49ers stadium

F-35 shown obsolete on previous posts

   The San Francisco 49ers, an NFL, american professional football, franchise long argued over how and where to build a new stadium, I would suggest floating it.  Their current stadium is located at Candlestick Point, directly along San Francisco Bay, so access to the water is relatively easy.

   The idea is to build a massive monolithic platform, a giant barge, out of concrete and guild the parking structure and stadium on top of it.  In order to recover the cost of construction, buildings would be built on top as well to produce additional revenue, A, B.  The platform might be 2000 ft by
10 000 ft.

   Concrete is weak in tension so post tensioning, placing wires in the concrete and tightening them to produce a compressive stress which under tension does not produce net tension, would be needed to overcome the stresses produced by waves.  2 dimensional post tensioning, as D, has been used.  For the stresses ina floating platform, 3 dimensional post tensioning , as E, must be used.  For 2 dimesions, the wires can be laid out and the concrete poured over them before i sets and the wires can be tensioned.  For 3 dimensions, the vertical wires would have to be held on a reel with 1 end attached to an anchor plate, before the horizontal wires are laid and the concrete poured, F.  The lowest level of horizontal layer could be pre-molded and cured before being set as blocks with lead wires placed in the4 wire guideways.  The lead wires would be connected as each block is placed.  When all the blocks have been placed the tensioning wire would be attached to the lead wire string and the lead wires pulled through placing the tensioning wires.
   For forming the concrete, large blocks of expanded concrete could be used as molds.  Expanded concrete is concrete with a lot of little air holes in it.  It is similar to styrofoam.  Normal concrete weighs about 150 pounds per cubic foot, pcf; expanded concrete can weigh as little as 10 pcf.  Sea water weighs 64 pcf.  The expanded blocks, when placed, with spaces between for pouring the structural concrete, would render the entire platform unsinkable.  Even if the platform broke into pieces each piece would float as the platform would weigh less than sea water and the expanded concrete would prevent sea water from flooding and sinking the structure.  In addition the expanded concrete would prevent buckling of the structural concrete walls and floors allowing the concrete to achieve its maximal compressive stress.

   At I, is shown a slightly different construction method.  The molded blocks are used as the lowest base but an additional layer of concrete which would be post tensioned would be poured on top of them before the expanded concrete blocks are placed.
   Where there is automobile parking, it might be a good idea to place expanded concrete above the structural concrete but below the actual parking surface.  That would allow for energy absorbtion in the expanded concrete in the event of a car bomb. The creation of a a sacrificial surface such as that would prevent damage to the structure, which would be very difficult to repair, H.
   At G is shown the possibility of building a convention center and exhibition space as an additional amenity.  The roof could span 1000 ft with a tied arch, a steel arch that has an additional continuous steel beam along its lower edge to counteract the reaction forces of the toes of the arch.  To the sides of the central space could bre additional conference rooms, offices and smaller group rooms.

   Over the parking areas, walkways could be built, they would provide a public access amenity.  This would be necessary to gain approval for the construction.  The walkways would be 100 ft wide to allow for cafes restaurants and shops.  They would be wide enough to also allow for the installation of basketball and tennis courts.   Across the walkway, J and L, a, there might be 4 supports for redundancy.   Along the walkway, K and L, b, the spacing of the column groups could be 50 ft to allow for vehicle parking nad access lanes.
   At M, is shown the floating structure connected to shore by arched bridges.  By floating the structure, it is immune to earthquakes, although the bridges and water, electric, gas and sewer lines could be damaged.
   At lower right is illustrated some consideration of wave action and bending stresses.  The Golden Gate restricts energy input form the Pacific Ocean and the large size of the4 bay dissipates the energy that does enter.  Waves produced by wind blowing over the bay are limited by the dimensions of the bay itself.
   Allowing for a 1000 ft long wave, a wave of 14 second period, which would be unlikely to ever be seen in the bay, the half-wave length would be 500 feet.  d could represent the distance between the centroid of the lower quarter wave and the upper 1/4 wave, maybe 400 ft.  For a sine wave, the area of water would be about 2/3 of the 1/4 wave length X wave amplitude, 1/2 wave height.  If the platform is 40 ft thick and the upper and lower concrete surfaces are 3 ft thick, for an allowable concrete strength of 3000 pounds per square inch, psi, 430 000 lbs/ foot square, psf, the maximum moment would be 40 X 3 X 430 000 = 52 000 000 pound feet.  For d = 400, 130 000 lbs of force over 250 ft, 1/4 of 1000 ft, = 520 lbs per foot.  But since the area is about 2/3, 1 1/2 X 520 = 780 lbs /ft.; Divided by 64, weight of seawater = 8 ft. X 2 to allow for both upper and lower waves portions, = 16 ft larger than any wave likely to be encountered in San Francisco Bay.
    The platform would have 2 X 3 feet of concrete for the upper and lower surfaces + the blocks underneath of maybe 2 ft for a total of 8 feet.  In addition there must be vertical structural walls, maybe 2 feet thick every 100 X 100 feet.  That would be 100 + 110 = 200 X 40 X 2 = 16 000, divided by 100 X 100 = 1.6 feet 8 + 1.6 = 9.6.  It takes 2.5 feet of water to displace 1 ft concrete, 9.6 X 2.5 = 24 ft. In addition ther is the expanded concrete 40 X 10 / 64 = 6.5 feet.  The total is 24 + 6.5 = 30.5 plus the weight of buildings.  Allowing 4 feet of concrete, which should be about 4 floors of building, that would require an additional 10 ft, total = 41 feet displacement.
   If the buildings equal 4 floors, they would be concentrated so that 30 floors would be built over 1/8 of the structure or 40 floors over 1/10.
   #0 feet of water = 1 ton or 1/2 cubic yard of concrete.  ! cubic yard of concrete costs about $150, labor adds a lot more cost.  If the cost is $1000 per square foot, the average for each floor of building would be $250/ square foot.  But not all the space is sellable, hallways elevator shafts, so the cost might be $330/ square foot.  With construction the total might be $700 / square foot.  For a 2000 square foot apartment, fairly large, the cost would be $1 400 000.   There would need to be 2 000 X
10 000 X 4 X 3/4 / 2000 = 30 000 apartments.  San Francisco has a population of about 600 000 or
240 000 dwellings.  In addition, some buildings could be used as hotels or office buildings, it might actually be affordable.

    To build the platform, a series of pipes in a herringbone pattern would be emplaced below the depth necessary to float the platform, N, O.   Then steel sheet piling would be driven to form a rough rectangle.  The area enclosed would have sand pumped in to fill above the high tide line.  The pipes could pump out water to consolidate the sand.  The loer blocks would be placed and the platform poured and allowed to set.  The sheet piling would be removed and water would be puped through the pipes to undercut the sand until the platform floats   The platform would then be towed into position.  The sand could then be restored to its original condition.
   One additional point is that the platform would block sunlight,killing sea grass where it is anchored.  There might also be a deficit of oxygen under the platform, so air might have to be pumped to prevent fish kills.
    The other question is what the life time of the platform would be, 50 years, at least should be obtainable.