Brent Ellmann Structural Option 200 Minuteman Park, Andover, MA Structural Consultant: Dr. Hanagan

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1 Structural Design: Goals: The original design of 200 Minuteman Drive was dictated largely by Brickstone Properties, the building s owner. The new design of 200 Minuteman Drive, with additional floors, also conforms to the owners goals (Goals 1, 2, and 4 below). Further goals have also been establish for the new design (Goal 3): 1. Maintain the all steel superstructure with a concrete slab on metal deck floor system, while adding additional floors. 2. Create an open office plan for future renovations and changes. 3. Improve performance of the floor system under vibration excitement due to walking for sensitive equipment, so that a greater variety of tenants may occupy the office space. 4. Allow for varying uses of the building by multiple tenants at once. The process of adding additional floors begins by using the Massachusetts State Building Code to establish the new building height and number of floors. The original floor system is then studied and any changes are made to improve the performance of the floor system for vibrations. Once the floor system is established, the columns, braced frames, and foundations can be designed, thus completing the structural design portion of this thesis. New Building Size: As mentioned in the Proposal section above, according to 780 CMR: The Massachusetts State Building Code, the maximum allowable building height for this type of structure is 85 feet, with a maximum of 6 stories. In order to maintain the 10 foot tall clerestory and 14-7 floor to floor heights, a total of 5 floors can be used. This results in an tall building, just under the 85 foot maximum height (See Building Section). The addition of two stories will add roughly 120,000 square feet onto the building, bringing the total area of the building up to 330,000 square feet of usable office space. The addition of the two floors will also increase the building weight by 12,390 kips. Page 16 of 96

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3 Floor System Design: In Brickstone Properties original design program for this building it was noted that the building will house research labs. Often times research labs have sensitive equipment in them, such as high powered microscopes, lasers, and finely tuned sensors. If a floor system used in this type of lab became too excited by vibrations, particularly those caused by employees walking around, these pieces of sensitive equipment could be adversely affected. The prevention of these disturbances of sensitive equipment is addressed in Design Guide 11, from the American Institute of Steel Construction. In Design Guide 11, the table below lists the criteria used to evaluate a floor system for vibrations. The table breaks down the levels of acceptable floor vibration velocities based on the sensitivity of varying pieces of equipment. In the vibration study carried out below it was assumed that the most sensitive equipment for this structure would be bench microscopes with 100x magnification Page 18 of 96

4 and laboratory robots. These pieces of sensitive equipment demand that the maximum vibration velocity be only 4,000 micro-inches per second. Design Guide 11 also classifies the vibrations caused by walking based on the speed of the walking excitation. There are three levels of walking; slow, moderate, and fast. Since this building is an office building, the chance that there will be any occupants running or walking very fast through the building is low. For this reason, in the following analysis only the slow and moderate velocities are considered, even though results for the fast walking velocities are given. The original basic framing plan consists of 30 x 30 bays. Typical bays consist of two W24 x 76 girders supporting three W18 x 35 beams (See Typical Floor Plan). Two more W18 x 35 beams connect directly into the columns, Page 19 of 96

5 completing a full bay. For this bay configuration, a four inch thick concrete slab on top of two inch deep 20 gauge composite steel deck, with 6 x 6 W2.0 x W2.0 welded wire fabric, creates the floor system. The table below shows the results obtained from a spreadsheet designed to carry out the vibration evaluation for 30 x 30 bays. Original Composite System Frequency (Hz) Deflection (in.) Beam Mode Girder Mode Combined Mode 5.51 N/A Mid-Bay Flexibility 4.58 x 10-6 in/lb Vibration Velocities (µin/sec.) System Max. Allow. Slow Walking 902 4,000 Moderate Walking 4,569 4,000 Fast Walking 20,563 4,000 As shown in the table above, the original system fails to meet the maximum allowable vibration velocities for moderate walking, but is sufficient for slow walking vibrations. Because the original composite system fails, a new floor system needs to be established and tested. It would be possible to continue to use a composite system by simply increasing the slab and member sizes until the vibration velocities meet the requirements. However, this would be an inefficient use of the composite action of the floor system. The process of evaluating floor vibrations does not depend on whether the system is a composite system or a non-composite system. The composite action of the floor has no bearing on the vibration characteristics of the system. It is the member and slab sizes that truly affect the vibrations in a floor. This means the final sizes found for a composite system, that meets the vibration characteristics above, would have the same final sizes as those found for a non-composite system that met the same vibration characteristics. Assuming this non-composite system meets the strength Page 20 of 96

6 requirements for gravity loading, the non-composite system would be the more efficient system because it would meet both the strength and vibration requirements, while requiring less labor and materials to install. The choice of a non-composite system here makes sense. The design of the non-composite system begins by designing a floor system that satisfies the strength requirements for the gravity loading. This design results in W16x31 beams spaced at 6-0 on center, with a 5 ½ slab on top of a 2 metal deck (See Bay Layout below). W27x84 girders complete the bay design. Using this bay set up in the same spreadsheet used before results in the vibration velocities shown in the following table. Non-Composite System Frequency (Hz) Deflection (in.) Beam Mode Girder Mode Combined Mode 5.89 N/A Page 21 of 96

7 Mid-Bay Flexibility 3.62 x 10-6 in/lb Vibration Velocities (µin/sec.) System Max. Allow. Slow Walking 667 4,000 Moderate Walking 3,377 4,000 Fast Walking 15,199 4,000 In this case the floor system satisfies the vibration velocities given by Design Guide 11. Based on the vibration study carried out above, the non-composite floor design shown here has been chosen to replace the original composite design. This change in floor system will not only improve the performance of the floor system under the owner s original program, but it will also open up more opportunities for more tenants. Before, any company that required the use of precise equipment in their labs did not have the option to rent space in 200 Minuteman Drive. Now that the floor system meets a higher level of precision, more companies can look at this building as a possibility for rental. For the owner, this results in more tenants and possibly charging higher rental fees for use of this facility. Column Design: 200 Minuteman Drive s layout results in two basic types of columns. The loading pattern on all the columns is the same, except for the columns that must support the added weight of the air handling units located on the roof. Below is a schematic showing the location of the air handling units on the roof. Page 22 of 96

8 The columns below these areas carry an extra dead load, from the air handling units, while the rest of the columns in the building do not. Two columns have been designed; one without the mechanical roof loading and one with the mechanical roof loading. The loads considered on these two columns are: Non-Mechanical Column: Roof Dead Load = 25 psf Roof Live Load = 30 psf Floor Dead Load = 80 psf Floor Live Load = 100 psf Mechanical Column: Roof Dead Load = 25 psf Roof Live Load = 30 psf Mechanical Dead Load = 180 psf Floor Dead Load = 80 psf Floor Live Load = 100 psf The following tables show the calculation of the loads carried by both the mechanical and non-mechanical columns. Since a five story column is impractical, the columns have been broken down into a lower three story column spliced with an upper two story column. The blue shaded part of the final table shows the loads on the upper column and the yellow shows the loads on the lower column. Page 23 of 96

9 Using the loads in the tables above, column sizes were chosen for the two different types of columns. The non-mechanical column is comprised of a three story W12x120, with a two story W12x72 on top. The two columns are spliced together at a point 4 feet above the fourth floor level. The mechanical column has a W12x136 column with a W12x87 column above, with the splice located at the same level as the non-mechanical column (See Typical Columns below). Page 24 of 96

10 The splices used in between the two columns are the same for both cases and are designed according to Manual of Steel Construction, Part 14. The splices are classified as Case I splices, based on the depth of the two members being spliced. On the upper column, filler plates are used to develop the full bearing on the lower column, and flange plates are used to connect the two columns together. The flange plates are 8 inches wide, 1-0 1/2 long, 5/8 thick, and are bolted to the columns. Braced Frame Design: The lateral force resisting system of 200 Minuteman Drive is composed of 10 braced frames placed throughout the structure (See Braced Frame Location Plan below). The center of rigidity and center of mass are roughly located in the center of the structure. The frames are symmetrically located about the center of mass in order to reduce the effects of torsion caused by the eccentricity between the center of mass and center of rigidity. By inspection, it can be seen that in this layout the critical frames will be those located in the far left and far right hand wings of the structure. These frames have the largest moment arm from the center of rigidity, and will have larger story shears from a lateral loading. For the purpose of this analysis, the two frames in the far left hand wing, one in the north-south direction and one in the east-west direction, will be inspected and designed for the worst case lateral loading. Page 25 of 96

11 In the case of 200 Minuteman Drive, the location of the structure plays a major role in determining the lateral loading carried by the braced frames. The wind loading for this structure is typical of a building located inland, with rolling hills and trees in the surrounding area. The basic wind speed is 90 mph and the site is an Exposure B site. In the north-south direction the wind pressure is 18 psf, while in the east-west direction the wind pressure is 15 psf. The table below shows how these numbers were found. Over the 80 foot height of the structure the wind pressure varies only 3 psf; therefore, it has been assumed that the wind pressure at the top of the structure is applied to the entire structure to simplify calculations. In the east-west direction the shear acting at each floor is 42 kips and 21 kips at the roof. In the north-south direction the shears are 138 kips and 69 kips, respectively. Typically the east coast is not seen as an active seismic region; however, the Massachusetts region is classified as an area of high seismicity, because it is near a fault line. 200 Minuteman Drive falls under Seismic Design Category B, having a short period spectral response acceleration of 0.30 and a one second spectral response acceleration of The site is classified as Site Class D, resulting in values of and for the short period design spectral response and one second design spectral response, respectively. The resulting story shears are displayed in the following table. Page 26 of 96

12 From inspection it can be seen that in the case of 200 Minuteman Drive the seismic forces are the governing lateral force. The structure is only 80 feet tall but has a large footprint of almost 70,000 square feet. This results in a very short, but heavy, structure. Seismic forces are directly proportional to the weight of the structure, so it makes sense that the seismic forces for this building are rather large. The combination of a short building, heavy building weight, and high seismicity results in the earthquake forces governing the design of the lateral resisting frames. At the 5 th floor the earthquake force is nearly four and a half times larger than the wind force at that floor. As mentioned before, for the purpose of this design, only two of the braced frames will be investigated. These frames, however, are located the farthest from the center of rigidity. Also, because these two frames are the most rigid frames, they carry the heaviest lateral load out of all ten frames. The table below gives the story shears for the two investigated frames. The frame labeled H-34 is the frame running in the north-south direction, and the frame labeled 3-HJ runs in the east-west direction. Using STAAD, the frames were designed while keeping three parameters in mind. First, the stresses developed in the members were inspected and kept Page 27 of 96

13 underneath the yield value of 50 ksi for A992 steel. In particular many of the cross brace members had to be evaluated multiple times before a size was found to prevent yielding. Also, the lower tier columns in both frames had to be increased from W12 x 120, as found earlier in the column design section, to W12 x 152. Secondly, the story drift and frame drift values were required to stay below 0.50 and 2.50, respectively, for a drift limit of L/360. Lastly, the width of the members used as braces could not be wider than 10. This was necessary to avoid the need to increase the depth of the walls where the braced frames are located. Frame 3-HJ originally had one steel tube cross brace through each panel, but that configuration drifted too much under the earthquake loads. To solve this problem a second steel tube cross brace was added to make an X in each panel (Shown below). The members are connected by a gusset plate at the center and at the columns. The new configuration met all the parameters laid out above. The overall frame drift is 2.40, and all story drifts remain below Just as important, the Page 28 of 96

14 stresses developed in all members stay below 50 ksi (See STAAD output in appendix). Frame H-34 is an Inverted K braced frame that utilizes W10 sizes for the braces. Just as before, the braces are connected to the columns and beams by gusset plates. Multiple sizes were tried in a STAAD model until the configuration below was found to satisfy the given parameters with the lightest members. The overall frame drift for this configuration is 2.43, below the 2.50 limit, and all the story drifts are under the allowable limit of Furthermore, the stresses found in the members, under the worst loading case, stay below the yield stress of 50 ksi. Page 29 of 96

15 Foundation Design: The addition of two floors onto the original design of 200 Minuteman Drive has increased the building weight by 12,390 kips. This increase means the foundation under the structure must be re-evaluated. Around the perimeter of the structure a strip footing is used to support the lower half of the curtain wall for the first floor. The loads applied to these strip footings have not changed, thus the original strip footings are still usable. The same holds true for the slab on grade used as the first floor. The soil and loading conditions have not changed, so the design of the slab on grade will not change. The square footings under all of the columns, on the other hand, will require larger and thicker sizes to account for increased gravity loads. Also, because of the increased lateral loading, the footings underneath the braced frames now experience large uplift forces, which cannot be compensated for by the dead weight of the structure. Where the braced frames are located, grade beams will be used, connecting the footings under the braces frames to the footings under the adjacent columns. There are three primary types of square footings found in this design. Just as with the column design, there are two different types of square footings; one for the columns supporting air handling units and one for those columns not supporting air handling units. The third type of footing is used under the braced frames. In this analysis, the footings under the two frames designed above will be the only braced frame footings considered. In all three cases 3,000 psi concrete, 60 ksi reinforcing steel, and engineered soil with a compressive strength of 6,000 psf will be used. Non-Mechanical Square Footing: A typical non-mechanical square footing must carry close to 500 kips of gravity load. The square footing size required to bear this loading on 6,000 psf soil is 10-0 x 10-0 x 2-0. The flexural reinforcement for this footing is #7s spaced at 12 on center, both ways. A 2-6 x 2-6 pier, reinforced with eight #7 dowels, evenly spaced around the perimeter, transfers the gravity load from the column down to the square footing (See cross section below). Page 30 of 96

16 Mechanical Square Footing: The footings for columns with air handling units above carry a dead load close to 650 kips. An 11-0 x 11-0 square footing reinforced with #7s spaced at 12 on center, both ways, was found adequate to support this weight. The footing is slightly deeper, measuring 2-6 deep, and it has the same 2-6 x 2-6 pier reinforced with eight #7 dowels, evenly spaced around the perimeter (See cross section below). Page 31 of 96

17 Braced Frame Squared Footing: The footings under the braced frames not only carry gravity loads, but also moment loads induced by the lateral forces. The results of the STAAD analysis, used to design the braced frames, also yields support reactions that the footings must resist. The support reactions for the two frames designed above were relatively similar; therefore the larger loading case, found in frame H-34, was used to design the footing. The footing was required to carry an axial load of 1,620 kips and a moment of 220 foot-kips. In order for a square footing to support these loads it must be 17-0 x 17-0 x 4-0. The reinforcing for this size footing is double layers of #6 rebar at 10 on center, going both ways. The pier size for this footing had to be increased from 2-6 to 3-0 in order to meet the bearing strength of the concrete. Twelve #7s are evenly spaced around the perimeter of the pier for reinforcement (See section below). Grade Beam Design: Under maximum lateral loading, one support of the two braced frames has an axial downward force of over 1,600 kips; however, the other support for that frame has an uplift force of 530 kips. The weight of the soil above the footing cannot withstand this large uplift force, so a grade beam is needed. To model Page 32 of 96

18 the beam it is assumed that pinned supports are placed at the locations all of the footings, except the one with the uplift force (See model below). Solving this model results in the moment diagram included in the figure below. As shown above, the design moments for the grade beam are 6,000 foot-kips and 3,900 foot-kips. It is important to note that the reactions developed in this model are sufficiently supported and developed by the designed footings. For example, in the model shown above the support at the far right requires a 200 kip downward force, which is supplied by the 630 kip axial load from the loads supported by the column. An initial size for the grade beam was determined based on the geometry of the footings and piers that are connected to each other by the beam. It is assumed that the bottom of all the footings will be placed at the same elevation, which means the top of the thicker footings underneath the braced frames are either 2-0 or 2-6 higher than the adjacent footings. The adjacent footings have pier sizes of 2-6 wide and the braced frame footings are 4-0 deep. Therefore, the first trial size was 2-6 x 4-0. However, it was found that the beam needed to be deeper and wider in order to fit the required reinforcing steel. Page 33 of 96

19 If the beam was made any deeper it would connect into the pier of the footings underneath the braced frames. The pier in these footings is 3-0 wide, so the new trial beam width was set at 3-0. The adjacent footings, though, only have 2-6 wide piers. To solve this problem and make construction easier, the piers of the adjacent footings have been increased to 3-0 x 3-0 piers. These changes resulted in a trial size of 3-0 wide by 5-0 deep (See Grade Beam Elevation below). This beam size is capable of carrying the 6,000 foot-kip load if a double layer of fifteen #9 reinforcing bars are placed in the top of the beam to carry the 6,000 foot-kip moment, and a double layer of nine #9 reinforcing bars are placed in the bottom of the beam to carry the 3,900 footkip moment (See cross section below). Page 34 of 96