Structural and Seismic Assessment

The historic Carnegie building is a single story with a complete basement. The building has a wood framed roof and floor with exterior concrete walls. The building withstood the 2001 Nisqually earthquake with minor cracks in the walls and interior plaster surface. Recommendations in this report for the seismic upgrade of the building are based on a life safety performance objective in accordance with the American Society of Civil Engineers (ASCE) 31-02 Seismic Evaluation of Existing Buildings.

 

Stacy Bartoletti, of Degenkolb Engineers, Seattle, completed a site visit and structural investigation of the existing building. The following report that he provided, describes the existing structural system and condition of the existing structural system, identifies structural seismic concerns, and gives preliminary recommendations for an upgrade, and separation of the 1968 Addition from the original structure.

 

Existing Building Description

The existing Carnegie Library is a single story structure with a complete basement and a partial mezzanine level. No original drawings for the building were available for review; however, the structural system could be observed through removal of interior finishes at one location and the open attic area. The building has exterior concrete Basement and Upper Floor walls and presumably has concrete spread footings. The exterior walls at the Upper Floor appear to be approximately 8 to 10 inches thick. (See lower photo on page 27.)

The concrete walls appear to terminate at the bottom of the ceiling framing. The ceiling is framed with 2x10 joists at 16 inches on center and beams, and is supported at the exterior by the concrete wall and in the interior by two columns. The roof framing consists of wood trusses spaced at 24 inches on center that span the entire width of the building between exterior walls with ¾ inch straight sheathing. It appears as though the ceiling framing is supported completely independent of the roof framing.

 

The roof framing is supported by wood stud framed pony walls landing on the exterior concrete walls as shown on the upper photo on page 27. Framing for the Upper Floor consists of 2x12 wood joists spaced at 16 inches on center with wood beams and diagonal floor sheathing. Framing for the Mezzanine level was not verified in the field; however, it is presumed to be wood framed with 2x joists.

The existing structural system appears to be in good condition. No signs of settlement, concrete deterioration, or wood rot were observed.

 

Structural Seismic Performance and Recommendations

The seismic lateral force resisting system in the building consists of the existing perimeter concrete shear walls and the wood roof diaphragm. During an earthquake it can be expected that some amount of movement and possible separation will occur at the ceiling and roof connection at the top of the concrete walls. When subject to seismic loading perpendicular to the face of a perimeter wall, the heavy concrete wall will move in and out of plane and will need restraint at the top. The current framing system and roof diaphragm does not provide sufficient restraint at the top of the concrete walls.

 

Other seismic concerns include the potential for clay roof tiles to fall off the building and the potential for the existing unreinforced masonry chimneys to fall during an earthquake. Tile falling from the roof and collapse of the chimneys can cause significant life safety concerns.

 

As part of the Snohomish Carnegie Library renovation, it is recommended that a number of seismic upgrades be completed to provide a life safe structure. These items are identified and described below:

  • Anchor the existing perimeter concrete walls to the roof and ceiling framing. This will likely include the addition of anchor bolts into the top of the wall along with hardware for direct connection of the framing to the wall. The anchorage can be installed from the top of the wall and completely concealed within the ceiling and attic space, so that there will be no visible changes to the exterior.
  • Provide a plywood diaphragm at either the existing roof level or the existing ceiling level at the Upper Floor. If the plywood diaphragm is provided at the roof level it will require the removal, salvage, and reinstallation of the existing clay tile roofing system. Plywood could be installed over the top of the existing ¾ inch straight sheathing. Given that the ceiling framing is independent of the roof framing, the plywood diaphragm could also be located at the top side of the ceiling framing below the roof trusses, or below the ceiling framing with removal and replacement of existing ceiling finishes.
  • Provide plywood sheathing and blocking on the existing stud pony walls from the top of the exterior perimeter concrete walls to the roof diaphragm level.
  • Provide direct anchorage for the existing clay tile roof. The existing anchorage for the clay tile roof is not known. If anchorage does not exist then it may be best to provide the plywood roof diaphragm in Item 2 at the roof level to address both conditions at one time. Investigation for clay tile roof anchorage would require roof access and removal of a predetermined number of tiles to look for wire anchorage and nails.
  • Remove or brace the existing masonry chimneys. It is assumed that both chimneys are of masonry construction above the roof line; however, this could not be confirmed. Bracing of the chimneys would require significant diagonal members back to the roof structure and perimeter walls. However, as neither chimney appears to exhaust existing systems, they may be removed. According to the architect, the north chimney appears to have little historic significance. The south chimney was once more elaborate, and clad in stucco, but it does not appear to have been an original character-providing feature.

Separation of 1968 Addition

Based on field observations and a review of the 1968 Addition structural drawings, it appears as though the Addition is primarily structurally independent of the original Carnegie Library building. The Addition roof framing consists of steel bar joists, wide flange beams, and wide flange columns. It is recommended that demolition of the Addition for separation from the Carnegie Library be taken to a grid line to allow complete removal of bar joists. Should the demolition stop short of a grid line, the existing bar joists will need to be shored, cut, and re-supported.