Third Avenue New York, NY. Structural Concepts/ Structural Existing Conditions Report September 30, Michelle L.

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1 Executive Summary The structural design of Third Avenue is composed of two way concrete slabs supported by a system of columns spanning an average of twenty feet. Lateral loads are resisted by a central core of shear walls supported on a mat foundation and extending through the full height of the building. The mat foundation provides the fixity needed for the shear walls to resist the overturning moments created by shear forces from wind and seismic loads as well as the shear loads that these forces create. The shear walls must be designed for the worst load case, which is not the same on every floor. At the top of the building, seismic loads control, but as the loads accumulate farther down the structure, wind loads take over. Spot checking certain elements of the building revealed at least a slight over design in all areas. The column loads from the designer were higher for both dead and live load than those calculated for comparison. This is due to some additional floor loads that do not appear on the drawings but were added into the designing engineer s column load take down. Extra loads could have been added to accommodate for future issues that are not clear from the drawings. Alternatively, this could be the engineer s way of adding in an extra safety factor. Live load discrepancy is due the engineer using higher live loads for the first five floors than those specified by the NYC Building Code. The NYC Building Code was the code used to design this building, but the engineer may have some additional information about the exact use of the retail and equipment room space that is not apparent from the drawings. Lateral load calculations based on the NYC Building Code showed that the shear walls may be over designed as well. However, these calculations only took into account the shear strength needed to resist to resist shear loads from wind and earthquakes. The extra shear wall area is most likely used to resist overturning moments that have not yet been analyzed in detail. The core is not centered in the east-west direction, so the shear walls may need to resist some torsional effects of this asymmetry as well. Page 1 of 10

2 Overall Structural System Third Avenue uses a flat plate reinforced concrete system. Gravity loads are supported by two way reinforced slabs that distribute the load into eighteen reinforced concrete columns. The maximum span between columns is approximately 20 feet. The building is supported on a 5000 psi concrete mat foundation varying from 40 to 60 in thickness. The slabs for the ground floor through floor twenty are made from 5950 psi concrete. Slabs for the cellar and floors twenty-one through the roof all use a concrete strength of 5000 psi. Thicknesses are nine inches for all floors except six and eight, which have slab thicknesses of twelve inches. The columns supporting the cellar are 5000 psi. Supporting the ground floor through floor twenty, the columns have a strength of 8000 psi. Above floor twenty, all columns have are made from 5000 psi concrete. Five of the columns stop below floor eight where the setback occurs. The remaining columns extend to the main roof. The only gravity resisting members for the elevator machine room and bulkhead, which are above the main roof, are the shear walls described below. Shear walls in the central core of the building resist lateral loads. These shear walls are constructed from 8000 psi concrete up to floor twenty. Above floor twenty, the concrete strength reduces to 5000 psi. The core surrounds the elevator and stair lobby. Therefore, where door openings occur, additional reinforcement is added above the opening or in the form on a link beam between openings. This core extends from the cellar all the way to the bulkhead roof. Codes Used The codes used for the structural design are Building Code of the City of New York, including the latest amendments ( NYC Building Code ) and American Concrete Institute Building Code Requirements for Reinforced Concrete ACE ( ACI 318 ) as modified by Subchapter 10, Article 5 of the NYC Building Code. Page 2 of 10

3 Typical Framing The typical framing is two way concrete slabs that distribute gravity loads to the columns. Shear walls brace the system to resist lateral loads. Below is a typical plan for floors 9-20 showing the location of the columns and the shear walls that support the loads. This plan also shows the slab openings that have an effect on the distribution of forces in the slab. Reduced scale plans for the ground floor through floor eight are included in pages A-10 through A-13 of the appendix in case they are necessary for further clarification. Page 3 of 10

4 Description of Structure Third Avenue uses a system of two way slabs, columns and shear walls to resist gravity and lateral loads. Columns span around twenty feet and vary dramatically in size. Several of the columns shift at some point in the building, causing very large cross-sectional areas at times. The columns walk from one location to another over the span of multiple floors. While column 10 shifts the load between the fourth and sixth floors, its dimensions increase from 36x20 to 62x20. In this area, the core of the reinforcement slopes in order to shift the load from the location above the column walk to the location below. Other columns have similar shifts at various locations. The most drastic shift occurs between floors five and eight, when column 12 moves to the exterior face of the cantilever and becomes column 18. The extreme tensions produced by such a large column walk require post-tensioning around that column in the eighth floor slab as well as an increased slab thickness. Typical slabs are 9 inches, whereas the eighth floor slab is 12 inches. Most columns throughout the building use either #11 or #9 bars for reinforcement. This is also true of the column walks. A core of shear walls surrounding the stairs and elevators resists the lateral loads in this building. The shear wall area decreases slightly as elevation increases. The largest shear from wind and earthquake loads occurs at the base of the shear walls, since the shear is additive from the top down. Therefore, this is where their area is the greatest. Concrete strength for the shear walls is 8 ksi near the base of the building and decreases to 5 ksi closer to the top. In the lower levels, vertical bars are #11 and horizontal bars are #4. As the story level increases, the vertical bar size decreases to #5 or #7 bars. At the lower levels, three shear walls resist the shear forces from loading on the wider north and south faces and two walls resist shear from lateral loads on the east and west faces. Toward the top of the structure, this reduces to two walls in each direction. Where openings for doors occur, link beams or extra reinforcing above the opening braces the wall so that it acts as a single unit rather than separate resistive units. All of the columns and shear walls are supported by a mat foundation below the cellar. The mat ranges in thickness from 42 inches to 60 inches. This acts as a fixed support to resist overturning moments from lateral forces when analyzing the building structure as a whole. Page 4 of 10

5 Required Loads Dead Loads Slab Self-Weight: psf for 9 slabs 150 psf for 12 slabs 300 psf for top roof Superimposed Dead Loads: Mechanical = 10 psf Partitions = 5 psf Flooring = 5 psf Total = 20 psf Cladding Loads (metal panel siding): 30 psf Live Loads (as specified by NYC Building Code see A-14 through A-16 of appendix) Roof: Snow Load = 30psf Concentrated Live Load = 200 lbs on area of 2 ft by 2 ft Interior Floors: Dwellings-Apartments = 40 psf Equipment Rooms = 75 psf Storage Light = 100 psf Exercise Room = 100 psf Telephone Equipment Room = 80 psf Retail Sales Basement & First Floor = 100 psf Page 5 of 10

6 Wind Loads (as specified by NYC Building Code see appendix page A-19) Refer to the wind load spreadsheets (A-1 and A-2) for the calculation of shear forces and overturning moments due to wind loads. Shear forces are shown on each floor and at mid-height of the double height floors (floors 25-30). The diagram to the right shows the pressure distribution over the height of the building. Page 6 of 10

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8 Seismic Loads (as specified by NYC Building Code) Refer to spreadsheet A-3 in the appendix for the calculation of seismic loads. The diagram shows the distribution of shear on each level. To find the design shear from seismic loads at any given level, all point loads above and including that level must be summed. Therefore, the greatest shear occurs at the base. The base shear in this building is 348 kips, as shown on the Seismic Loads spreadsheet. Page 8 of 10

9 Spot-Check Slab The two-way slab design was checked through a comparison to the CRSI design handbook. A more detailed analysis using a sophisticated computer program would be more useful for this kind of check. However, more exposure to such programs is necessary before a more detailed check can be completed. For a 9 inch slab with no drop panels, a 60 psf load and columns spaced at 20 feet, the CRSI handbook requires 12 #5 top bars in the column strip and 10 #4 bars in the middle strip. Bottom bar requirements are 10 #4 bars in the column strip and middle strip. The loads used to find these values were factored according to ACI since the book is based on these values rather than the 2002 load factors. However, CRSI only provides designs for 4 ksi concrete. The slabs in this building are 5 ksi concrete or stronger. The basic top and bottom reinforcement is #4 bars at 12 inches. Added bars, mainly #4 and #6 bars are called out on the drawings as needed in the column strips. Shear Walls Shear walls were checked for base shear only. Spreadsheets in the appendix show the calculation of shear due to wind loads and seismic loads. The spreadsheet titled Design Shear (A-4) combines the shear from wind and earthquakes to find the worst case factored load on each floor. The base shear values from this spreadsheet were used to determine the design loads for the shear walls supporting level two. The shear was then distributed to the shear walls acting in that direction according to tributary area. It appears that the shear walls were designed to resist more than just these lateral forces, based on the shear calculations included in the appendix. Additional shear wall area may have been needed to resist overturning moments from wind and earthquake loading. Column Column number 1, the bottom left column in the framing plan, was checked using PCA Column. Refer to the appendix for the spreadsheet used to determine column loads as well as a printout from PCA Column. Loads were calculated through the method of tributary areas. The loads calculated were slightly lower than those on the drawings. All loads for comparison are service (unfactored) loads. The column was designed for a live load of 109 kips and a dead load of 836 kips. The calculated loads used to check the column design were 76 kips live and 790 kips dead. Live loads are lower than those calculated by the engineer because the calculations took into account required loading given by the NYC Building Code rather than the higher estimates used by the engineer. For instance, the code requires 100 psf for retail areas on the basement and first floors of buildings. A live load of 200 psf was used for the design of this floor. Additionally, the engineer increased the live Page 9 of 10

10 load for the mechanical rooms to 150 psf rather than the 75 psf specified by the NYC building code. The majority of the floors were designed for the normal 40 psf required for apartments, but the additional loads used by the engineer on the first five floors account for the discrepancy in live loads. These loads may have been increased for a specific unique use of this space or just as an extra safety precaution. The dead load used for design is also higher than the dead load obtained on the attached spreadsheet. Reasons for this include changes in column sizes between the time that the initial loads were calculated and the drawings were released as well as additional allowances for dead loads on the upper levels above where the columns stop. Once again, this may have been done as a safety factor or other unknown reasons. Since it is not clear if there were specific reasons for the extra loads, the column was analyzed for both sets of loads. As expected, it was over designed for the loads calculated on the attached spreadsheet. However, based on the loads from the drawings, the column is still over reinforced based on axial loads alone. The required reinforcement was the same for both cases. While the column design calls for 6 #11 bars, PCA Column shows that 6 #9 bars would be sufficient. The extra reinforcement could be required to resist bending moments from the slab that were not included in this analysis. Refer to pages A-8 and A-9 in the appendix for column loads and results from PCA Column. Page 10 of 10