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Cast Connex sources Cast construction part: Cast Connex Corp. worked with its metal casting supplier to produce a construction frame connector that met several design criteria.

Cast Connex Corp., Toronto, had an idea for a new way to connect steel construction frames. But it had to satisfy a critical design concern to do it. Concentrically braced frames are widely used in steel structures because of the increased stiffness, low cost and ease of construction they offer compared to other lateral reinforcement systems. The diagonal bracing members of braced frames are often hollow structural sections because of their compression load efficiency and aesthetic appearance. These braces, which come in a range of section sizes, are either welded or bolted into place in the field. Rectangular hollow section brace members are easily mill-fabricated and connected to the steel frame through tailored reinforced gusset-connector-brace systems. But the rectangular braces suffer from reduced ductility, notch toughness and compression strength because of residual stresses and stress concentrations in the section corners. Circular hollow section members avoid this corner effect but pose different connection challenges (Fig. 1).

[FIGURE 1 OMITTED]

With the help of the metalcasting process, researchers at the Univ. of Toronto, Toronto, Canada, were able to overcome these challenges and developed a new connector design for circular hollow section members. Cast Connex licensed the proprietary technology and worked with its metal casting supplier in a collaborative design effort for rapid, effective design development, process optimization and quality control management. The final steel casting connector offered a mechanical design with integrated features, reduced stress concentrations and isotropic properties, a functional design with reduced detailing and fabrication time, well-controlled welding methods, simplified assembly with straightforward field bolting, and a standardized design with wide application to different load requirements, eliminating the need for one-off connector designs.

Meeting Stringent Requirements

A structural connector in building construction has to meet stringent building code requirements, be easy to design for a range of structural forces and configurations, have low production cost, and provide simple field fabrication.

The steel connector casting developed by Cast Connex consists of a solid round section transitioning into two flat plates that are bolted to a gusset plate. Bolt attachment to a single gusset plate accommodates simple fabrication, construction and site erection. It also eliminates the need for field welding and its inherent variability (Fig. 2).

[FIGURE 2 OMITTED]

The heavy end of the connector is tapered to accept hollow round braces of different wall thickness for complete joint penetration welding. Cast Connex developed the steel connector in four sizes to connect to different outer diameter braces (4, 5.5625, 6.625 and 8.625 in. [10.16, 14.13, 16.83 and 21.91 cm]). The four connectors range in size from 14 x 7 to 27 x 14 in. (35.6 x 17.8 to 68.6 x 35.6 cm) with weights between 41 and 320 lbs. (18.6 and 145.1 kg).

A structural connector in building construction must meet the following mechanical strength requirements:

* ultimate tensile strength: 550 MPa (80 ksi);

* tensile yield strength: 345 MPa (50 ksi);

* percent elongation: 22% in 50 mm;

* reduction in area: 35%.

The steel used for the Cast Connex connector was required to have Charpy impact strength of 27 J at -20C and had to be weldable, with the finished weld having Charpy impact strength of 27 J at -30C. The surface finish requirement for the part was 450 RMS. The quality assurance requirements also were extensive, covering both casting process parameters and comprehensive mechanical testing and nondestructive evaluation of the castings.

The Casting Design Issues

Cast Connex selected Pacific Steel Casting Co., Berkeley, Calif., to produce its steel connectors. The casting design team (Cast Connex design engineers and Pacific Steel casting engineers) focused on three imperatives:

* design for performance

* design for production/castability

* design for cost

The requirements for performance, casting production and cost were closely interconnected. Three casting design issues played a major role in meeting the three design imperatives:

1. Select a steel alloy that met the mechanical requirements and reliably produced the four different sized castings in sound condition.

2. Select a molding method that produced the required dimensional tolerances and surface finish in a cost-effective manner.

3. Develop a casting and mold design that produced flaw-free, within-tolerance connectors at the best cost.

1. Steel Alloy

The Cast Connex design engineers originally considered three different steel casting alloys, as specified by ASTM standards: A27 Grade 70-40, A148 Grade 80-50 and A958 Grade 8620.

A27 Grade 70-40 steel is the strongest grade of A27, but the designers recognized it does not meet the mechanical property requirements for the steel connector. The alloy's ultimate tensile strength of 70 ksi was below the 80 ksi design goal, and its tensile yield strength of 40 ksi was below the 50 ksi goal. The elongation of the alloy met the 22% requirement, but the reduction in area value failed to meet the 35% target. In addition, the chemistry and heat treat specifications were not tight enough to permit precise control of the alloy for this demanding application.

A148 Grade 80-50 steel met all the mechanical property requirements for the steel connector, with an ultimate tensile strength of 80 ksi, tensile yield strength of 50 ksi, elongation of 22% and reduction in area of 35%. But the broad chemistry and open heat treat specifications were not tight enough to permit precise control of the alloy for the application.

A958 Grade 8620 steel met all the mechanical property requirements for the steel connector, with an ultimate tensile strength of 80 ksi, tensile yield strength of 50 ksi, elongation of 22% and reduction in area of 35%. The alloy's full maximum/minimum specification for chemistry (0.18/0.23 carbon, 0.60/1.0 manganese, 0.3/0.6 silicon, 0.35 maximum phosphorous, 0.04 maximum sulfur, 0.4/0.7 nickel, 0.4/0.60 chromium and 0.15/0.25 molybdenum) and heat treatment (austenitize at 1,650F, air quench and temper at 1,150F) ensured that the casting would meet specifications. So, the design team selected A958 Grade 8620 steel for the connector.

2. Molding Method

In addition to selecting the appropriate alloy, the casting design team worked together to determine the best molding method, which affects the surface finish and dimensional tolerances of the finished casting. The three most critical dimensional features in the connector were:

* The uniformity and the +/- tolerance of the center slot opening. If it were too narrow, the gusset wouldn't fit into the slot; too wide and the bolted plates wouldn't have surface contact with the gusset.

* The position, alignment and diameter of the bolt holes on the two plates.

* The angle, depth and uniformity of the weld shoulder on the cylinder.

(The dimensional tolerances for the castings were specified by the ISO 8062 Grade CT8. The A grade specifies the tolerances per the basic casting dimension.)

Cast Connex and Pacific Steel engineers considered two types of molding methods to meet the requirements for the steel connector: green sand and nobake sand molding. The designers weighed the relative capabilities, advantages and costs of the two molding methods and attempted to choose the one that took into account the as-cast tolerance and finish requirements and level of detail while achieving the lowest possible cost.

Green sand molding can meet the baseline requirements for the connector for surface finish, detail level and cost. But for the longer 27-in. heavy connector, green sand molding has a larger dimensional tolerance (0.12 in.) than the target tolerance requirement of 0.1 in.

Nobake sand molding also can meet the baseline requirements and offers improved surface finish. In addition, nobake molding can hold tighter dimensional tolerances at larger dimensions than green sand. It meets the target tolerance of 0.1 in. for the larger dimension, so the designers determined it was the best choice for the connector.

3. Casting Design and Mold Orientation

One of the advantages of the casting process is that detailed features can be produced in a near-net shape configuration. This capability can reduce or eliminate expensive machining steps.

The center slot of the connector is a key feature because the gusset has to fit cleanly but tightly into the slot for field assembly. Rather than machining 10-in to 18-in. cuts, the center slot was formed with a sand core. Cores are sand shapes inserted into the mold to produce interior features in the casting.

Another design choice for the connector was whether and how to produce the bolt holes in the connector plates. Bolt holes can be drilled into the finished casting or produced in the casting with features on the sand core and then finish drilled after casting. The connector did not have a standard bolt-hole configuration. In actual use, each structural designer chooses the bolt size, count and configuration based on the structural loads with which he must work. With that in mind, the designers decided to produce the connector without bolt holes and let the field engineers drill the holes required for their design.

Finally, in mold design, the orientation of the part in the mold is an important factor in producing a sound casting. The part should be oriented in the mold so that metal flow is uniform and even in the different sections of the casting and the core can be securely positioned in the mold. In the case of the steel connector, the metalcasting engineer had two options for orientation in the mold--vertical or horizontal.

In the horizontal orientation, metal would flow first into the lower plate and then into the top plate, producing non-uniform flow and fill into the two plate sections. In addition, the core forming the center slot would be oriented horizontally and tend to "float" in the melt, producing variation in the slot geometry.

In the vertical orientation, the designers determined metal flow would be uniform, with the two vertical plate sections filling evenly together. The center core also was oriented vertically and securely seated in core prints (seating slots) in the top and bottom molds. The designers therefore determined this was the preferable orientation for producing a flaw-free casting.

Producing the Part

The steel connectors are cast in a four-step process. First, the molten steel (2,850F) is poured into the assembled sand mold and allowed to cool and solidify in the mold. After cooling, the casting is removed and shaken out of the sand mold. Next, the risers are cut off and the riser stubs and flash lines are ground smooth. Finally, the casting is shot-blasted for improved surface appearance.

Heat treatment of the connectors is a critical process step to produce the required microstructure, mechanical properties and Charpy impact values. The castings are heat treated per the ASTM A958 specification (1,700F austenitizing, water quench and 1,150F temper cycle).

Following heat treating, the outside faces of the two plates are milled to the required flatness and thickness. The weld shoulder is an as-cast feature, while the holes in the connectors are machined by the end users to match their bolt pattern and connection configuration in their structural design.

Quality assurance for these structural components is driven both by building codes and specifications and requirements. Testing and inspection in production includes tensile tests and Charpy V-notch tests on test blocks from each heat, visual examination per ASTM A802, magnetic particle examination per ASTM A903 and ultrasonic examination per ASTM A609. First article components also were tested radiographically to the procedures prescribed in ASTM E446 and ASTM E186.

About the Authors

Carlos de Oliveira is chief executive officer of Cast Connex Corp., Toronto, Canada. Ryan Grant is a technical sales representative for Pacific Steel Casting Co., Berkeley, Calif. Steve Gonczy, Gateway Materials, Mount Prospect, III., compiled the material for online publication.

[ILLUSTRATION OMITTED]

RELATED ARTICLE: What Is Cast Connex Corp.?

Cast Connex Corp. manufactures standardized and customized cast steel structural components for building and bridge structures. The company retains the exclusive license rights to intellectual property developed at the Univ. of Toronto for cast steel structural connectors. Cast Connex products include a range of pre-engineered cast steel connectors for use with hollow structural section members, as well as structural engineering design and design-build services for custom cast steel components.

Carlos de Oliveira, Cast Connex Corp., Toronto, and Ryan Grant, Pacific Steel Casting Co., Berkeley, California Compiled by Steve Gonczy, Gateway Materials, Mount Prospect, Illinois
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Author:Gonczy, Steve
Publication:Modern Casting
Date:Sep 1, 2010
Words:2049
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