structural engineer’s finite element software program, and the geotech- nical engineer utilizing PLAXIS model, a geotechnical finite element software. The structural model generated a preliminary contact pressure distri- bution. Terracon used the PLAXIS model and the preliminary con- tact 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 punch- ing 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 asso- ciated 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 accommo- date foundation settlement during construction. The client and design team decided the expected settlements were acceptable, and the estimates allowed the design of appropriate con- nection 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 (www.terracon. com), 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.
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 foot- ings, 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 col- umns, 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 founda- tions 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 de- sign, Terracon teammembers 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 eco- nomical 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 pro- cess between the structural engineer running the SAFE model, the
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august 2018
csengineermag.com
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