How to Specify Cast Steel Tree Nodes

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September 25, 2019

How to Specify Cast Steel Tree Nodes

Cast steel nodes are frequently utilized at the junctions between structural members to simultaneously address the requisite structural loading on the junction and provide an aesthetically pleasing form. In such structural configurations where multiple branches extend from a shared origin, steel casting manufacturing enables an economical means of forming transitional geometry.

The specification of cast steel tree nodes in contract documents is very straightforward when a delegated design approach is utilized.

To begin with, we recommend that you include our sample specification for custom designed castings in your project’s Division 5 specification section. This specification language clarifies roles and responsibilities of the steel casting supplier and structural steel fabricator; stipulates the various casting design and production submittals; establishes qualifications for the casting supplier’s quality inspectors, engineers, and manufacturing qualifications; and provides information related to various casting design parameters. Various aspects of this specification will be referenced in the reminder of this explainer.

Next, it is necessary to diagrammatically show and call out the cast steel nodes on the structural drawings (and perhaps also on the architectural drawings if the cast nodes are to be architecturally exposed). It is neither necessary nor advisable for building design practitioners to fully dimension and detail the cast nodes in the contract documents, as casting design requires a thorough understanding of casting manufacturing methodologies and constraints, metallurgy, non-destructive examination practices and acceptance criteria for steel castings, machining means and methods, tolerancing, and related structural steel fabrication. However, there are some particulars that must be shown in the contract documents to adequately convey design intent and to appropriately transfer design responsibility for castings to the casting supplier, as described below.

Below is an excerpt from structural drawings prepared by BVH Integrated Services for Steffian Bradley Architects in relation to an entranceway canopy at Baystate Noble Hospital in Westfield, Massachusetts.

The following key items are shown on this drawing:

• Identification of the node as a steel casting

• Centerline geometry of the steel framing

• Extents of the cast node (in this case, the length of the cast node’s “arms” was provided, as measured from the node’s central workpoint)

• Identification of the means by which the structural elements are to be adjoined to the cast node (in this example, the SEOR has indicated that the round HSS members are to be welded to the cast node)

• Structural loading that the node must be engineered to be capable of resisting (note that loading can also be provided in tabular format on the drawings or via alternative digital information exchange – i.e. force export from analysis software or an analysis model file)

• Clear identification of elements to be designated as Architecturally Exposed Structural Steel and which category per AISC or CISC (in this

example the cast node, the round HSS column members, and the WT members of the canopy roof are all simply designated as “AESS” as this project

preceded the adoption of AESS categories in the AISC code)

• Any features or shaping constraints that are to be respected or are desired should also be indicated on the drawings (although not applicable in the above example, features including reveals (described below) or particular shaping characteristics that the design team is looking to achieve with the casting should be illustrated and, when necessary or appropriate, also dimensioned)

When establishing the centerline geometry of the steel framing and the extents of the cast node, consideration should be given to weld access. For instance, in the above example the cast node’s “arms” need be long enough to enable welding of each of the upper HSS members to the node all the way around the circumference of the HSS member.

PROTIP: Eccentricity can be introduced into the centerline geometry of the steel framing to reduce the required overall size of the cast node, as illustrated in the example below from The Leaf at Canada’s Diversity Gardens, as designed by Architecture 49 with KPMB Architects and Blackwell Structural Engineers. These framing eccentricities should be incorporated into your structural analysis model of the steel framing so that secondary bending and shear forces can be captured in the structural analysis of the frame.

Some additional items to consider when cast steel nodes and the associated structural steel framing will be architecturally exposed:

Casting Surface Finish

Selection of casting surface finish should be made keeping in mind the coating system that will be applied to the architecturally exposed structural steel incorporating the casting as well as the viewing distance – that is, the shortest distance between a building occupant and the casting in question. If castings will only be coated with a thin-build coating system, surface finish is of particular importance. If the castings will be coated with a thick-build, multi-layer coating system or with intumescent coatings, the casting’s surface finish is less critical. Similarly, if building occupants cannot get up close to the casting, its surface finish is again less critical.

The typical surface finish options that are available from most foundries are those described in ASTM standard ASTM A802. ASTM A802 standard references “SCRATA Reference Comparator Plates” which are visuotactile plates that are to be compared against the surface of the casting being examined. There are different sets of comparator plates for each type of visual discontinuity (i.e. thermal metal removal marks, grind marks, weld repairs, gas porosity, etc). ASTM A802 provides four options – level 1, 2, 3, and 4 – with level 1 being the “best” surface finish and 4 being the “worst”. Section 2.2.A.2.b. of our sample specification for custom designed castings calls for the surface finish of the castings to comply with ASTM A802 Level 1. Contact CAST CONNEX for other, higher quality surface finish options that we have developed for cases where surface finish is particularly critical.

Grinding of Welds

When castings are employed in AESS, and particularly when viewing distances are within 20-feet, it is quite common for designers to stipulate that welded joints between the casting and incoming structural steel members be ground smooth and flush. Section 2.2.A.2.c. of our sample specification for custom designed castings articulates this requirement, but also provides the option to make use of automotive body filler (i.e. “bondo”) to assist in masking the welded joint when appropriate. 

Architectural Reveals

Purposeful reveals incorporated into a cast node’s design can be used to accentuate the node and to visually break the cast steel from the adjoining structural steel, which can offer some leeway with respect to the required surface finish of the casting. Reveals can be recessed or raised and are typically provided on the casting, proximal to the welded joint preparation. Reveals also draw the eye away from the location of the welded joint between the casting and the incoming structural member, thereby also providing some leeway with respect to the masking/grinding smooth of the welded joints. This is particularly helpful if field welding is required, as blending of field applied welds can be challenging. 

Exterior Shaping

The exterior shaping of cast nodes impacts the visual appearance of the frames in which they're used. For example, pictured below are 3D printed models showing three different exterior shaping configurations for the same centerline

geometry. Note how the node's shaping impacts the way that the members appear to "flow" in the structure. Ultimately, KPMB Architects opted for the shaping which created a more vertical articulation in the steel framing. 

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