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Architecturally Exposed Structural Steel (AESS)

San Diego Library Aess 775X519
San Diego Central Library - San Diego, CA

Specifying Hot-Dip Galvanized AESS Elements is Simplified through Proper Communication & Specification Practices

Hot-dip galvanized (HDG) steel has many benefits beyond protecting steel from corrosion. It has an attractive aesthetic appeal due to its neutral gray color palette that provides an excellent contrast to surrounding building materials and landscaping elements. The HDG coating (zinc) enhances the appearance unfinished steel while providing a durable, corrosion resistant, maintenance-free and sustainable coating. Additionally, hot-dip galvanized steel can be top-coated with paint or powder coatings to provide limitless appearances all while increasing the corrosion protection beyond what the HDG coating can provide alone. As a result, architects are turning to HDG steel to provide a unique, attractive aesthetic in their AESS projects.

When utilizing hot-dip galvanized steel in AESS projects, additional requirements beyond the typical ASTM galvanizing specifications (ASTM A123, ASTM A143, ASTM A153, ASTM A384, ASTM A385, ASTM A780) are necessary both before and after galvanizing to meet the aesthetic requirements for each AESS category. The majority of products galvanized to these standards do not require heightened aesthetics; therefore, the presence of unattractive surface conditions are allowed as long as they do not affect the corrosion resistance of the coating or intended use of the product. To maximize hot-dip galvanized appearance for AESS a combination of these ASTM standards and specific AESS practices are necessary.

Using the AESS C Category

The AESS categorical system was developed to provide clear expectations between the specifier and the fabricator to achieve the desired architectural finish. However, the requirements defined in the AISC/CISC categories (1, 2, 3, 4) do not address all the nuances of hot-dip galvanized coatings. Some practices required for each of these categories will have little to no effect on the initial galvanized coating appearance, whereas other, additional practices can greatly improve it (Figure 1).

Aga Hdg Aess Steel Sample 2 775X265
Figure 1: Left to Right: AESS C/1, AESS C/2, AESS C/3, AESS C/4

For example, the nature of the galvanizing process can result in aesthetic limitations in overall uniformity, smoothness, coating repairs, and the contouring and blending of mill markings, welds, and seams. These topics are discussed in detail in the AGA publication Hot-Dip Galvanized Architecturally Exposed Structural Steel Guide.

To define both practical exemptions and additional requirements necessary to achieve an equivalent aesthetic for each AESS category, it is recommended to use the Custom (AESS C) category. A modified AESS Category Matrix Table for HDG (Table 1, below) is provided as a guide and is modeled after the AISC/CISC AESS Category Matrix. The use of AESS C is not limited to these practices, but those noted using an “x” are highly recommended to maximize HDG appearance. Responsibility for each category characteristic in Table 1 shall be mutually determined by the galvanizer, fabricator, and erector prior to fabrication.

Modified AESS Category Matrix for HDG
I.D.Category CharacteristicsAESS C/4
1.1Surface preparation to SSPC-SP 6xxxx
1.2Sharp edges ground smoothxxxx
1.3Continuous weld appearancewaivedwaivedwaivedwaived
1.4Standard structural boltsxxxx
1.5Weld splatters removedxxxx
2.1Visual samplesmock-up req'dmock-up req'doptionaloptional
2.2One-half standard fabrication tolerancesxxx
2.3Fabrication marks not apparentxxx
2.4Welds uniform and smoothxxx
3.1Mill marks removedxx
3.2Butt/plug welds ground smooth & filedxx
3.3HSS weld seam orientated per contract documentsxx
3.4Cross sectional abutting surface alignedxx
3.5Joint gap tolerances minimizedxx
3.6All welded connectionswaivedwaived
4.1HSS seam not apparentxoptional
4.2Welds contoured and blendedx
4.3Surfaces filled and sandedx
4.4Weld show-through minimizedx
Custom Characteristics for Batch Hot-Dip Galvanizing Beyond ASTM A123 Requirements
C.1Weld show-through minimized again after HDGxxxx
C.2Flame cut edges grind 1/16" including cut facesxxxx
C.3HDG skimmings removedxxxx
C.4Washers underneath bolt headsxxxx
C.5HDG repair material: TBCxxxx
C.6Special care in fabrication and erectionxxxx
C.7HDG zinc splatter removedxxxoptional
C.8Plug and smooth round vent/drain holesxxoptionaloptional
C.9Wet storage stain prevention proceduresxxoptionaloptional
C.10HDG cosmetic touch-ups using ZRP sprayxoptionaloptionaloptional
C.11Low silicon welding electrodesoptionaloptionaloptionaloptional
C.12Increased venting/drainage designs for HDGxxx
C.13HDG dross smoothed to projections < 1/16"xxoptional
C.14HDG runs smoothed to projections < 1/16"xxoptional
C.15Drilled vent/drain holes onlyoptionaloptionaloptional
C.16HDG handling marks smoothed to projections < 1/16"xx
C.17Progressive dip galvanizing avoidedxx
C.18Soft/padded slings, lifting devices, and restsxx
C.19HDG roughness smoothed to projections < 1/16"xoptional
C.20Welds contoured and blended again after HDGx

Table 1: Modified AESS Category Matrix Table for HDG (Modeled after the AISC/CISC AESS Category Matrix)