Iano's backfill

Structural Innovations In Seattle Highrise

Banking On A Museum (ENR, July 17, 2006) describes the construction of Seattle's Washington Mutual Center-Seattle Art Museum Downtown highrise, a 42-story office tower/museum expansion with innovative structural design and construction, including:

The building is the tallest to date in the US to use performance-based methods for its seismic design. Rather than adhere to the prescriptive seismic design requirements of the building code, structural engineers Magnusson Klemencic Associates performed a more complex and lengthy analysis of the building structure to prove that their alternative design could perform to the same levels. The result is structure with fewer architecturally intrusive elements.

The building is also the first in the US to use buckling restrained braces (BRBs). In this case, 44 BRBs within the first thirteen levels of the structure link the relatively slender concrete core to a pair of outlying concrete-filled steel pipe columns. This linked composite structure increases the core's effective depth, thereby increasing its stiffness and reducing overturning forces.

Where the main structural core is located eccentrically relative to the lower, larger floor plates, additional braced and moment frame structures provide balanced lateral force resistance to these portions of the structure.

Steel reinforcing in the "ductile concrete" core was so densly placed that it was cast with self-consolidating 10,000 psi concrete. In addition, the concrete mat foundation, constructed 95 feet below grade, ranged from 7 to 14 feet thick.

Apart from the structural design, other interesting aspects of this project include unusual financial and development relationships between the building's two banking and museum tenants, provisions for structural changes in floor configurations as occupancy between these two tenants changes over time, and a variety of innovative techniques applied to the construction process.

This article is good case study in the real-world interaction of the many forces that shape buildings and the interesting designs that can result.

More Info
Tall, skinny ... stable: Using novel technology, S.F. tower should resist quakes, gales (SFGate.com, July 2, 2006) discusses another innovative tall building designed by engineers Magnusson Klemencic Associates, this one in San Francisco. The companion Back Story also links to video and a podcast, in which engineer Ron Klemencic discusses the building's design.

Buckling Resistant Braced Frames

Technology Triumphs (Modern Steel Construction, September 2005) describes the use of "buckling restrained braces" in steel frame construction. These composite steel members are used as diagonal, lateral force resisting members in a steel braced frame. In comparison to conventional steel members, BRBs have significantly greater energy absorbing capacity and result in a structure better able to resist extreme seismic events.

BRBs are comprised of three main parts:
1) An inner steel member acts as the primary loadbearing component. This element is intended to resist the axial forces--both tension and compression--that are generated in the brace when the structure is subjected to lateral forces.

2) An outer steel tube surrounds the inner member.

3) Mortar fills the space between the outer tube and inner member. A coating on the inner member prevents bonding between the inner member and the mortar so that the inner member remains free to slide within the mortar.

Under the influence of lateral forces, a building frame naturally distorts, with columns tending toward out of plumb, and normally rectangular column/beam bays becoming parallelogram shapes. In the case of a braced frame structure, this distortion is resisted by  diagonal braces within the frame which experience axial tension or compression under these conditions. Under the extreme loading  that may accompany a major seismic event, such braces may be subjected to repeated cycles of stress reversals, well beyond the elastic limits of the material.

In the case of conventional steel bracing, the ability of the bracing members to to resist seismic forces under such conditions is limited by the tendency of the member to fail in buckling. Once buckling occurs, the member's load resisting capacity becomes compromised, and it may no longer be able to effectively resist additional stress cycles.

In contrast, with buckling restrained braces--as the name suggestions--the combined effect of the outer steel tube and the mortar infill is to restrain the brace against buckling failure. Under high compressive forces, the BRB will yield plastically, but not undergo gross geometric instability. In this way the brace remains intact and capable of absorbing additional cycles of stresses.

First developed in Japan, BRBs are gaining acceptance in North America as well. In comparison to conventional structural steel bracing members, the major benefit of using BRBs in a braced frame are its greater strength and energy absorbing capacity with less weight. Additional benefits may include simplification of bracing member connections, and reduced foundation loads.

More Info
Seismic framing technology and smart siting aid a California community college (Tech Brief, Architecture Record, 08.05) describes the use of buckling restrained braces in the San Bernadino Vallue College of California.

Calatrava's Turtle Bay Sundial Bridge

What price beauty? According to Metropolis Magazine (Buying The Bridge, November 2004) the final cost for Santiago Calatrava's footbridge over the Sacramento River for the town of Redding, California was $23.5 million, far above the original $3 to $5 million budgeted for the project.

The bridge connects the city's Turtle Bay Museum with parkland on the river's opposite shore. The bridge's 700 foot long by 23 foot wide deck is supported by a sculptural, inclined steel pylon and 14 cable stays. The steel pylon was fabricated in Vancouver, Washington and shipped to the site in sections weighing 30 to 40 tons each. In order to minimize the bridge's environmental impact, it avoids setting foot in the river or even casting shadows into the river's sensitive salmon spawning grounds. Yet the pylon does function as a sun dial, casting its shadow onto the large plaza formed at its base.

According to Modern Steel Construction's Sun Sculpture (October 2004), the project was not just a financial challenge for the client, but also a constructional challenge. The project required approximately 500 construction drawings to be completed by the construction team based on preliminary design drawings provided by Calatrava. This documentation work included:

• 3-d modeling of the original design
• Sophisticated mathematical adjustment of the 3-d model providing cambering to counteract dead and live loads on the structure
• Development of detailed descriptions of each of more than 1200 steel plates, including different angle cuts on each edge in preparation for full-penetration welding to adjacent plates
• Preparation of several scale models and an animation to assist with visualization of the bridge and its construction sequencing

Complicating this work was the fact that the pylon is a double-walled structure, with the non-parallel inner and outer walls. Detailing took almost 2 years to complete.

Recent News In Structural Steel Design

Steel Connection Design
30 Good Rules for Connection Design, Modern Steel Construction, May 2004, discusses principles of economical steel connection design. Some examples:

• Limit the number of bolt diameters used (to reduce errors in fabrication or the field).
• Avoid different grade bolts with the same diameter.
• Avoid overhead welding.
• Limit maximum fillet weld size to 5/16-inch (the maximum size that can be completed in a single pass). Longer, smaller thickness welds are preferred over shorter, thicker welds.

For structural designers and others interested in gaining a better appreciation of steel connection design friendly to fabricators and erectors, this article is a good reference.

Architectural Exposed Structural Steel
Architecturally Exposed Structural Steel, Modern Steel Construction, May 2003, discusses guidelines for the design and specification of exposed steel structure. This lengthy article includes extensive sample specification language, commentary, color photographs, and detailed cost data. This article is recommended reading for architects and specifiers concerned with this type of construction.

The information contained in this article is also available on the American Institute of Steel Construction web site's AESS Guide Specification page. This information has also appeared as a continuing eductation series article in the 06.04 issue of Architecture Record magazine.

Propriety Steel Connection for Seismic Load Conditions
SOM receives patent for novel seismic structural joint, Building Design & Construction, describes a new structural steel joint system designed and patented by architecture/engineering firm Skidmore, Owings & Merrill. The "Pin-Fuse" joint is a hinged connection that remains rigid under moderate structural loads. However under extreme seismic load conditions, the joint may rotate while dissapating the dynamic energy of the seismic forces. According to the article, the Pin-Fuse joint can be used with either steel or concrete structural frames, and should allow reduced structural frame member sizes in comparison to alternative design strategies for such extreme loadings. (At the time of this writing, this article could be viewed online here.)

More Info
For more on the fundamentals of steel connection details, see pages 386 - 395 in the textbook.

Chicago's Millenium Park

Chicago Team Struggles To Impose Order On Chaos, ENR, February 9, 2004, describes the challenges faced in the construction of Chicago's $475 million music amphitheater designed by Frank Gehry. According to the article, success of the project depended on "computer enhanced" techniques such as three-dimensional solid object modeling, "net" meetings conducted over the World Wide Web, high-tech surveying of in-place construction, and computer aided fabrication.

The article describes some of the positives and challanges of working on a structure with complex geometry. On the plus side, steel system RFIs were reduced to 125, from an estimated 10,000 if computerized modeling had not been employed. On the other hand, misalignments in fabricated supports required as built modeling of the in-place structure so that it could be tested against the original design model, and then descrepencies identified and corrected.

The article continues on at some length regarding structural design and analysis, constructability issues, temperature- and load-related movements, steel erection strategies, panel fabrication, and more. For those interested in the new digitally driven design and construction methods, this article is a good reference.

Integrated Steel Design

To Help Save Time, Structural Engineer Wears Harder Hat, ENR, February 2, 2004, describes Tacoma Washington's use of innovative CAD modeling and fabrication techniques to preorder structural steel for the 279,000 square foot Mt. Tahoma High School. On this project, engineers Putnam Collins Scott Associates developed a digital three-dimensional object based model from which steel detailing and fabrication could proceed. This process effectively removed steel procurement and detailing from the project's critical path and shaved close to three months off of the project's construction schedule.

The ENR article discusses both the risks and potential benefits of these so-called integrated steel design (ISD) techniques. On the Mt. Tahoma High School project, they were highly successful. In addition to reductions in the construction schedule, the engineers claim that there were only 13 RFIs related to structural steel (an unusually low number for a project of this size); of over 2900 anchor bolts only four for one base plate required modification; and of over 15,000 bolted connections, there were no mismatches.

These techniques also create new risks. With ISD, the owner's design team takes responsibility for steel detailing, instead of the general contractor and its suppliers. Additionally, with ISD, steel detailing information flows directly from the design model to the fabricator, without separate review by the general contractor or steel fabricator. This requires a steel designer sufficiently knowledgeable regarding steel fabrication and erection.

Learning From the Northridge Earthquake

Northridge Aftermath: Aftershocks Continue, ENR, January 26, 2004, dicusses the impact of the 1994 Northridge Earthquake on structural design practice even today, 10 years later.

One of the most interesting aspects of this earthquake event was the unexpected discovery of failed steel beam to column connections in 100 or more buildings involved in the earthquake. These failures occurred in buildings of diverse ages, and in many cases, in locales that experienced only mild ground movements during the quake. These unexpected, brittle connection failures have resulted in major rethinking of the design of steel connections in welded moment-resisting steel frames.

This article also discusses areas where some experts argue improvements are still needed. These include improved dialog between structural engineers and earth scientists, as well as attending to older unreinforced masonry and non-ductile reinforced concrete structures.

In the aftermath of the unexpected framing failures, a joint venture of the Structural Engineers Association of California, the Applied Technology Council, and the California Universities for Research in Earthquake Engineering, aka SAC was formed with the goal of "investigating the damage to welded steel moment frame buildings in the 1994 Northridge earthquake and developing repair techniques and new design approaches to minimize damage to steel moment frame buildings in future earthquakes". For those wishing to know more on this topic, portions of SAC's Interim Guidelines also make interesting reading.

Deciphering ASTM Standards for Structural Steel W-Shapes

[View within mini-mill melt shop where recycled steel is prepared for processing into new steel]

A recent article on this site, Structural Steel Materials Standards, described recommended ASTM standards for specifying various structural steel shapes and components. For those not regularly involved in the reading and writing of specifications, deciphering the meaning and significance of ASTM standards can be a challenge. As a way of introduction, the following provides some explanation regarding ASTM standards commonly applied to structural W-shapes.

Material Properties
One of the important functions of specifying steel standards is to define the material properties of the steel used on a project. The following table summarizes some commonly used designations for structural steel and several properties of the steel specified. As can be seen from the table, by specifying an ASTM designation (and in some cases also a Grade), minimum structural properties of the steel are established:

ASTM Designation	Minimum Yield Stress (ksi)	Minimum Tensile Stress (ksi)
A36	36	58-80
A572 Grade 50	50	65
A572 Grade 60	60	75
A572 Grade 65	65	80
A992	50-65	65

Other materials properties not shown in the above table may also influence the choice of steel type. For example, even though A992 steel and A572 Grade 50 steel appear comparable in the chart above, the relatively newer A992 designation is preferred for other aspects of its material definition.

Method of Manufacture and Cost
Differences in ASTM structural steel standards also reflect changes in steel manufacturing processes. For many decades, structural steel was manufactured mostly from raw materials and was formulated to meet the requirements of ASTM A36. Higher-strength steel was only specified where the need for its superior structural properties justified the significant additional cost associated with such material.

Today, most structural steel is manufactured in so-called mini-mills. By relying on scrap steel as the primary raw ingredient for manufacturing new steel, these newer mills are able to produce higher strength alloys such as ASTM A572 or A992 at lower cost than traditionally manufactured A36 steel. Consequently high-strength steel is now routinely specified for structural W-shapes.

How Are Standards Created and Enforced?
ASTM standards are developed through a consensus process involving industry stakeholders such as producers, consumers, users, government bodies, and researchers. ASTM itself is a not-for-profit organization. It has no enforcement mandate, and the standards it publishes are strictly voluntary.

ASTM standards may become defacto standards when they are adopted by the trade association that represents a particular industry. For example, in the case of the structural steel standards dicussed in this article, definitive recommendations for their use are found in the American Institute of Steel Construction's (AISC) LRFD Manual of Steel Construction (3rd Edition).

ASTM standards may change from voluntary to required when they are adopted by reference in building codes or other regulations. For example, the 2003 International Building Code makes AISC's design standards mandatory in paragraph 2205.1 General, which reads in part:

The design, fabrication and erection of structural steel for buildings and structures shall be in accordance with either the AISC-LRFD, AISC 335, or AISC-HSS...

In this way, ASTM standards that are part of the referenced AISC standards become mandated through the building code. (In other cases, the building code may directly reference ASTM standards themselves, rather than indirectly referencing them through other publications as in this example.)

Structural Steel Materials Standards

Are you Properly Specifying Materials?, Modern Steel Construction, January 2004, provides guidance on application of ASTM standards to specifying structural steel and related components. According to this article, preferred standards include:

Standard	Application
ASTM A992	Structural W-Shapes
ASTM A572, A913	Structural W-Shapes with higher yield and tensile strength than A 992
ASTM A588	W-Shapes of "weathering steel"
ASTM A36	Structural M-, S-, HP-Shapes, Channles, Angles, Plates, Bars, Threaded Rods
ASTM A53 Grade B	Steel Pipe
ASTM A500 Grade B	Round and Rectangular Tubes (Hollow Structural Sections)
ASTM A325, A490	High-Strength Bolts
ASTM A307	Common Bolts
ASTM A563	Nuts
ASTM A436	Washers
ASTM A1554	Anchor Rods

Information regarding steel grades and yield stresses is also provided.

For those who write or read structural steel specifications, this is a useful article. Companion articles in the same issue address availability of various shapes and grades, designing with high-strength grades, and related topics.

More Information:
Followup article Deciphering ASTM Standards for Structural Steel W-Shapes will provide some basic guidance on making sense out of ASTM structural steel standards.
Steel alloys, shapes, and fasteners are discussed on pages 374 - 382 of the textbook.
ASTM Standards and their summary scopes can be viewed online at ASTM Standards Search.

Ongoing NIST Research Into 9/11

Reliving 9/11, With Fire as Teacher, New York Times January 6, 2004, describes ongoing research by the National Institute of Standards and Technology into the causes of the collapse of the World Trade Center towers. Aspects of the NIST efforts discussed in this article include:

• Full-scale mockups and fire-testing of office workstation clusters to better understand the intensity and behavior of the fires that eventually weakened the building structure and lead to the collapse of the towers
• Testing of steel samples from salvaged structure to determine its strength characteristics under very rapid loading as was experienced during the planes' impacts
• Development of a detailed model of how heat from the fire seeped into structural elements and affected the structures' strength and stability
• Impact resistance testing of fireproofing insulation to determine the extent to which it may have been dislodged from structural elements during the intial impacts, thereby leaving the steel more vulnerable to the subsequent building fire

According to the article, NIST's findings could lead to new safety recommendations for ordinary high-rise construction. It is also expected to significantly advance the science of fire and how it affects structure.