Steel castings scrape the sky.
* The current uses of steel castings show they can meet the needs of the construction industry. The challenge for steel metalcasters is getting their customers to understand that.
* Detailed within are examples of what steel metalcasters and their customers need to know to be able to produce castings for building construction applications.
Steel cast components often are overlooked in the construction of new buildings. While typically used in safety-critical applications of harsh, demanding environments to carry significant loads, they are welded into a fabricated structure. But in doing this, steel cast components are not given the chance to live up to their full potential.
As steel metalcasters, you must understand the limitations design engineers have placed on steel cast components. Only through this understanding can steel metalcasters equip themselves with the knowledge to overcome this obstacle and increase cast component use in this industry.
Through their uses in other applications, steel castings have displayed the capability of providing unlimited geometric possibilities. In industrial equipment, steel castings are used as connectors to perform demanding tasks while holding other structural elements together. If they were put to this same use in building construction, they could prove to be an attractive option to improve the overall performance of structures while lowering the total cost. The key to bringing steel castings into the mainstream of building construction is knowledge.
This article looks at what steel metalcasters need to know about the requirements of the building construction industry and provides some examples of how steel castings meet those requirements. First, it establishes a foundation of steel use in other applications and then relates it to the construction industry. This knowledge allows metalcasters to go to their customers and show them what they can do.
How do we know steel castings can be successful in building construction applications? The answer--they have already succeeded as connecting components in high-stress environments. The most common application of steel castings is couplers for the railroad industry (Fig. 1). The electric utilities use large, dedicated trains to move coal from the Powder River Basin to the Midwest. These trains are commonly made up of 120 freight cars that weigh 286,000 lbs. each, for a total train weight of more than 13,000 tons. This train is powered by two sets of 12,000-hp. locomotives for a total of 24,000 hp.
[FIGURE 1 OMITTED]
This whole system is connected in the center by one set of steel castings in the form of a coupler, illustrating that steel castings are capable of safe, economical, reliable, simple and repetitive performance.
In fittings for pipes, castings not only provide a flow path, they are a structural connector. The fitting is a successful structural component as long as it outlives the pipe. Since the cast steel fitting cross-section is maintained larger than the pipe cross-section, it assures the pipe will fail first.
So if connections are a problem and steel castings offer an answer, why are castings not being used?
Most of the problems in using steel castings in building applications seem to be related to a lack of understanding on both sides. Steel metalcasters do not understand the requirements or needs of the building construction industry. At the same time, designers, fabricators and erectors may not understand the benefits of castings.
Making a Connection
Connections are a critical feature of building construction. While they rarely account for more than 5% of total structure weight, they are typically 60% of the cost. Moment and special bracing connections are especially difficult and costly. The breakdown of the cost for steel construction in 2001 was 25% material, 33% shop, 28% erector and 14% other. But labor-related costs exceeded 60% of the cost of construction.
Modular cast connectors could reduce labor and erection costs, improve performance, decrease erection time, easily transfer loads from one shape beam of column to another, enhance reliability of the connection, and reduce engineering and detailing costs. In seismic applications, they could cost-effectively improve the safety and reliability of the structure. For special architectural features, steel castings could provide innovative and attractive transitions between shapes of unique designs.
Good connections must support the loads and satisfy the cede and specification requirements. At the same time, they must perform safely and economically, and be simple and repetitive. Ideally, they will fabricate and erect easily while minimizing the labor required.
The most common troubles with connections include:
* fit up and access at the site;
* failure to clearly satisfy the code requirements;
* incorrect interpretation of drawings;
* lack of needed information on the drawing (e.g. loads);
* poor match between member sizes;
* high cost of fabrication and erection.
Steel castings are capable of meeting these requirements. They also can be attractive as connectors because of the freedom they provide in the design process. Good casting design allows weight to be reduced, cost to be lowered and performance to be improved. Castings also work more effectively in big sweeping curves and non-uniform sections with complex geometry.
One example of steel castings providing shape and performance in a structure is their use as nodes in offshore oil platforms (Fig. 2). The casting is designed to perform in a demanding, high-stress environment and a corrosive atmosphere. It weighs 20% less than a fabricated connection and moves the welds to the hollow structural sections outside the high-load regions of the structure to prevent failure of a welded joint in a high-stress region. The steel cast nodes are designed so the pipes drive the loads into the casting. The casting geometry and section size are tailored to handle the requirements of the application.
[FIGURE 2 OMITTED]
When producing castings for use in building construction, it is important to remind customers not to believe everything they hear. It is a popular notion that cast steel is brittle because the cast iron commonly used in automotive and household goods cracks easily. Customers may need to be reminded that the properties of steel are very different from iron.
Steel castings can meet or exceed the ductility, toughness and weldability of rolled steels. Designers generally think of design requirements in terms of strength, but the design is commonly constrained by modulus, fatigue, toughness of ductility.
Increasing the strength of steel normally reduces the ductility, toughness and weldability. It is often more desirable in a steel casting design to use a lower strength grade and increase the section size of modify the shape. The design freedom makes castings an attractive way to obtain the best fabrication and material performance as well as the required stillness and strength.
Rolled sections of steel have their structure elongated in the direction of rolling. The strength and ductility is improved in that direction but they are reduced across the rolling direction. The lack of a rolling direction in steel castings gives them uniform proper ties in all directions. Rolling steel cold can add strength, but it reduces ductility and toughness. Cast steel grades achieve the same made off by alloying and heat treatment.
The dominant material used in building construction is carbon steel be cause of its reliable properties, low cost and ease of fabrication. One common grade used for building construction in rolled sections is ASTM A36. The use of steel castings is permitted in building construction using material from either ASTM A27 grade 65-35 of ASTM A148 grade 80-50.
The properties of carbon steel depend on the composition and heat treatment. Because designers use yield strength as a basic property in design, material is often ordered to higher strength without considering the advantage of using a lower-strength material with optimum ductility and weldability.
Since the load carrying cross-section can be increased to accommodate lower strengths, the casting can be supplied in the highest ductility with strength levels that are compatible to the rolled structural shapes. This use of cast carbon steel in its optimal condition ensures the casting will perform safely and reliably and that excessive loads will cause failure to occur first in the rolled section.
Already In Use
While steel castings are not widely used in building construction, there are some instances where they have been put to good use. One example is the development of a modular connection (Fig. 3) in a bridge. A self-aligning beam-to-column connection was designed to improve safety and productivity in erection.
[FIGURE 3 OMITTED]
This connection used a wedge-shaped extension on the beam that slid into a wedge-shaped slot on the column. The production of the complex wedge and slot was accomplished with steel castings. It was subjected to full-scale mechanical performance tests. When loaded beyond the design requirement, it deformed plastically but did not rail catastrophically.
A gusseted reinforced "L" bracket was designed as a carbon steel casting and was tested for earthquake retrofitting of damaged and undamaged structures in California (Fig. 4). The connection was designed to be installed where welds had failed by bolting it to the bottom of the beam-column connection. A prototype was cast, tested and approved by the State of California. The test demonstrated that the cast connector would survive the maximum load required.
[FIGURE 4 OMITTED]
An example of steel casting's advantages for high-performance complex connections is shown in some recent work by Robert Fleischman at the Univ. of Arizona for designing seismic connectors. Since castings can have non-uniform walls and contain complex features, they can be designed to locate the strain deformation of a loaded structure.
A cast modular node was produced that looks nominally like a reinforced welded connection.
In reality, the casting process allows the intersection of the beam and column to be increased while the column and panel section can be tailored to absorb the deformation with little transferred load to the beam or column. The welds can be made outside of the node. This panel zone part, as-cast and after test, is shown in Fig. 5.
[FIGURE 5 OMITTED]
A cast modular connector also was developed and evaluated using finite element modeling and casting solidification simulation to provide an effective design (Fig. 6). A full-size test subassembly was fabricated and exceeded the requirements of the FEMA 350 cyclic test protocol.
[FIGURE 6 OMITTED]
Other regions of the world also are using cast components to achieve unique structures. Figure 7 shows Stuttgart Stadium, Stuttgart, Germany, and a cast node for tubular hollow structural sections that is welded into a structure and a complex cast base.
[FIGURE 7 OMITTED]
Steel castings provide new opportunities for lower costs, improved performance and unique designs. The ability to repeatedly make complex shapes allows for the design of modular connectors that are reasonable in cost and reduce shop and erection costs. The ability to tailor geometry, customize steel properties and integrate castings by welding allows for improved performance. And the freedom of geometry, size and complexity allows designers unprecedented flexibility.
Because of this freedom, steel castings offer architectural and structural flexibility that can challenge building designers' imaginations. The challenge for metalcasters, however, is getting their customers to understand that.
This article was adapted from an article in the Spring 2004 issue of Engineered Casting Solutions.
For More Information
"Castings in Building Construction: Breakthrough Opportunity," A. Johnson, 2002 Steel Founders' Society of America Technical & Operating Conference.
"Mechanical Properties of Cast Carbon and Low-Alloy Steel," Steel Founders' Society of America, Engineered Casting Solutions, Summer 2003.
Raymond Monroe is the executive vice president and David Poweleit is a senior design engineer at the Steel Founders' Society of America, Crystal Lake, Ill.
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|Article Type:||Cover Story|
|Date:||May 1, 2004|
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