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<?xml version="1.0" encoding="windows-1252"?> |
<ownershipDocument> |
| <schemaVersion> X0306 </schemaVersion> |
| <documentType> 4 </documentType> |
| <periodOfReport> 2020-02-14 </periodOfReport> |
| <notSubjectToSection16> 0 </notSubjectToSection16> |
| <issuer> |
| | <issuerCik> 0000352825 </issuerCik> |
| | <issuerName> FOSTER L B CO </issuerName> |
| | <issuerTradingSymbol> FSTR </issuerTradingSymbol> |
| | </issuer> |
| <reportingOwner> |
| | <reportingOwnerId> |
| | | <rptOwnerCik> 0001375886 </rptOwnerCik> |
| | | <rptOwnerName> Kelly Brian H </rptOwnerName> |
| | | </reportingOwnerId> |
| | <reportingOwnerAddress> |
| | | <rptOwnerStreet1> 415 HOLIDAY DRIVE </rptOwnerStreet1> |
| | | <rptOwnerStreet2></rptOwnerStreet2> |
| | | <rptOwnerCity> PITTSBURGH </rptOwnerCity> |
| | | <rptOwnerState> PA </rptOwnerState> |
| | | <rptOwnerZipCode> 15220 </rptOwnerZipCode> |
| | | <rptOwnerStateDescription></rptOwnerStateDescription> |
| | | </reportingOwnerAddress> |
| | <reportingOwnerRelationship> |
| | | <isDirector> 0 </isDirector> |
| | | <isOfficer> 1 </isOfficer> |
| | | <isTenPercentOwner> 0 </isTenPercentOwner> |
| | | <isOther> 0 </isOther> |
| | | <officerTitle> Sr. Vice President </officerTitle> |
| | | <otherText></otherText> |
| | | </reportingOwnerRelationship> |
| | </reportingOwner> |
| <nonDerivativeTable> |
| | <nonDerivativeTransaction> |
| | | <securityTitle> |
| <value> Common Stock </value> |
| </securityTitle> |
| | | <transactionDate> |
| <value> 2020-02-14 </value> |
| </transactionDate> |
| | | <deemedExecutionDate></deemedExecutionDate> |
| | | <transactionCoding> |
| <transactionFormType> 4 </transactionFormType> |
| <transactionCode> F </transactionCode> |
| <equitySwapInvolved> 0 </equitySwapInvolved> |
| </transactionCoding> |
| | | <transactionAmounts> |
| <transactionShares> |
| <value> 1794 </value> |
| </transactionShares> |
| <transactionPricePerShare> |
| <value> 18.03 </value> |
| </transactionPricePerShare> |
| <transactionAcquiredDisposedCode> |
| <value> D </value> |
| </transactionAcquiredDisposedCode> |
| </transactionAmounts> |
| | | <postTransactionAmounts> |
| <sharesOwnedFollowingTransaction> |
| <value> 32623 </value> |
| </sharesOwnedFollowingTransaction> |
| </postTransactionAmounts> |
| | | <ownershipNature> |
| <directOrIndirectOwnership> |
| <value> D </value> |
| </directOrIndirectOwnership> |
| </ownershipNature> |
| | | </nonDerivativeTransaction> |
| | <nonDerivativeHolding> |
| | | <securityTitle> |
| <value> Common Stock </value> |
| </securityTitle> |
| | | <postTransactionAmounts> |
| <sharesOwnedFollowingTransaction> |
| <value> 531 </value> |
| </sharesOwnedFollowingTransaction> |
| </postTransactionAmounts> |
| | | <ownershipNature> |
| <directOrIndirectOwnership> |
| <value> I </value> |
| </directOrIndirectOwnership> |
| <natureOfOwnership> |
| <value> L.B. Foster Company 401(k) Plan Shares </value> |
| </natureOfOwnership> |
| </ownershipNature> |
| | | </nonDerivativeHolding> |
| | </nonDerivativeTable> |
| <derivativeTable></derivativeTable> |
| <footnotes></footnotes> |
| <ownerSignature> |
| | <signatureName> /s/ Brian H. Kelly by Heidi Loeffert, attorney-in-fact </signatureName> |
| | <signatureDate> 2020-02-18 </signatureDate> |
| | </ownerSignature> |
| </ownershipDocument> |