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

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1 Executive Summary: 200 Minuteman Park currently stands as a three story commercial building. The original design was limited to three stories by an industrial zoning law in Andover, Massachusetts. If this zoning law is ignored, the maximum height of the building, set out by 780 CMR: The Massachusetts State Building Code, is eighty five feet or six stories. Maintaining the original clerestory and floor to floor height allows for a five story building to fit under the maximum height of eighty five feet. This change in the design will necessitate changing the column sizes, foundation sizes, and lateral bracing. The original composite beam floor system will be retained, and braced frames will still be used as the lateral force resisting system. The mechanical and electrical systems will also be redesigned for the new larger building size. In order to maintain the multi-tenant capabilities of the structure, a change in the HVAC and electrical systems design schematic will be required to allow for many tenants at once. All of these changes to the building systems will directly affect the construction of the structure. The increased schedule and materials will result in a larger budget and change in construction sequencing. The increase in building height will also change the crane used on the job. Page 1 of 17

2 Background: 200 Minuteman Park stands as a 200,000 square foot Class A office building in Andover, Massachusetts, worth roughly $15,000,000. Although the building has a large square footage, it stands only three stories above ground plus a roof that contains two mechanical penthouses (See Building Section). In plan this office building Page 2 of 17

3 appears to be a large semi-circle, with the flat side of the building facing to the south and the curved façade pointing towards the north (See Building Plan). Page 3 of 17

4 The southern wall faces a small courtyard with a pond and park, while the northern façade faces a road. The southern façade is comprised of metal cladding and low-e reflective glazing, designed to reduce energy costs and improve thermal efficiency. The curved northern façade also has the same low-e reflective glazing, but instead of the metal cladding there is a limestone brick veneer in between the glazing. In the center of the structure a large atrium, with a clerestory at the top, has been placed to allow natural light to penetrate into the interior of the building and to separate the structure into two halves. Early on in the design of this building the owner, Brickstone Properties, wanted to allow for future use by multiple tenants; therefore the building has two separate mechanical penthouses, each supplying half of the structure. The systems found in 200 Minuteman Park are typical of an office building. The building is fully sprinklered and has an addressible fire alarm system. On the roof, 6 air handling units sit, three each in the two mechanical penthouses, to provide ventilation and cooling. On the ground floor there are 2 natural gas-fired cast iron boilers that provide the heating for the structure. The power supplied to this building comes from utility transformers at a voltage of 480/277 volts. Sticking with the idea of allowing for multiple tenants, there is a dual main-tie-main switchboard, with each switchboard having a 2,500 amp rating. 200 Minuteman Park has a structural system composed primarily of wide flange steel beams, girders, and columns, with composite concrete on metal deck floor systems. The columns and large girders in 200 Minuteman Park are ASTM A572 Grade 50 steel, Page 4 of 17

5 steel tubes or pipes are ASTM 500, Grade B steel, and the rest of the steel in the structure is ASTM A36. Concrete footings and piers used in the foundation have a minimum required strength of 3,000 psi after 28 days. The concrete used in the rest of the building, i.e. interior slabs, exterior slabs, and walls, has a required minimum strength of 4,000 psi after 28 days. Concrete strip footings, ranging in size from 1 x 2 to 1 x 2-8, were used around the perimeter of the building, while the steel columns are supported on concrete footings and piers of various dimensions (See Foundation Plan). The piers are all 24 x 24 square and extend up to the base of the concrete footings located below the columns. 200 Minuteman Park has no basement or sub-terraineous levels; therefore the first floor is a four inch thick concrete slab on grade reinforced with 6x6 W1.4 x W1.4 welded wire mesh. Page 5 of 17

6 The basic framing plan consists of 30 x 30 interior bays and an exterior arc that is broken down into 30 sections. A typical bay consists of two W24 x 76 girders supporting three W18 x 35 beams (See Typical Bay Layout). Two more W18 x 35 beams connect directly into the columns, completing a full bay. The arc is comprised of a series of W24 x 55 and W24 x 131 girders that link the arc s steel tube columns. For this bay configuration, a four inch thick concrete slab on top of two inch deep 20 gauge Page 6 of 17

7 composite steel deck, with 6 x 6 W2.0 x W2.0 welded wire mesh reinforcement, creates the floor system. The roof framing consists primarily of 22K6 open web joists spanning between W24 x 55 girders. In a typical bay there are 4 joists between the girders and two joists that frame directly into the columns. Sitting on top of the joists is one and a half inch thick 20 gauge galvanized wide rib deck that is continuous over three spans. In the center of the structure two mechanical penthouses sit, housing the six total air handling units and four smoke exhaust fans. In the areas where these mechanical units sit, the framing changes from K series joists to non-composite W18 x 35 beams to accommodate for the increased dead load (See Roofing Plan). Page 7 of 17

8 The lateral force resisting system used in this structure is a series of braced frames placed in the east-west and north-south directions of the building. As can be seen from the provided building plan of 200 Minuteman Park, there are 10 braced frames placed throughout the structure (See layout below). Typical frames are composed of two W10 columns and a variety of W24, W10, and TS compression and tension members (See Page 8 of 17

9 Typical Lateral Bracing Elevations). The loads which these compression and tension members must carry are listed on the drawing. The geometry of the frames varies by their position in the structure, because at the location of some of the braced frames, openings are needed for a loading dock and other passage ways. Problem Statement: 200 Minuteman Park was developed by Brickstone Properties to be a commercial office building. However, the area in Andover where the building has been placed was zoned as an industrial park. According to Andover s strict zoning laws, any building sitting on an industrially zoned area cannot exceed three stories. This zoning law explains why 200 Minuteman Park was designed to only be three stories tall, when by Page 9 of 17

10 780 CMR: The Massachusetts State Building Code it could have been at least six stories, or a maximum height of eighty five feet. Proposed Solution: The strict zoning law will be ignored, for the purpose of this proposal, and the building will be redesigned under the limitations set forth by 780 CMR: The Massachusetts State Building Code. Using the maximum height of eighty five feet as a starting point, ten feet must be reserved for the clerestory placed at the top of the atrium. That leaves seventy five feet for, at the most, six stories. In order to maintain the current floor to floor story height of 14-7, five stories can fit within the remaining seventy five feet. This change adds two more floors onto the structure, along with almost 120,000 square feet. The total floor area of the redesigned structure is now 330,000 square feet, for five stories, with a floor to floor height of 14-7 (See Building Section below). Adding two stories and 120,000 square feet onto this structure will obviously change all the systems, i.e. structural, mechanical, electrical, etc., of the building. Although the floor system will remain the same, since the loading on the floor does not change, the sizes of the columns, footings, and lateral bracing will change considerably. Also, since the HVAC system will have to be enlarged to carry the added loads, more roof-top air handling units will be required. This means part of the roof will also require a change in design to carry the increased dead load of the air handling units. Accompanying the increased HVAC loading will be the increased electricity needed to Page 10 of 17

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12 service the increased lighting, mechanical, and other extra electrical needs of the two new floors. In order to address these changes in the systems, a majority of the building will have to be redesigned and resized. In particular, the lateral bracing for this building will require an in-depth analysis to find a new solution for resisting the increased horizontal forces for this structure. In order to maintain deflection limits of the building, it may be necessary to change the one-bay braced frames into two-bay braced frames, in order to carry the high seismic loads. Solution Method: The new design of the structure will be carried out using the LRFD Method. The original structure had a combination of A36 and A992 steel; however, for this design it will be assumed that all the steel used in the structure will be A992 steel. The floor system will remain as a composite beam system, with a four inch slab on top of two inch decking. The roof system will be composed of the same K-series joists as before, but in certain areas the roof system will change to account for the extra air handling units. This entire design will be completed using RAMSteel to design the gravity resisting elements of the structure. The lateral bracing will be designed using either STAAD or ETABS. The lateral forces will be re-evaluated for the taller and heavier structure, then applied to various trial braced frames to find which will be best suited for the structure, paying particular attention to the deflection of the frames. The breadth requirements will be met by resizing the air handling units and heating elements of the building to meet the increased demands. The type of lighting in Page 12 of 17

13 the building will remain the same; however, the switchboards will need to be resized and new ones will need to be added in order to handle the higher demand of electricity for the added lighting and mechanical loads. The construction of the larger building will also be looked at to find out how adding two floors will change the cost and schedule of the building. Tasks and Tools: Task #1: Develop Model in RAMSteel a) Original floor system will be retained because it was owner specified 1) A992 steel with a maximum deflection of one inch 2) Four inch concrete slab on two inch decking-composite floor 3) Four inch thick slab on grade for first floor. b) Upper floors will require special attention and modification to allow the atrium to remain and extend the extra two floors. (Maybe easier to add two floors underneath current second floor.) c) Dead loads and Live loads same as original design. 1) Dead Load = 80 pounds per square foot 2) Live Load = 100 pounds per square foot (includes partition load) Task #2: Design Lateral System a) Find new lateral forces on building - Wind or Seismic b) Design new braces in either STAAD or ETABS (Assume relative Page 13 of 17

14 stiffness of braces remains the same.) c) Depending on results, determine if changes need to be made (i.e. more frames, stiffer connections, bigger frames) Task #3: Design Foundation a) Find forces acting on square footings under columns from the outputs of the RAMSteel and STAAD/ETABS models. b) Design of square footings underneath the columns for the braced frames will require special attention to carry the large overturning moments; option of changing the square footings into one large strip footing may need to be looked at. c) 3,000 p.s.i concrete on top of 6,000 p.s.f engineered back fill used; General Failure Factor of Safety will be four. Task #4: Design of HVAC a) Determine new HVAC loads for increased building size. b) Maintain original HVAC design; just increase number and/or size of air handling units, boilers, and other equipment. c) Find total number and sizes of new equipment and determine if the original structure needs to be redesigned to carry the larger dead loads caused by more mechanical equipment. Task #5: Design of Electrical System a) Determine new electrical needs for increased building size. Page 14 of 17

15 b) Maintain the original floor by floor design, including the lighting style, to meet the owner s wish to use the building as a multi-tenant facility. c) Resize the main switchboards to account for increased electrical demand; may want to change set up to maintain an efficient method to allow for multiple tenants. Task #6: Cost and Schedule a) Develop new basic schedule for the larger building size. b) Find a new cost of the larger building using Means. Task #7: New Drawings a) Create new drawings and schematics to represent the new building design using AutoCAD. Schedule: Page 15 of 17

16 Breadth Topics: Increasing the building size by two floors has a direct impact on every aspect of the design and construction process of this building. Originally the air conditioning and heating of the building was handled by six air handling units and two boilers. The addition of the two floors will require either larger air handling units and boilers, or the addition of more units and boilers onto what is already in place. The change of air handling units will also require resizing some of the main supply ducts to the building to carry the increased air supply. Just as with the mechanical systems, the electrical systems will need to be increased to carry the larger loading. The addition of two more floors of lighting, receptacles, and mechanical equipment requires more electricity to be supplied to the building. This means the main switchboards must have an increased rating, and the wires used to supply these switchboards may need to be enlarged in order to maintain a safe system. Because the owner wishes to maintain a building capable of housing multiple tenants, the choice of placing separate switchboards on each floor will be investigated as a viable solution. Obviously adding two floors onto the original design will mandate a change in the construction management of the building. The schedule will be lengthened and the cost of the building will increase. The original design called for the columns to be the full height of the building. This allowed the structure to be erected in lifts that were the full height of the building. Now that the building is five stories, more than one lift is needed Page 16 of 17

17 because a five story column is impractical. The process of erecting the steel and floor system requires an evaluation, which includes determining the size and placement of the crane needed for the taller building. Page 17 of 17