STRUCTURAL OPTION
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Condensed Final Report (without appendices)
Executive Summary
The original nine story hotel, the Residence Inn, located in downtown Norfolk, VA, features 160 luxurious suites, designed to accommodate the needs of the extended-stay guest. Each suite features separate living, sleeping, and food preparation areas, complete with fully-equipped kitchenettes. The hotel recently opened after being constructed over the past year, for a total cost of approximately $22 million.
Structural systems of the building as originally designed include structural reinforced cast-in-place two-way slabs, columns, and shear walls, all eventually resting on grade beams and precast concrete piles at the foundation.
One of the goals of this thesis was to create a new signature brand for Marriott specifically designed to accommodate the needs of the business traveler, adding an office-suite dimension to hotel-style living. The idea of this mixed-use building is to ease the burden for the traveling professional by providing separate residence and office suites, thus enabling them to conduct business conveniently and in a professional manner, while maintaining a sense of personal life and pleasure. Architectural plans and 3-D renderings were developed to illustrate the intentions of the design and its interface with the hotel below. In the process of meeting this goal, green roof spaces were also designed and a signature lighting scheme for the shared conference rooms was created.
Due to the confined nature of many sites, including this one, the only plausible solution to an expansion is vertically. Therefore, the additional load created by the two story expansion required that gravity columns be re-designed accordingly.
With the introduction of just two additional floors projecting vertically from the originally designed structure, corresponding to a 22% increase in overall building size, lower level column sizes were found to increase on average by 30%. This result indicates a diminishing return on gravity structural systems. Perhaps a more economical solution would have been to expand the footprint of the building to accommodate the additional program requirements; however, for this particular site, this would not have been a feasible alternative. Almost all column designs were governed by slenderness, or a tendency for the columns to buckle due to unbraced lengths between stories. Small increases in unbraced lengths with similar axial loads were noted to have a tremendous impact on the strength of the column to resist buckling. Moments induced in the columns due to drift were found to be most significant at the lower stories, due to the increased affect with larger axial loads found there. In general, the results were as expected: increased column sizes to resist larger gravity loads.
In addition, a second goal was identified to gain experience in seismic design of building structures. Since Marriott’s network of lodging is so expansive and constantly growing, it would be beneficial to have a prototypical structure that could be used in a number of different locations throughout the United States. This would reduce the amount of re-engineering required of similar buildings. The current location of the Residence Inn by Marriott is downtown Norfolk, Virginia, where seismic activity is relatively low. In order to develop a prototype for the structural systems for more locations across the United States, the structure would need to be designed for additional seismic loads. Increased mapped spectral response acceleration parameters of 50% and 15% of gravity for the short and long period accelerations respectively and a more severe Seismic Design Category D were used as criteria for the design to ensure that the structure is capable of being located in the most locations. Wind pressures are already relatively high in this region, but were increased to be applicable for more coastal regions.
After the new lateral loading criterion was developed, the building was analyzed and lateral resisting shear walls were re-designed to meet the new demands. Extensive use of the computer modeling program ETABS was used for this portion in order to satisfy the MAE requirement. A number of assumptions were necessary to proceed with the designs, which are discussed further in the report.
Based on the analysis, it was found that Special Reinforced Concrete Shear Walls were required by code, as opposed to the Ordinary Reinforced Concrete Shear Walls of the original design. The results of the shear wall re-design indicate that in general, special reinforced concrete shear walls require a special boundary element design, which, in many cases, causes a significant increase in material, particularly reinforcement. Although architecturally the re-design had little effect, the hidden increase in strength and ductility directly correlated with an increase in cost of structural systems of approximately 2.1% or $91,500 for this design in Norfolk, VA. Assuming that structural engineering costs are reduced by two thirds by taking advantage of a design that is, for the most part, ‘pre-engineered,’ it is estimated that for each new reproduction of the prototype, a savings of $11,500 could be realized. For obvious reasons, some structural engineering would be required that takes into consideration the particular site for which the prototype would be located for design of foundations and checks for more critical conditions than were assumed by the prototype. Since the cost savings realized would be small relative to the total cost of the building ($22 million in Norfolk, VA), moral consideration must play a role in the decision of whether or not to consider using the ‘over-designed’ prototype for less critical locations, as there is a significant associated increase in the use of non-renewable resources and energy to produce the excess steel reinforcement. Otherwise, this exercise proved to be a valuable one, where experience in shear wall design for high seismic loading was obtained. The possibility of a prototype structure for the Executive Residence Inn remains and would certainly be valuable to a company such as Marriott, to whom economy could be realized, especially after a number of reproductions.
MAE Acknowledgement
For a majority of lateral system analysis, the finite element program ETABS was utilized. The use of this program to understand and obtain critical design loads for shear walls shall be used to fulfill the MAE requirement. Additional work in the area of seismic design of concrete structures shall also be used to fulfill this requirement.