analysis of steel structures under seismic...

Analysis of Steel Structures Under Seismic Stress Fall 2011

Engineering ReportAnalysis of Steel Structures under Seismic Stress

IntroductionThis report will analyze the benefits and flaws of common steel building methods under seismic stress and describe techniques to maximize earthquake resistance. The system of Steel Reinforced Concrete—Wall and Frame buildings will be discussed, with a more specific focus on Steel I-beam construction. The interrelation of these building techniques to structural integrity during seismic events will be developed, primarily concerning high-rise buildings experiencing a variety of forces beyond simple compression and tension. Buildings of great height are constantly affected by wind-stress forces, with an increased threat from seismic motion. As urban population increases, taller buildings will need to be constructed to meet the demand for housing and office space within cities. This report will provide an analysis of current methods and make suggestions for future improvements.

Analysis of Current Techniques Two common methods of building are frame structure and wall structure. These two techniques are used throughout the world for a variety of environments and budgets. The highlight of this comparison is the behavior during seismic events of wall structure compared to frame structure.

Wall Structure Wall structure utilizes solid walls reaching from the building foundation through the entire height. The materials used in this construction method often consist of masonry units or poured concrete that is reinforced by steel rebar dowels. This method is inexpensive to construct; no specialized production is required and the materials may be bought in large quantities, even in rural areas. In addition to the low price, concrete has very high strength in compression, but without reinforcement, tensile strength is very low (9). When proper reinforcement is applied, concrete is a very adaptable building material that may be used to create multiple storied structures. A building with reinforced concrete walls extending throughout the entire height of the structure is extremely rigid to torsion forces. However, during seismic activity, this rigidity becomes a major flaw of reinforced concrete wall construction. The stiffness of the structure transmits the seismic forces vertically throughout the height of the building. While the lower stories may remain very stable, the upper levels experience magnified displacement in comparison (6). Even with large amounts of reinforcement, an earthquake of great enough magnitude could cause structural failure in the upper stories of a reinforced concrete wall building.

Frame StructureFrame structure uses a combination of vertical columns and horizontal beams to create the skeleton of the structure. The columns will often run the entire vertical height of the building, while the beams span between the columns to create a platform for the floor slab. Reinforced concrete is often used in frame construction, once again offering an inexpensive and easily available building material (9). When built with concrete, the positions of columns and beams are outlined with forms which will hold the concrete in place while curing. All of the columns and beams of the individual floors are placed monolithically—in one simultaneous action—to provide optimal integrity of the each component and the connecting joints. In the upper floors, the thickness of the beams and columns may be tapered to account for the decrease in applied loads—a principle that will be further explored in the steel I-beam section. When all the floors have been poured, columns and beams form a matrix of support (Fig. 1) that determines the characteristics of the entire structure.

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Figure 1 Frame Structure (1).

This matrix is strong, acting very flexible compared to the rigidness of reinforced concrete wall construction. Yet, as reinforced concrete wall construction exhibits displacement in the upper levels during a seismic event, frame construction may articulate in the lower portion of the building (6). When a frame building is subjected to seismic energy, each joint will flex to help dissipate the energy. When the capacity for absorption is reached, the seismic force begins to echo through the structure, returning to the base levels and amplifying the force that the lower joints experience. A structural failure that often happens during seismic activity upon improperly built frame structures is a collapse of the first story supports. When the base columns begin to wobble, the compression strength of concrete is no longer optimized, causing the first level supports to crumple. Often the damage is not catastrophic (Fig. 2), but there are extreme cases (Fig. 3)

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Left: Figure 2-Mild seismic damage to a reinforced concrete frame building (3).

Above: Figure 3-A soft story collapse of a reinforced concrete frame building with brick infill (2).


This sort of damage may be counteracted by applications of calculated design, but the threat of the failure remains nonetheless.

Steel Structure The two previous sections have discussed the construction of wall and frame structures with the use of reinforced concrete. This section will outline the use of steel in modern construction applications.

Steel I-beam Frame ConstructionSteel Frame construction is composed of the same elements as Reinforced Concrete Frame: vertical columns, horizontal beams, floor slabs and joints. A major difference between concrete and steel is their tensile strength. Reinforced concrete construction requires steel rebar reinforcement in order to give the structure the tensile capacity to span distances and stay standing.

As steel is primarily what makes this tensile strength possible, logically, larger components of steel will have similarly increased tensile strength. Unlike concrete, steel is not poured into the structure form monolithically. Beams and columns assembled in steel production plants must be brought on site and fastened together to form the structure. A current method is to assemble columns extending through multiple levels. The initial set of columns ends between floors, and are welded and bolted to the next set of columns (Fig. 4). As with concrete frame, the upper floors of a steel framed building there have less weight for the vertical members to support than there is on the bottom floors—for this reason, thickness and size of the columns may decrease with each joining (Fig. 4 &5).

The technique of decreasing the column size saves money by conserving steel, as well as reducing the mass load on the rest of the structure. A key aspect of this joint placement is not only from an efficiency standpoint,

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Left: Figure 4-Column Joint between Floors

Right: Figure 5- Decreasing Column Size at joint


but from a structural strength perspective as well. Supporting the floors that the columns rise up through are horizontal beams. In steel frame construction, four of these beams form a junction at the columns (Fig. 6).

Figure 6: This picture also displays a floor joist intersecting the right hand beam

Now consider; if the column joint were to be placed floor level, there would be five joints bonded together at these junctions--should a building with this design be subjected to seismic forces, a large amount of stress would be applied to the joints of the columns, as well as the beams. In current building practices, this sort of stress is avoided; yet, a massive amount of force is concentrated by the change of a joint placement. What if this force of stress could be harnessed to produce energy that would increase the building's seismic protection? In this next section, I will review the techniques for seismic energy dissipation and propose my intent to develop a method of harnessing structure-damaging energy.

Passive Seismic Energy Control DampenersCurrently, methods of dissipating seismic energy are passive in nature. Available to be built into a new structure or retrofitted into an existing building, the control dampeners disperse the force of the earthquake by allowing the building to move. These systems may be put to use in reinforced concrete wall structures, reinforced concrete frame structures, or steel I-beam constructions. With skyscrapers, Tuned Mass Dampeners in the upper stories are used to counteract wind induced sway—as the building moves one direction the dampener moves in the opposite, generating a converse force and limiting the building sway. The allowed movement of the passive control dampeners operates on the same principle as the Tuned Mass Dampeners —the passive control dampeners are placed between the building foundation and the bottom floor supports, allowing the mass of the

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building to move with counteractive force. Currently, common application of these passive systems take the form of polymer bearings that provide the entire building the freedom to wiggle under seismic stress, which averts the forces instead of absorbing them within the structure (5).

A less common, but more controlled method of seismic counteraction uses a sliding principle. The building is placed on top of a system of many small rails--allowing enough side to side motion to occur to deflect seismic energy from the structure (5). The judgment on which system to be used in an existing building is based off of the style of the structure.

A short rigid building would benefit from the multidirectional deflection of the bearing system, whereas the sliding system would be better suited to a more ductile building with stabilizers for shear forces already built in. When a new structure is being built, these components may be integrated with the design to yield even better results. Nonetheless, neither of these systems provides a way to absorb the energy from an earthquake. The potential energy of an earthquake is massive. The next section that this report will discuss is a method to convert seismic energy into electricity.

Regenerative Control SystemThe proposal of regenerative control systems involves implementing passive control systems at the base of the building, and using pressurized fluid to generate electricity from the movement. As a seismic event increases in power, more resistance may be applied to the system, generating more energy as a result (5). This energy may be directly applied to other seismic controlling dampeners within the building, or stored within low-friction fly wheels for future use.

Redirecting the power of natural forces as a measure of protection from those same forces is a massive step forward in the field of seismic control. Despite this massive innovation, there is still room for improvement. If regenerative control devices could be placed on every floor, the force reduction capabilities would be limitless.

Regenerative Control Devices are still in infancy, with none currently in use in existing buildings. Over the course of the coming years I intend to further explore this idea—researching techniques, sketching designs, and creating models to find methods that will best be able to harness the power of earthquakes for human use.

Conclusion

The methods of reinforced concrete wall construction and reinforced concrete frame construction each behave differently under seismic stress. Using thoughtful design, the problems may be accounted for and the benefits may be optimized. Steel I-beam construction is a more expensive method, which affords greater ductility and design variables. All of these systems may be made to better resist seismic forces through the use of passive control dampeners. As technology continues to improve, the feasibility of regenerative control systems will increase, allowing the energy of seismic forces to be used in the protection of structures. Earthquakes may be devastating when ill-prepared for, but each event increases our knowledge of the way structures behave when subjected natural seismic force. So long as humanity continues to innovate, no problem is insurmountable.

Additional Photographs of Structure Study: (All Steel photos are of the Old Mill Office Building, 6200 S. Cottonwood, UT, taken by Jeffery Thomas)

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Report reviewed by Richard Silva, P.E., Structural Engineer, Seismic Safety Program Manager, National Park Service, U.S. Department of the Interior

Works Cited1. Bazzurro, P. (2009, July 14). Nonlinear Dynamic Analysis--a Step Advance in Assessing the

Vulnerability of Buildings to Earthquake Ground Motion. Air Worldwide.

2. Girty, G. (2008). Earthquakes. Retrieved 2011, from Notes on Planet Earth Version 3.0:

http://www.geology.sdsu.edu/visualgeology/geology101/index.htm#

3. Miyamoto, H. K. (2009). The 2008 Sichuan Earthquake. Structure , 18.

4. Moehle, P. J., & Mahin, A. S. (1998, January). OBSERVATIONS ON THE BEHAVIOR OF

REINFORCED CONCRETE BUILDINGS DURING EARTHQUAKES . Retrieved September 2011,

from National Information Service for Earthquake Engineering.

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5. Richards, G. (2010). Movers and Shakers. Engineering & Technology , 38-41.

6. Sullivan, T. J., & Priestley, M. J. (2005). Development of an innovative seismic design procedure for

frame-wall structures. Journal of Earthquake Engineering , 279-307.

7. University, Purdue. Earthquake Hazard Infromation. 2011

<http://web.ics.purdue.edu/~braile/edumod/eqphotos/eqphotos1.htm>.

8. Waikato, U. o. (2011). Earthquakes. Retrieved 11 14, 2011, from Science Learning:

http://www.sciencelearn.org.nz/Contexts/Earthquakes/Sci-Media/Video/The-base-isolation-principle

9. Yakut, A. (2007). Reinforced Concrete Frame Construction. Middle Eastern Technical University , 1-8.

10. York, Research Foundation of the State of New. How Buildings Respond to Earthquakes. 2010.

November 2011 <http://mceer.buffalo.edu/infoservice/reference_services/buildingRespondEQ.asp>.

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