TECHNOLOGY & tRENDS
LEADERSHIP IN ENERGY AND ENVIRONMENTAL DESIGN (LEED)
1101 New York Avenue
The Leadership in Energy and Environmental Design (LEED) Green Building Rating System was developed by the U.S. Green Building Council (USGBC) to establish standards for environmentally sustainable construction. LEED promotes a whole-building approach to sustainability by recognizing performance in five major areas: sustainable site development, water efficiency, energy/atmosphere, materials/resources, and indoor environmental quality. Big business is embracing green building, governmental bodies are rewarding sustainable efforts, and individuals around the world are making everyday decisions about materials and methods that will impact generations to come.
CONCRETE AND LEED
Development in material and techniques keep concrete a leader in the green building movement.
Using concrete can increase the number of points awarded to a building in the LEED system. LEED designers recognize concrete's ability to create an effective thermal envelope, reduce sound transmission and provide a durable, safe and healthy living place. Concrete provides unlimited flexibility in all applications and because concrete is designed for each specific project, it generates minimal waste.
Concrete parking garages within buildings can be used to limit site disturbance, thus helping to maintain existing natural areas that would otherwise be consumed by paved parking. Additionally, cast in place structures provide sufficient design capacities to support green roof systems.
LEED promotes the use of materials with recycled content such as fly ash, silica fume, and slag cement. All of these materials are used in concrete construction and are considered post-industrial. Furthermore, using recycled concrete or slag instead of extracted aggregates would qualify as post-consumer. Concrete construction supports the use of local materials and reduced transportation distances since ready-mix plants are generally within 50 miles of a job site.
Miller & Long is committed to assisting its clients in meeting targeted LEED goals. We recognize the value of sustainable designs and the contribution that the concrete industry can provide.
For further information please visit the following websites:
U.S. Green Building Council at www.usgbc.org
Portland Cement Association at www.cement.org
An alternative to the traditional method of rebar protection (epoxy coating) is the use of a corrosion inhibitor. Corrosion inhibitors are liquids added to concrete during the batching process. They chemically inhibit the corrosive action of chlorides on reinforcing steel and prestressed strands in concrete. They also promote strength development of the concrete.
Corrosion inhibitors are recommended for all steel-reinforced, post tensioned and prestressed concrete that will come in contact with chlorides from deicing salts or a marine environment. Examples are parking garage decks and support structures, bridge decks and prestressed members, and structures in marine environments. Corrosion inhibitors are patented systems containing calcium nitrite, which interacts with the embedded steel in concrete to prevent salt attack. By chemically reacting with the reinforcing, a barrier is formed which prevents chloride penetration. Recommended addition rates range from 2.0 to 6.0 gal/cy. Corrosion inhibitors also increase the early strength of concrete mixtures and may have accelerating action on setting time.
Corrosion inhibitors provide another option for protection of concrete structures. We recommend consultation with a Structural Engineer to determine if an inhibitor is a practical option for your project.
Studrails provide simple, practical solutions for the control of punching shear in concrete structures. Studrails efficiently reinforce the concrete in high shear zones around concentrated loads by forming an open structured embedded truss.
Different studrails are manufactured to meet the requirements of individual applications and the system is totally reliable and cost effective. Studrails allow increased design freedom and flexibility.
SECURITY & PROTECTION
Due to recent events, many new structures are being considered for design against progressive collapse. Progressive collapse is defined as the spread of an initial local failure from element to element, eventually resulting in the collapse of an entire structure or disproportionately large part of it.
The inherent mass and stiffness characteristics of reinforced concrete offer distinct advantages over other building materials such as steel and timber under blast loading. Reinforced concrete structures are better able to resist the overall shock due to local disintegration caused by a blast. It is imperative that new buildings, which may be subject to sudden failure, be designed to provide force protection features that protect and ensure the safety of their occupants.
Miller & Long has provided input to many clients and design firms regarding various options for progressive collapse protection. We understand the necessities and challenges of today’s environment and are committed to providing structures that meet security goals.
MIXED USE STRUCTURES
Bethesda Theatre - Residential
Consolidation of living space, retail, entertainment and parking has led to the emergence of mixed-use structures. Urban environments as well as planned suburban communities have experienced the development of these multi-use designs. The various uses generally share a common foundation/footprint with specific design defined as the structure rises.
The varied uses create challenges in structural design due to multiple dimension bay spacing and loads. Differing floor heights and transfer conditions require designers to integrate uses and structural systems in such a way so that economy is maximized.
Flexibility of concrete designs permits the integration of multiple layouts and structural systems necessary for the creation of the mixed-use product. Concrete systems also allow easier modifications to be made during the construction process.
Miller & Long has constructed a variety of mixed-use structures in both urban and suburban settings. We continue to provide input and analysis regarding economy and logistics to our clients during the development process to ensure a successful project.
LONG SPAN OFFICE STRUCTURES
THE DEMAND FOR LONG SPANS
Recent market trends have created a demand for usable floor space that is not interrupted by vertical structural members (columns and shear walls). The "open floor plan" provides the flexibility to create the desired floor layout for the individual tenant without being subjected to restrictions by the structural elements. Typically, the structural floor plate is open in width from the corridor to the building edge and in length for the entire building. Most of these structures have typical floors of approximately 25,000 to 30,000 square feet and are five to eight stories high. However, long span systems work well for structures with typical floors exceeding ten stories. Clear span dimensions from corridor to building edge are typically 40 to 45 feet and vary in the short span from 20 to 30 feet.
PAN / SKIP JOIST, SHALLOW-WIDE DROP AND UNIFORM FLAT SLAB DESIGN
Generally, to achieve the desired long span in a concrete structure, three basic designs are considered – Pan/Skip Joist, Shallow-Wide Drop and Uniform Flat Slab. All are one-way slab systems that can be modified to fit a specific bay spacing.
The Pan/Skip Joist system utilizes either steel or fiberglass pans which vary in widths of 30", 53" and 66". Post-tension cables are generally used in beams, pan ribs or both. Depths of pans are usually 16" to 20" with a 4 ½" to 5 ½" concrete slab on top of pans.
Shallow-Wide Drop systems are a continuous drop/beam, which extends for the entire length of the long span with a 6" to 9" slab between the beams. This system has post-tensioned reinforced beams with either mild steel or post-tensioned reinforced mid-spans. The width of the drop/beam varies depending upon bay size but is generally 42" to 72" wide and 16" to 18" deep (including slab). Beams may also be utilized in the short span direction, but they are generally not necessary.
Uniform flat slab designs maintain a constant slab thickness throughout and incorporate column capitals that project no more than twelve inches (12") from the column face. Slab thicknesses generally range from ten inches (10") to twelve inches (12") with a column capital thickness maximum of twelve inches (12"). Studrails may also be considered when analyzing punching shear at columns.
ADVANTAGES OF SHALLOW-WIDE DROP SYSTEMS
Westpark Corporate Center
In the vast majority of cases involving long span floor plates, the Shallow-Wide Drop system has been proven to be advantageous over the Pan/Skip Joist system. First, the Shallow-Wide system reduces floor height. On average the Shallow-Wide system is six (6) inches thinner than the Pan/Skip Joist system. This savings of floor-to-floor dimension reduces building skin costs as well as heating and cooling costs which, in the case of the latter, are continual. Second, the Shallow-Wide design reduces labor costs. This is due to the fact that layout and handling of pans is eliminated. In some cases, this labor reduction may result in a shorter construction schedule. Third, the concern for rent and availability of pans is eliminated by the Shallow-Wide system. Since pans must be rented, the cost of pan use can vary based upon availability. The rent itself is an added cost since deck formwork is required whether pans are used or not. In addition, floor size, layout and schedule may not allow a significant re-use of pans thus increasing rental expenses.
ADVANTAGES OF UNIFORM FLAT SLAB DESIGNS
Uniform Flat Slab designs gain the same advantages that Shallow-Wide systems have over Pan-Skip joist designs. The one major exception is that they reduce floor to floor height more than shallow-wide designs. It is this fact that makes the flat slab more economical, in some cases, than the shallow wide design. The simplicity of the flat slab design is also beneficial when designing mechanical systems for long span structures.
VARIOUS SHALLOW-WIDE AND UNIFORM FLAT SLAB DESIGNS
The design of a Shallow-Wide Drop or Uniform Flat Slab system will vary due to a variety of factors. However, the most significant factor in determining the design of a structure is bay size. Since most long span floor plans demand a 40 to 45 foot clear span in one direction, the column spacing in the short span direction assumes greater importance. As the dimension of the short span increases so does the size of the beam or thickness of the slab in the long span. Additionally, post-tension cable and steel densities increase. Generally, the short spans are limited to 20 to 30 feet, however the desire for large corner bays may increase the dimension to the range of 45 feet. In these cases, a rather large intermediate beam or thickening of the slab is required to control slab deflection.
Post-Tensioned Cable Designs – Bonded/Unbonded
Post-Tension Cable designs may be bonded or unbonded and may be used in both beams and slabs. Bonded cables are cables that are bundled into PVC or galvanized metal ducts. The cables are then stressed and grout is pumped into the duct that creates a monolithic cable system. Bonded systems have a slightly higher cable density, but may reduce the amount of rebar required in slabs and beams significantly. Unbonded cable systems are monostrand cables encased in greased PVC sleeves that are independent of each other. Unbonded tendons will require additional steel but may save time due to the fact that the grouting operation witnessed in the bonded design is eliminated.
Post-Tensioned and Mild Steel Hybrids
Preserve at Tower Oaks
The preference to eliminate post-tension cables from mid-spans has led to the design of post-tensioned and mild steel hybrids. These designs have either bonded or unbonded post-tension cables in the drop/beams and only mild steel in the slabs between the beams. Absence of post-tension cables in the slab enables future core-drilling of slabs without the possibility of striking post-tension cables. This design is more cost efficient in conditions where the short span dimension is held close to 20 feet. Steel densities in the slab increase significantly as the short span dimension approaches 30 feet. This is due to the fact that post-tension cables control deflection at long spans more efficiently than mild steel. Hybrids are possible with flat slab designs but are not generally considered due to design complexities.
SHALLOW-WIDE/FLAT SLAB COMPARISIONS
The major advantage in any long span design is the ability to provide open floor plans. This can be achieved with a variety of designs. Thus, the next logical step is to provide this layout with the most economical system based on a client's specific needs.
Cost variances can only be analyzed once specific floor plans have been submitted. However, the direction of costs can be provided by general descriptions of floor plans. Most often, the shallow-wide drop and uniform flat slab designs are the most economical long span systems. They are well suited for standard office loads.
When comparing systems, several factors should be considered. They include weight of structure on the foundation, formwork, rebar and post-tension cable densities, floor to floor height and hybrid design. Multiple options may be considered utilizing both shallow-wide and uniform flat slab systems. A structural floor cross section indicating a system's specifics at the interior and exterior bays will be sufficient to provide a comparative analysis
Shallow Wide Long Span Design
Uniform Flat Slab Long Span Design