section, subject to the approval of the corporation based on its determination that adoption of the method by the plan would not significantly increase the risk of loss to plan participants and beneficiaries or to the corporation.

((B)) The corporation may prescribe by regulation standard approaches for alternative methods, other than those set forth in the preceding paragraphs of this subsection, which a plan may adopt under subparagraph (A), for which the corporation may waive or modify the approval requirements of subparagraph (A). Any alternative method shall provide for the allocation of substantially all of a plan’s unfunded vested benefits among employers who have an obligation to contribute under the plan.

((C)) Unless the corporation by regulation provides otherwise, a plan may be amended to provide that a period of more than 5 but not more than 10 plan years may be used for determining the numerator and denominator of any fraction which is used under any method authorized under this section for determining an employer’s allocable share of unfunded vested benefits under this section.

((D)) The corporation may by regulation permit adjustments in any denominator under this section, consistent with the purposes of this subchapter, where such adjustment would be appropriate to ease administrative burdens of plan sponsors in calculating such denominators.

((E)) ** .—** Notwithstanding paragraph (1), a plan may be amended to provide that the withdrawal liability method described in subsection (b) shall be applied by substituting the plan year which is specified in the amendment and for which the plan has no unfunded vested benefits for the plan year ending before .

((d)) ** Method of calculating allocable share of employer of unfunded vested benefits set forth in subsection (c)(3); applicability of certain statutory provisions**

((1)) The method of calculating an employer’s allocable share of unfunded vested benefits set forth in subsection (c)(3) shall be the method for calculating an employer’s allocable share of unfunded vested benefits under a plan to which , or a continuation of such a plan, applies, unless the plan is amended to adopt another method authorized under subsection (b) or (c).

((2)) Sections 1384, 1389, 1399(c)(1)(B), and 1405 of this title shall not apply with respect to the withdrawal of an employer from a plan described in paragraph (1) unless the plan is amended to provide that any of such sections apply.

((e)) ** Reduction of liability of withdrawn employer in case of transfer of liabilities to another plan incident to withdrawal or partial withdrawal of employer** In the case of a transfer of liabilities to another plan incident to an employer’s withdrawal or partial withdrawal, the withdrawn employer’s liability under this part shall be reduced in an amount equal to the value, as of the end of the last plan year ending on or before the date of the withdrawal, of the transferred unfunded vested benefits.

((f)) ** Computations applicable in case of withdrawal following merger of multiemployer plans** In the case of a withdrawal following a merger of multiemployer plans, subsection (b), (c), or (d) shall be applied in accordance with regulations prescribed by the corporation; except that, if a withdrawal occurs in the first plan year beginning after a merger of multiemployer plans, the determination under this section shall be made as if each of the multiemployer plans had remained separate plans.