Building a Strong Foundation


    Modeling lays the groundwork for a successful hospital project in Orlando.

    In the budget-driven and time-sensitive health care industry, reducing the project delivery schedule can produce significant cost savings and make these critical facilities available to the public sooner. The recently constructed Florida Hospital for Women in Orlando, Fla., demonstrates how collaboration between the owner, key design partners, and construction team can open opportunities for unique design and construction solutions while also expediting construction. In the case of this project, the relationships and communication between the project leaders led to a final project design saving the client more than $1 million and allowed the facility to open two months earlier to begin serving the community.

    The new Florida Hospital for Women facility, a 12-story, 430,000-square-foot tower, has 322 beds with three floors of 108-bed capacity reserved as shell space. Opened in January 2016, the facility has 14 labor and delivery suites, 13 operating rooms for obstetric and women’s services, 12 postpartum care beds, mother-baby beds, and high-risk beds. The operating rooms accommodate DaVinci Surgical System equipment for robotic minimally invasive surgery.

    Structural engineering design

    A cast-in-place, conventionally reinforced 12-inch concrete flat plate was selected as the primary structural framing system. An alternate structural floor and roof system proposed was a 27-inch-wide module joist system. Level 2, where the operating rooms are located, required a 24-inch-deep joist and beam system to meet the strict vibration criteria for micro-surgery operating rooms.

    Regarding column design, specifying a higher strength steel for the larger bars eased congestion and saved placement costs by reducing the required number of bars. A “blade” shear wall was required at the south end of the building to supplement the stair and elevator cores to maintain torsional rotation within allowable limits. An elevated pedestrian bridge connecting the new hospital to the New Bedford Tower on the south end and a one-story connector along the south elevation of the existing children’s hospital were also added.

    The new facility is situated on a tight urban site. It is bordered by a busy rail line on the west, an existing 40-year-old hospital building on the north (Andersen Wing), a 10-year-old children’s tower on the northeast corner, a public street on the south, and the main hospital entrance on the east. These active adjacent structures and connections to the new facility between the existing site buildings and features added significant complexity to design and construction.

    The hospital’s proximity to the rail line resulted in the need for the train-induced vibration to be considered in the design. Patient rooms and operating rooms needed to meet vibration guidelines for design and construction of health care facilities. A base isolation system was initially proposed to isolate the building from train-induced vibrations, but this was cost prohibitive. However, subsequent analysis showed that the 12-inch flat slab was sufficient to damp out transient vibrations for all conditions except the Level-2 operating rooms. These areas of the structure were stiffened to meet the more stringent requirements for micro-surgery.

    At the areas where the new hospital connects to the existing Andersen Wing at the ground and second levels, the new floor slab was designed with a recess to be leveled with the Andersen building after initial mat settlement to assure a smooth transition between the two structures.

    Geotechnical engineering and foundation design

    The geotechnical field explorations included SPT borings and CPT soundings to about 100 feet deep. The subsoil consisted of medium dense sand followed by medium stiff clays, and silty to clayey sands underlain by the Hawthorne Group soils.

    Foundation studies for many high-rise buildings in the general project area have shown that for column loads in the range of 1,000 to 1,500 kips, shallow foundations, in the form of spread and combined footings, should be designed using a net allowable bearing pressure of 4 ksf to 5 ksf to provide tolerable total and differential settlements.

    For column loads of 1,500 to 2,000 kips, continuous footings, usually in one direction and sometimes in both directions, have performed well. For column loads of 2,000 to 2,500 kips, a continuous mat at average and relatively uniform contact pressures of 2 to 2.5 ksf have provided cost-effective foundation support.

    Foundation soil improvements using vibro-replacement stone columns, geopiers, soil mixing, rigid inclusions, or similar techniques have been utilized for relatively high loads since they provide higher bearing pressures than those usually recommended for shallow foundations for comparable total and differential settlements. Beyond column loads of about 3,000 kips, deep foundations in the form of driven or Auger-Cast-In-Place replacement or displacement piles have typically been used and proved to have an economical advantage over shallow foundation systems for higher loads.

    The new facility would subject columns to loads of up to 4,000 kips. Such loads are routinely founded on deep foundations, extending 75 feet or more into the ground to tip in the Hawthorne Group soils. Conventional shallow foundations most probably will create excessive settlements under such heavy loads.

    At first, deep foundations were the preferred choice by the project team due to the heavy loading and to ensure minimal total and differential settlement. Next, a solid mat foundation was proposed by Terracon to be evaluated.

    Understanding both the geotechnical and structural aspects of the design, Terracon team members progressed beyond a typical geotechnical engineering study to an interactive design with the structural engineer, Kevin Casey, P.E., (Paul J. Ford), in collaboration with the contractor (Brasfield and Gorrie), and the owner’s representative, Mohammed Alai, AIA (Florida Hospital). The goal was to design the most economical foundation system to support the building. Settlement analysis was performed utilizing numerical modeling to provide an accurate estimation of the subgrade reaction modulus to be used in the structural analysis.

    After considering several options at the preliminary stage, Terracon proposed the use of a solid mat foundation. The design of a mat for such heavy and variable loading conditions required an iterative process between the structural engineer running the SAFE model, the structural engineer’s finite element software program, and the geotechnical engineer utilizing PLAXIS model, a geotechnical finite element software.

    The structural model generated a preliminary contact pressure distribution. Terracon used the PLAXIS model and the preliminary contact pressures to estimate settlement, redefining the contact pressure estimates using aspects of slab rigidity, subgrade reaction, and soil-structure interaction. The revised subgrade moduli across the mat were input into the structural model to estimate new settlement and contact pressures, and this iterative approach was repeated until estimated settlements from SAFE and PLAXIS became congruent.

    The iteration produced a final mat design with predicted maximum settlements of 2.5 inches at the interior core decreasing to about 1.5 inches along two adjacent buildings. An iterative structural design was used to determine the least cost mat by varying concrete strength and mat thickness. The limiting design parameter was maintaining punching shear stress below the allowable for un-reinforced concrete.

    Construction phase

    The contractor determined the cost of the mat foundation would be slightly less than deep foundations. A more substantial benefit associated with the mat foundation alternative was reduced construction time, significantly reducing costs and allowing the hospital to open the facility sooner.

    The challenge of accepting the mat foundation was the risk of building settlement. The client and design team needed to decide if accepting the risk of having a potential building settlement of 2.5 inches was worth a savings of two months in the project schedule.

    Geotechnical analysis predicted the majority of the settlement as being elastic, with greater than 90 percent of the settlement occurring in the first year of construction. The design team detailed a slight recess in those areas connecting to Andersen to accept a leveling topping that could be applied to assure a smooth transition between the structures. Architectural detailing considered the possible need for this “ramp” between the buildings. Design details were incorporated to accommodate foundation settlement during construction.

    The client and design team decided the expected settlements were acceptable, and the estimates allowed the design of appropriate connection points to existing structures. Settlement monitoring confirmed settlements were well within the maximum values predicted.

    Amr Sallam, Ph.D., P.E., M.ASCE, a senior principal at Terracon (, was the principal geotechnical engineer for the Women’s Hospital of Florida. He has been practicing geotechnical engineering in the United States and overseas for 24 years. His specialty includes consulting for shallow and deep foundations for high-rises, heavily loaded structures, bridges, wind turbines foundation design, deep and staged excavations, and more. He is expert at utilizing 2D and 3D finite element software to optimize foundations. Sallam has been involved in designing and reviewing many of Orlando’s most prominent high-rises, sports facilities, and airport facilities.