Fourth Avenue Building FEMA-178 Seismic Evaluation
2. BUILDING DESCRIPTION
The roof of the penthouse is constructed of untopped roof deck supported by structural steel purlins and beams. The penthouse floor, second floor and third floors are constructed of concrete topped composite steel deck supported by non-composite structural steel purlins and beams. The beams frame to structural steel columns supported by individuals spread footings on a basic 24' x 36' grid.
Lateral wind and seismic forces are resisted by concrete shear walls around the various stair and elevator shafts and by the basement walls at the lowest level.
The south structure is a three level cast-in-place concrete structure, approximately 381' x 239' in plan, supporting a small structural steel building at the far north end. The building was designed in 1974. The entire structure except the small steel building is below grade. The northern 130' was designed to support a future office tower of approximately 18 floors.
The ground level slab is constructed of a 13 3/4" thick flat slab with drop panels and column capitals on a maximum 36' x 36' grid. The south half of the ground level slab is constructed of post-tensioned slabs, beams and girders on a maximum 30' x 60' grid. This portion of the structure was designed to support a 720 psf super-imposed load. The two below grade levels are constructed of two-way joist (waffle) slabs on a maximum 36' x 36' grid.
The structure is founded on individual spread footings except that the northern 130' of the structure is founded on a 4 1/2 foot thick mat footing. Lateral seismic forces are resisted by perimeter cast-in-place concrete basement walls along with a few miscellaneous interior concrete and CMU walls.
The structure over the north end was designed as a portion of the future office tower. The structural steel purlins and beams support a concrete topped 3" composite steel deck. The steel columns under the entire future office tower extend to the mat footing. Lateral forces above grade are resisted by welded by moment frames and by the basement walls below grade.
The structures contain none of the deficiencies, which these evaluations seek. Both structures can resist the forces prescribed by FEMA 178.
4.0 UNIFORM BUILDING CODE ANALYSIS
The south structure can easily resist the forces prescribed per the criteria for zone 3 of the 1994 Uniform Building Code. All lateral force-resisting elements appear to be detailed in conformance with that code.
The one story steel structure can resist the forces prescribed per the criteria for zone 3 of the 1994 Uniform Building Code. Details of the welded moment frames are not in conformance with current practices, but the stress levels in critical welds are less than 10% of the theoretical strength and thus are adequate.