Welcome to Lourdes Diaz's AE Senior Thesis e-Portfolio
Thesis Proposal
Paseo Caribe's structure is reinforced cast in place concrete. The floor system is a one way 8 inch post tensioned slab. This system was selected for both its strength and the minimum slab thickness. However, according to UBC 1997 seismic requirements control the design of the structure. The maximum story shear due to seismic is about 5 times larger than that due to wind. Seismic forces are proportional to the weight of the building. Three quarters of the total weight of the building is in the slab. If the weight of the slab can be reduced, the large seismic lateral forces will also be reduced. This will reduce the amount or shear wall resistance required for lateral loads. A reduced amount of shear walls will further reduce the total weight of the building. It will also allow for a more open – wall free - floor plan as the bearing capacity of the walls is reduced due to a lighter slab dead weight. Furthermore, this reduction in weight will have a positive impact in the foundation of the building.
Therefore, a lighter weight floor structure must be designed for both parking garage and apartment units to resist dead loads and minimum design live loads of 60 psf (40 psf live + 20 psf partitions) for the apartments and 50 psf for the parking garage as specified in ASCE 7 – 95 Table 4.1. The new floor structure must be able to span 27 feet while meeting deflection criteria of l/360. The total depth of the slab should be kept to a minimum while maintaining a floor ceiling envelope of less than 18”. These will accommodate for a ceiling height of 8 feet 6 inches with a floor to floor height of less than 10 feet. This maximum floor height will allow the building to meet the height limit requirements for Seismic Zone 3 specified by UBC 1997 Table 16-N of 160 feet for a bearing wall system with concrete shear walls (Item 1.2.a).
Two alternate systems are going to be investigated to minimize the weight of the building, the magnitude of the lateral loads; further reducing the amount of shear and bearing walls in the floor plan. The first alternate system is a pre-cast hollow concrete system with high strength concrete for columns and bearing walls. The second alternate system will explore the use of steel. Steel is not widely used in Puerto Rico. Due to the lack of available materials and skilled labor it is an expensive and lengthy (due to procurement) construction method. However, for purposes of this educational research it will be assumed that the materials and labor are readily available. The system will look at a composite steel floor system. This alternative will explore a composite steel deck and Smartbeam system.
The proposal layouts the background and foundation on the current structure and the problems to be addressed. It also outline the requirements for the new proposed systems and a list of preliminary steps to follow for a complete design. Finally, two breadth topics are discussed and a analysis is presented on their impact on the alternate structure.
Breadth Topic 1 : Construction and Cost Saving Impact
The objective of this breath analysis is to determine whether the pre-cast concrete structural system will be more viable than the designed post-tensioned system. Both systems are considered to be high cost compared to conventional concrete construction. An overall cost impact will be determined as a percentage of the overall project thought constructability reviews, schedule variances, and cost-impact. The overall cost impact will consider actual changes in budgeted costs and cost associated with saving from impacts on schedule and general conditions.
Breadth Topic 2 : Impact on Floor Vibrations and Mechanical Damping
The objective of this breath analysis is to determine the impact of the lighter hollow pre-cast concrete system on the absorption and transmittance of impact forces and vibrations over the current solid cast in place floor system. Major internal sources of vibration loads in a residential building are due to oscillation of machinery and the passage of vehicles. Therefore, considering the vibration characteristic of the new floor system is important for this project because of the residential floors are located above the 10 level parking garages and also the mechanical systems are on the upper two stories.
Astudy of the hollow-core system based on the published paper “Vibration
Criteria for Assembly Occupancies” by Allen, Rainer, and Pernica to
determine the lowest acceptable fundamental frequency of the floor system.
The procedure is based on the density of occupancy and appropriate forcing
frequency to determine and acceptable limiting acceleration ratio at the
center of the floor. The lowest acceptable fundamental frequency will be
compared to the actual natural frequency of the floor structure.