Typical Altered Column Grid Layout: Level 6

b. Initialization: Floor System Consideration

The decision of whether to redesign 750 East Pratt in concrete (similar to the substation construction), or steel (similar to the tower construction), was based initially upon the type of floor system which could most consistently and most effectively handle the relatively low gravity loads of the office levels as well as the higher and more dynamic loads of the electrical substation equipment and vehicular traffic.

The effectiveness of such a floor system is essential. On the office floors, where people spend their days and the operation of sensitive computer equipment must be considered, vibration is a key issue. Substantially larger loading conditions exist in the substation equipment rooms and must be accounted for. The diaphragm action of the floor system must also be preserved for lateral efficiency.

Consistency is desirable so that constructability and simplicity may be achieved. Although different floor systems might be ideal for different situations, the most desirable system in this case would be one that could effectively handle BOTH the low and high loading cases without being over conservative in any situation.

Floor system alternatives were considered for both concrete and steel superstructures. Calculations further supporting these results are tabulated in my preliminary reports.

i. Concrete System Alternatives:

Two concrete floor systems are well known for handling large point and dynamic loads: the waffle slab system and the post-tensioned beam system. The disadvantage that both of these systems has is inflexibility. Neither allows flexibility with penetration location, or is easily altered after construction, should future tenant requirements demand it. In a generic open plan office building, flexibility is important to the rentability of the space. The strength of these systems is not necessary or economical for the loading conditions of the office levels. Concrete as a whole does not offer the flexibility required for the specific requirements that 750 East Pratt poses.

ii. Steel System Alternatives:

Two steel floor systems were also considered for this building's needs. The original composite deck on W-shape beams was an obvious consideration, as was an alteration substituting open web joists for the W-shapes. Open web joists offer a lower strength-to-weight ratio than traditional W-shapes, but after researching several cases involving these joists, I decided against implementing them in my design. Open web joists are difficult to fireproof, and tend to be deeper than W-shape members with similar structural properties. Open web joists offer considerable advantages in extremely tall buildings where every pound counts, but in a 20-story office building these benefits did not outweigh the disadvantages.

a1. Design According to AISC LRFD

a2. P-Delta Effects Included

a3. Rigid Diaphragm Effects Included

a4. Dual-System Effects Considered In E-W Direction

a5. Building Considered Sidesway Inhibited

Braced Framing Layout

 

The braced frame system was the most rigid in layout, as the braces will interrupt the open office layout if they are not adjacent to either the core or exterior walls. Braces were located where they would be least obstructive to the architectural functions of the building. See note below regarding brace selection.

Moment Framing Layout

 

The rigid moment frame system was much more flexible in layout, and so the number and placement of moment frames necessary to optimize the design could be iterated through trial and error using the RAM Frame software for comparison purposes.

Combination Framing Layout

The combination braced-moment frame system is one that utilizes the most effective qualities of both above systems. Using the results and assumptions described below, an optimized combination of the two systems was designed.

Note - Initially, four braced frames and two fixed moment frames resisted lateral forces in the E-W direction (similar in orientation to the braced and moment framing layouts shown above), but after checking for strength and deflection, two additional moment frames were added, leading to the final design.

Note - A preliminary comparison of chevron, V-, and X-braces was performed in STAAD to estimate what type of brace would be most effective in this case. Chevron braces were found to be the most efficient in that they reduced drift while reducing the effective length (and thereby increasing the effectiveness) of the lateral girders that they supported. Knee braces were used in place of chevron braces where window obstruction would be undesirable. The results of this comparison are tabulated in Appendix E.

i. Braced Frame System

The braced frame system was modeled in RAM as shown. Due to the open plan architecture of the office levels, braces could only be implemented where shown graphically above. The braces at grid lines 4 and 11 may be objectionable to the owner as well, but were included as a possibility for optimization. Regardless, no column size up to W14 satisfied the gravity loading conditions of the building. The gravity columns must carry proportionally higher loads because the pinned connections in the braced frames do not transfer moments to the lateral columns. I decided not to attempt to increase the column size as the goal of this exercise is to minimize structural member sizes as compared with the original design. Due to this failure, the braced frame lateral system was rejected.

ii. Moment Frame System

The moment frame system was modeled in RAM as shown. Moment connections were kept to the minimum to reduce costs, as these connections are very costly to construct. Fixed moment frames do not interrupt an open plan and are contained solely in the plenum space of the building. Due to the high moments that members in this system are required to carry, they can become very deep, and can potentially raise plenum depth to undesirable levels. Deep members can be used on the exterior walls of a structure without presenting this problem. Using RAM Frame, deflection for the N-S direction of the building proved to be extremely high. Increasing girder sizes in the frames to W40x149s did not solve the problem, as the deflection due to the critical seismic load still exceeded 24" (well over the 8.14" allowable). Using heavier beams than these in a 20-story building is unusual and will increase material costs to unwieldy levels. Moment frames should be used only when braced frames cannot be used.

iii. Combination System

As both braced and moment lateral load resisting systems had their advantages and disadvantages in this case, the third alternative to lateral system design was to combine the two types of braces such that their respective advantages could be exploited. Because the moment frame system allowed such high deflection in the N-S direction and because there are only two practical locations to place lateral force resisting frames in this narrow section, a braced frame lateral system was chosen for this direction. Although a system composed completely of moment frames would work in the E-W direction, moment frames are so much more expensive than braced that I opted for a combined system consisting of two braced frames at the exterior walls and two at the core, supplemented by four moment frames where the open plan does not allow braces. Implementing this system in RAM Frame and optimizing the design proved that this system is a very feasible one for the unification of the 750 East Pratt structure.

RAM Frame Model of Combination Framing System

d. Analysis: Comparison with Original Design

Once the final lateral system design was modeled and analyzed in RAM Frame, several checks were made in order to determine the performance of the system and compare it with the original.

i. Check for Strength

Whether or not the building satisfies serviceability requirements, strength criteria must be satisfied to ensure the safety and integrity of a structure. In order to achieve this, the structural members were first designed for gravity in RAM Beam and RAM Column. These results were imported into RAM Frame and analyzed using Drift mode and checking per LRFD standards. Each failing member was individually optimized until the building no longer failed due to inadequate strength. Where slenderness was a problem in several columns in the lower levels of the building (where story heights are larger and loading conditions are more demanding), bracing and stiffeners were modeled in the form of intermediate beams at half the storey height of the failing column. In the case that bracing would be preferable to stiffeners, these 'beams' were omitted where vehicular traffic would be obstructed by them. Only in these cases do the columns reach sizes of greater than W14x398.

ii. Check for Deflection

Once it was determined that no members in the design would fail due to strength criteria, drift was checked and found to be a concern. In order to reduce the drift of the structure to within H/400, or 8.14" in this case, moment frame member weights were iteratively increased until this criteria was satisfied. The sizes of structural members that were not rigidly connected to columns (as they are in the four rigid moment frames) were not increased, as their connections were assumed to be pinned and thus could not physically affect the lateral drift of the building. The depth of the girders was limited to W24 so as not to increase plenum depth beyond that of the original design. Once the girders in the moment frames reached W24x146 in the E-W direction, a drift of 8.09" was achieved, and the design was considered optimized. This is by no means an unwieldy shape, and is acceptable by weight and depth standards for my purposes. Actual member sizes used would, of course, depend on fabricator availability at the time of construction to maximize economic efficiency.

Deflection x100 in E-W Direction	Deflection x100 in N-S Direction

Several factors that can demonstrate the improvements made on the design of a building include a general member size comparison and cost analysis.

Due to the differences in building size and material, the total weight of steel used in each respective design will not give a meaningful comparison of the amount of steel material saved by redesign. Rather, a takeoff of steel beams on a typical floor was performed (columns were ignored because column size was not drastically altered). On a typical office level floor (level 6) the takeoff results are as follows:

• A typical floor in the 750 East Pratt redesign is approximately 28,000 sf. Including the interstitial and penthouse levels, there are 20 levels of structural steel framing. This corresponds to a total square footage of 560,000 sf. (versus the original 360,000+ sf. of steel framing), or according to the above estimation, 2800 tons of steel. (The original building also included concrete as well as steel framing on the original three floors and roof of the substation.)
  
  
• Typical composite decking consists of 2", 20-gage deck. For a building this size, at least 500 squares of decking will be used, so the material cost per square foot is $0.81. RS Means 053 10 42700
  
  
• There are 6 fixed moment connections per moment frame and 4 moment frames per floor (plus 3 intermediate bracing 'floors') which corresponds to 552 moment connections in the redesigned structure.
  
  
• There is approximately 1200 linear feet of steel framing per floor which corresponds to about 95,000 square feet of steel to be fireproofed. Spray-on fireproofing costs $0.42/sf. RS Means 072 554 0400
  
  
• There are 826 lf of HSS bracing per E-W brace and 2067 lf per N-S brace. 5x5x0.25 tubes weigh 15.62 #/ft; 6x6x0.25 tubes weigh 19 #/ft and are the cutoff between 15-20 #/ft @0.72/ft and >20 #/ft @ 0.66/ft. There are no tubes smaller than 5x5x0.25 in the bracing system of the building. RS Means 051 110 0400/0600
  
  
• The area of the original concrete-framed floor plan is 22,600 sf. (67,800 square yards) per floor, and the poured-in-place slab is 6" thick. Normal weight concrete is $126/cy including forms and reinforcing steel. RS Means 033 130 1950

i. Color Rendering

Color rendering is highly dependant on the type of lamp used. Metal halide lamps were used in all luminaires at the pedestrian level. Almost all varieties of white-light metal halide lamps have good color rendering properties which are achieved by a phosphor coating. Warmer or cooler colors can be achieved depending on preference. Metal halide lamps have color rendering abilities superior to any other type of HID lamp. Because of these properties and the high efficacies (75 to 125 lumens/watt), metal halide lamps were chosen to light the walkway.

ii. Glare/Reflected Glare

The decorative luminaires selected for the walkway were altered using 3D AutoCAD, resulting in matching wall-mounted fixtures. To reduce glare at the pedestrian level, these luminaires were placed at least 16' from ground level. While not overbearingly high, they no longer pose a direct glare problem. Reflected glare, on the other hand, is a positive effect where a metallic glass façade across from a river create specular reflections from the decorative luminaires.

iii. Shadows

Shadows are at a minimum. Light is distributed evenly over the ground level, as is demonstrated in the false color images below. The pedestrian bridge and centralized seating area have a slightly smaller illuminance, creating a more intimate atmosphere in these areas while retaining the required illuminance level. This effect is demonstrated by the rendering shown.

The shadows on the central portion of the façade also serve to accentuate the architectural illumination above the pedestrian area.

Rendering

False Color Image

The pole fixtures at the pedestrian level are Bega 8135MH pole fixtures placed at 30' except at the center, where a slightly lower illuminance is desired. They are lamped with 70 watt metal halide bulbs with a luminous flux of 5000 lumens. The wall mounted fixtures are custom fixtures created to mimic the Bega pole fixtures, and are lamped likewise. They are positioned symmetrically at the column lines of the building, and the pole fixtures line the edge of the river to cast specular reflections on the water as well as effectively light the pedestrian traffic areas. An illuminance of 10 footcandles (100 lux) is achieved on the walkway with the exception of the aforementioned shadowed areas.

The floodlights at the roof level are lamped with 175 watt metal halide bulbs with a luminous flux of 14000 lumens. This high intensity accounts for the harsh wash desired on the columns as compared with the other architectural features as well as the large light loss factor associated with an exterior lamp with such an inconvenient location for maintenance.

All of the lamps involved in the exterior illumination of 750 East Pratt are metal halide bulbs. This provides more convenient maintenance and the opportunity to buy in bulk, both of which reduce the long term cost of the system.

The IESNA/ASHRAE 90.1 table 9.3.2 states that the power limit for a building façade is 0.25 watts per square foot of illuminated façade area. Power usage of the lamps is tabulated as follows (based on a front façade square footage of 87250 sf):