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Fwd: HOT-DIP ZINC COATINGS AND OVERLAPPING SURFACES

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I AM SEEKING ADVICE ON THE STANDARD PRACTICE
FOR FABRICATING STRUCTURES TO BE HOT-DIP
GALVANIZED.  THE STRUCTURE IN MY CASE IS A LIGHT
WEIGHT STEEL TRUSS BRIDGE EXPOSED TO THE
ENVIRONMENT OF THE OHIO RIVER, TOP AND BOTTOM
CHORDS OF WT8x13 SECTIONS, WITH VERTICAL AND
DIAGONAL MEMBERS OF ANGLES WELDED TO THE
STEM OF THE WT SECTIONS (SINGLE DIAG. ANGLES
2.5x2.5x1/4 AND DOUBLE VERTICAL ANGLES 2x2x3/16). 
THESE CONNECTIONS NECESSARILY CREATE
OVERLAPPING SURFACES BETWEEN THE WELDED
MEMBERS INTO WHICH THE MOLTEN ZINC CANNOT
FLOW.

REFERRING TO ASTM A-384 AND ASTM A-385 I FIND THE
FOLLOWING ADVICE ON OVERLAPPING SURFACES:
		
ASTM A 384, Sect. 4.4 states: ?All edges of tightly
contacting surfaces should be completely sealed by welding.
 This will prevent rusting of the surfaces which are so
connected that molten zinc cannot circulate through the
crevices to galvanize the contacting surfaces?.
	
ASTM A 385, Sect. 5., states: ?Overlapping or contacting
surfaces that have not had all edges seal welded are
undesirable.  When the distance between the overlapping
surfaces is less than 3/32 in., these surfaces will not
normally be wet by molten zinc. ... Such uncoated surfaces
cause a rust staining after exposure to the environment?.

THIS SEEMS TO ME TO BE VERY STRAIGHT FORWARD. 
THE ASTM STANDARDS CLEARLY EXPLAIN WHAT
SHOULD BE DONE AND WHAT TO EXPECT
(CORROSION) IF SUCH MEASURES ARE NOT TAKEN. 
HOWEVER, DETAILS SUCH AS DOUBLE ANGLES
WELDED TO THE STEM OF A WT SECTION, OR TO A
CONNECTION PLATE PREVENT ACHIEVING A
COMPLETE SEAL WELD.  

MY QUESTIONS ARE:

1.  WOULD LACK OF GALVANIC PROTECTION BETWEEN
THE OVERLAPPING SURFACES OF THE NODE
CONNECTIONS APPRECIABLY SHORTEN THE
RELIABILITY OR SERVICE LIFE OF THE STRUCTURE?

2.  IS THE ADDED COST OF SEAL WELDS JUSTIFIED?

3.  IN CASES SUCH AS THAT OF DOUBLE ANGLES,
WOULD ADEQUATE PROTECTION BE PROVIDED BY 
"SEALING" THE INACCESSIBLE EDGES WITH A ZINC
SOLDER STICK?

4. WHAT OTHER INSIGHTS CAN YOU OFFER?

THANK YOU, LEROY SMITH



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From: LEROY SMITH <LEROYS(--nospam--at)mail.orh.usace.army.mil>
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Cc: LEROYS(--nospam--at)mail.orh.usace.army.mil
Subject:  HOT-DIP ZINC COATINGS AND OVERLAPPING SURFACES
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