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FEATURED
PROJECT
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LEWIS STORE, FREDERICKSBURG, VA
The second floor was investigated for the future use as an office space with light storage, bookcases, meeting space, etc., with a required minimum live load of 100 psf (pounds per square foot), and dead load of 25 psf (total of 125 psf). The attic was evaluated for use as a potential storage space only, as the area does not lend itself for use as offices. The attic was investigated for a future live load of 50 psf. Upon completion of the assessment, S. Harris & Co. provided professional structural engineering and architectural design for the occupancy of the second floor. The project includes the reinforcement of the beams and girders on the second floor, the structural enhancement to the attic collector beam, and the conversion of the first floor corridor walls to bearing walls. HISTORICAL BACKGROUND The Lewis Store is one of the oldest surviving urban retail buildings in the United States. It was constructed in 1749 by John Lewis, a leading Virginia merchant and planter. Lewis was the brother in law of George Washington, as he married Washingtons sister, Elizabeth. The building is distinguished by the use of stone quoins at the corners, a feature rarely seen in small brick buildings, and carved stone lintels. These and other design elements make the Lewis Store an architecturally important urban store surviving from the colonial era. The Historical Fredericksburg Foundation, Inc. plans to operate a replica of an eighteenth-century store in the restored store as an innovative historical and educational attraction. OBSERVATION AND ANALYSIS Several field visits were made in order to assess and determine the load capacity of the existing floor joists. The floor boards above the summer beam and at the locations around the masonry ledges were removed on the second floor to expose the structure of the floor.
Second Floor There are two types of connections of joists to the summer beam, which are found in an alternating pattern. The joists were measured at 3 1/8 x 8 3/8 typical, spacing varies from 21 Ό to 27 Ύ, and were observed to have pulled their tenons out of the mortises in the summer beam from 1 to 1 5/16. The pins that were driven into the mortis and tenon joints were in some cases broken or missing all together. To support these vulnerable joist connections, steel stirrups, were installed to hold the joist and summer beam connection in place. These stirrups were installed during a renovation construction project in 2001.
Because the joists were pulled out
of their mortises in the summer beam, steel stirrups have been installed to keep
this condition from worsening. These stirrups give the joists the necessary
support at the connection to the summer beam. It is important to note that the
two joist connections at the far sides of the building, as well as one within
the corridor did not have these stirrups installed. At the masonry walls, the joists sit in a pocket approximately three to four inches deep. The joists are also supported by a masonry ledge, approximately 4 ½ wide. The ledge is continuous on all sides of the building. This brick set back provides an excellent support point for the joists at the masonry wall.
From this analysis, the summer beam was determined to be grossly inadequate to carry the required load of 50 psf for the office space and 80 psf for the corridor due to the large mortis and tenon joints cut into the beam. The summer beam requires remediation over its entire length to increase its loading capacity. A timber girder spans along the N-S direction of the building, and collects and transfers the load of the summer beam. The girder, measured 8 Ό x 11 ½, is supported by the masonry ledges at its ends. Due to the compromised cross sectional connection at the summer beam, the girder was calculated to be incapable of providing the necessary bending and shear capacities if it were acting alone. Deflection points of the second floor plan were measured along the collector beam and along the existing floorboards at each window opening. From observation, the floor plan resembles a bowl shape, with the highest points on the outside of the floor plan, and the lowest point at the center of the floor. After measurements were taken it was determined that the maximum deflection along the collector beam was approximately 10 inches from the west side of the building to the lowest point at the center of the floor. Attic
The collector beam for the attic, measured 4 Ύ x 8, also bears on the masonry ledger provided by the brick wall setback. Another rod and steel support assembly similar to the second floor support was installed at 13-9 from the west wall to carry the attic collector beam.
The beam is notched approximately
every 16 to receive the attic floor joists. Due to the large size of the notch
to accommodate a joist on either side of the beam and the number of notches for
the joist connections, the attic collector beam is compromised beyond any
reasonable use of the space. The current configuration of the beam and joists in
the attic can support a load less than 20 psf. Two supplemental steel assemblies were built, in 2001, to support the attic and second floor. The two wide flange beams, W12x19 and W12x26, cradles, and pairs of Ύ diameter rods were installed to support the summer beam on the second floor and the collector beam of the attic. The W12x19 beam was determined to be inadequate to carry the load of the second floor. The W12x26 beam was found to be adequate to carry the loadings of the attic floor. However, both of these steel beams are not appropriately laterally braced and are therefore incapable of carrying the required loads. RECOMMENDATIONS Second Floor S. Harris & Co. designed an intricate steel truss system, capable of withstanding live and dead loads totaling 125 psf, to reinforce the summer beam. Steel trusses, by definition, are structures formed by a group of members arranged in the shape of triangles. In this instance, the trusses are composed of a series of turnbuckles, clevises, threaded rods, and steel plates. The trusses are designed to relieve the summer beam of the incoming loads from the joists.
The upper portion of the truss consists of a bent Ό thick steel plate, which rests on top of each joist. The steel plate has three components: a turned up section, an overhang, and a flat plate. The turned up section is bent in order to prevent movement and to allow each individual truss system to come in direct contact with the next truss in the sequence. The overhang serves to connect the top flat plate, acting in compression, to the upper clevis.
The upper clevis is a steel plate bent into a channel shape that connects the overhang of the top steel plate to the diagonal threaded rod (plan view, below left). The threaded rod, 1 1/8 diameter, withstands the tensile loads of the truss, and connects the diagonal turnbuckle to the lower diagonal clevis.
The lower clevis connection consists of three steel plates that are bent into a channel shape, Ό thick. The lower clevis is broken down by an outer clevis, an inner clevis, and a diagonal clevis (above). A 5/8 diameter pin connects all three clevises at one point ( plan view). A hole must be drilled through the bottom portion of each joist in order to insert a threaded rod. These threaded rods tie each joist together, and are terminated at each turnbuckle on the horizontal lower axis. The lower turnbuckle and threaded rods are the primary source of tension in the truss system.
The N-S timber girder will also require additional support to increase its loading capacity. S. Harris & Co. recommends that the reinforcement of the first floor corridor wall be converted to a load bearing wall. The conversion requires the removal of plaster on one side and installing additional wall studs and structural posts on each side of the door openings. Additional support will have to be provided in the basement in order to distribute the loads generated from the new bearing walls located directly above on the first floor. New 4x4 posts and 2x2 square footing foundations will be placed in the basement in order to accommodate these loads from the first floor. In addition, leveling the second floor requires the removal and disposal of all existing floorboards. The leveling 2x8 pieces will be nailed to the side of each joist, and will rest on the masonry ledges, raising the floor a total of 10 inches.
This design requires minimal alteration to the original collector beams, girder, joists, and masonry ledges. Although the irregular joist spacing lends itself to highly detailed steel fabrication, the design is elegantly simple in the fact that each individual truss can be fine-tuned by the precise tightening of a turnbuckle. Attic The top flange of the wide flange steel beams installed in the attic will need to be braced laterally at the same locations that the bottom flange is already braced. If both beams are braced, the W12x16 will adequately provide a live load capacity for the attic floor of 50 psf. However, the W12x19 will still not be able to provide the 50 psf required for the light attic storage. The top flanges are proposed to be braced by building up the existing formwork with additional 2x4 material. A steel strap tie will be fastened to the built up formwork and wrap the top flange of the beam.
The attic collector beam is proposed to be reinforced with three separate steel plates, ½ thick, running the entire length of the beam. The existing steel rods that span from the second floor to the attic collector beam will be removed, and the rods will be reused to connect the new steel plates to the collector beam. CONSTRUCTION After a thorough investigation and design phase, the renovation improvements to the Lewis Store began in March 2006. The structural renovations in the basement have been completed, and the renovations in the attic are well under way. The second floor steel truss system in currently being assembled. The contractor has laid out the steel members and has started to install the clevices, turnbuckles, and bent up plate sections. The total truss assembly is estimated to take about a month, and then the initial turning of the system will occur shortly thereafter.
The second floor of the Lewis Store will be habitable with the structural reinforcements previously described and include architectural finishes consistent to that of the first floor. The Historic Fredericksburg Foundation, Inc. will be able to utilize the second floor as their office space, on a level surface. The aspects of this design require minimal modifications to the original fabric of the structural system.
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The
Historic Fredericksburg Foundation, Inc. retained S. Harris & Co. to investigate
and determine the structural condition of the Lewis Store, which is
located in Fredericksburg, Virginia. The focus of the investigation was to
assess and calculate the load capacity for expanded use of the second floor,
second floor corridor, stairwells, and the attic space. 
A
summer beam, which is original to the construction of the building, runs east to
west in direction and is approximately 8 ½ x 10 3/8 with some variation.
Significant wood rot is noticeable at 11-8 from the east wall, where Ύ
diameter rods have been dropped down from the steel assembly in the attic to
support this degraded point in the beam, thus forming a hinged support at this location.
These rods were also installed with other structural improvements in 2001. The summer
beam could be capable of handling
the loading of the second floor; however, the beam has been compromised by the
alternating joist connections. The mortises
created in the summer beam are invasive and tunnel almost through the entire
width of the beam at these connections. The joist connections into the beam have
diminished the cross sectional area of the summer beam so that this member
cannot withstand the load capacity in either bending or shear.
The
original joists in the attic show significant reduction in their capacity
brought on by the joinery detail into the collector beam. The joists measure 2
Ύ x 8, and are spaced approximately 16 on center. Like the floor joists on
the second floor, the attic joists also sit on a masonry ledge where the brick
masonry is set back. Each joist is notched at its end to fit into the collector
beam. The actual depth of each joist where it is notched into the collector beam
is only 3. The joinery detail has reduced the shear capacity of the joists to
an extent that has greatly minimized the space for any use unless additional
support is incorporated into the system.
