Proposed Structural Design Alternative

 

                Within the Washington D.C. market, concrete structural systems are the primary system specified.  While a composite steel system was chosen so that there would be some continuity between the existing and new structures, the rectangular floor plan lends itself to a concrete post-tensioned solution.  In an attempt to explore a burgeoning structural system in lieu of a more conservative alternative, an engineered wood products and heavy timber structural system will be studied. 

Within the past two decades, wood structural systems of the solid sawn heavy timber and engineered wood products varieties have become increasingly popular internationally and within the design community in an attempt to utilize the only major renewable structural material while increasing construction efficiencies.  While wood has been utilized for centuries in both Asia and Europe, it has recently enjoyed a revival in the design communities in Northern Europe, Japan, New Zealand, Canada, and the Northwestern United States.  Wood’s low structural weight, affordability, availability, and ease of construction give it a structural, financial, and sustainable advantage for low to mid-rise construction. 

The LiUNA Headquarters Expansion can take advantage of some of the strengths of wood design.  The rectangular floor plan allows for perpendicular framing of structural members.  The typical office space occupancy aligns with previous case studies such as the Bullitt Center in Seattle, the Tamedia Office Building in Zurich, and T3 currently under construction in Minneapolis.  The carbon-sequestering properties of wood present unique opportunities for holistic stainable design beyond typical LEED standards and would help enhance the existing LEED design of the project. 

 

Proposed Design Solutions

 

Bay Re-Design

 

                The original bay sizes and live loads inhibit the practical application of a wood system.  Initial studies in the alternative systems study with the full 40 span’ indicated that a timber frame design were not feasible due to impact on structural depth.  Therefore, an additional column line used only in the substructure was utilized in the superstructure.  In the redesign of the structural grid, impacts on the parking garage arrangement were considered and minimalized as much as possible.  The structural grid proposed eliminates two parking spaces total, but these locations can be used as bicycle or motorcycle parking spaces in keeping with the design intent of providing for transportation methods that limit fossil-fuel consumption and are adapted to urban commutes.  The proposed floor plan re-design is shown on the following page.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Gravity System

 

                 Initial investigation into reducing the live load from the original assumed 100 PSF to the typical 50 PSF with 15 PSF for partitions and accounting for live load reductions indicated two benefits.  First, CLT spans were sufficient to support the gravity loads without purlins.  Second, the depth of the girders was reduced 16 ½”.  To further reduce the structural depth, a structural steel saddle connection will be designed to keep the top of girder and top of CLT slab at the same elevation as shown. 

In the redesign of the gravity system, a standard steel saddle connection bracket for the CLT floor and girder will be designed to provide a simply-supported connection using either timber rivets, bolts, or screws bearing on a steel plate.  The necessity of stiffened connection will be investigated if a traditional unstiffened seat yields in flexure.  Based on knife plate glulam connections at columns, a simply supported connection for girders will be designed. 

 

Lateral System

 

                The proposed framing plan allows for the gravity and lateral systems to be separated from one another in the East-West direction.  This flexibility will allow for several possible lateral force resisting systems.  In the East-West direction, the original architectural intent was to preserve un-redistricted views through the building and curtainwall.  Within the four original moment frames, the girders were controlled by serviceability criteria under gravity considerations while the overall frames were controlled by deflection criteria of H/400. 

 

                The proposed floor plan introduces a fifth moment frame, thereby reducing the amount of load distributed to each frame.  The primary design solution will be to develop a wood moment frame system.  An important design aid will be the “Seismic Design of Glulam Structures” by A.H. Buchanan and R.H. Fairweather.  In an attempt to create a “tree” style connection between the perpendicular gravity and lateral systems, the ideal solution will be to create a bolted-bolted moment connection utilizing the Z parallel and perpendicular qualities of wood in combination with steel limit states for the respective materials.   In the event that a solution cannot be found in wood, then the existing steel moment frame can be compared to a concrete frame for construction and cost efficiencies. 

 

                Initial studies indicate that the bolted-bolted proposal is possible.  Using the updated structural grid and wind loads found in Notebooks A and C, wind loads were distributed using a flexible diaphragm analysis to each frame.  The worst case design loads were found to be 87K’ on the beams and 81 K’ on the columns at the second floor framing column F-N.3.  A portal frame analysis approach digests these moments into axial and shear forces that have to be resisted at the connection.  The beams and columns were sized for maximum moment and found that 14 ½” x 10 ½” of various rated strengths were adequate. By utilizing the double shear force parallel and perpendicular strength capacity values in Table 12I, the number of required bolts for each connection were found.  The limit states for plates and bolts used in steel design were also checked.  A schematic design representation of the connection is shown below in combination with the gravity CLT and girder system. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                 Notebook Submission C discussed the differences between the original design approach and that assumed for this thesis.  The existing LiUNA Headquarters system is considered to be independent, and the expansion building must be designed to withstand the loads it will receive.  While the original Braced Frame 1 in the expansion building was adequate to resist all of the lateral loads in the North-South direction, there was no alternative load path.  In response to these findings, the proposed lateral redesign will utilize dual lateral resisting systems along both the Western and Eastern edges.  A vertical truss system framed in wood will be compared to a CLT shear wall system for design efficiency. 

 

Construction Breadth

 

                While there are construction contractors that specialize in the construction of heavy timber buildings, the majority of those on the East Coast are located in New England and typically build one to three story structures.  In order to build something the size of the LiUNA Headquarters Expansion Building and to sustain a growth in the timber construction industry, more workers will need to become familiar with the timber erection process.  In an attempt to expedite the process of getting workers the necessary skills to become timber framers, parallels can be drawn between current industry standards and those of timber construction.

 

                While timber framers typically are classified as carpenters, the skills required are more akin to a cross between those possessed by ironworkers and pre-cast concrete workers.  Columns and beams are lifted into place using rigging and crane procedures similar to those used for steel erection, connections are made using bolts and steel plates, and CLT floor and wall panels are set into place like precast floor and wall components. 

 

                In order for the LiUNA Headquarters Expansion heavy timber redesign and other large scale projects across the country to be feasible outside of the geographic regions of New England and the Pacific Northwest, it will be necessary to complete the following:

 

1. Outline the construction sequence

2. Identify the primary rigging, erection, and connection tasks

3. Draw parallels between the tasks required and which workers currently in industry have the skillset to complete them

4. Analyze potential differences between traditional construction methods and those in timber and evaluate potential impacts on crew size, necessary training, and time to complete each task

5. Determine approximate crew sizes for each main task and time required using RS Means

6. Create an approximate structure erection schedule

 

Mechanical Breadth

 

                 In the past decade, wood buildings have grown in acceptance by the design and construction community.  Heavy timber buildings with educational and commercial occupancies such as the Bullitt Center and T3 in Seattle and Minneapolis, respectively, in addition to podium and stick construction in the Eastern United States have helped showcase some of the possibilities of designing with wood.  The increased focus has brought new questions about the safety of wood buildings in response to fire hazards. 

               

                 Currently considered a flammable material and able to contribute to fire fuel loads under typical building codes, wood is currently limited to a low-rise usage in most jurisdictions.  Passive and mechanical measures provide hopeful opportunities for wood buildings to still meet the spirit of the building code.  Passive measures such as the encapsulation method or charring method count on the properties of drywall layers or the wood itself to limit the spread of fire.  While drywall has a proven track record of being used to provide fire-rated separation, the predictable burning and self-extinguishing rate of fire in wood can be counted on to limit fire growth.  Furthermore, sprinkler systems can provide protection for the entire building.