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Dewey Struc Report5a! RYAN S. HELLWIG, PE ! STRUCTURAL ENGINEER ! January 11, 2012 Laura Fitch, AIA, LEED AP Kraus-Fitch Architects, Inc. 110 Pulpit Hill Rd. Amherst, MA 01002 lfitch@krausfitch.com Re:Feasibility Study - Renovations and Repairs Dewey House 4 Neilson Drive Smith College Northampton, Massachusetts Proposed Scope of Project Structural Work a.Structural Improvements (1)Structural analysis of existing roof structure is required for higher snow loading due to significant increase in thermal insulation values. (a)Reinforcement of existing roof structure may be necessary. (2)During the renovations of the bathroooms, some water damage may be discovered due to leaks or previous cutting for piping. (a)Reinforcement of existing floor joists may be necessary. (3)Repair of 8+/- foot long section of wall in the corner of conference room 105. (4)Reinforcement of floor in front of the Secretary’s Office. b.Structure for New Elevator / Lift Structural Description & Condition Assessment The original building is over 175 years old. Over the course of its life, it has been modified many times, including having been moved twice. No complete, accurate structural drawings exist. The building consists of two wings: one approximately 3700 square feet on two floors towards the southeast, which appears to be older construction and is considered the original part of the building; another almost 3400 square feet on three floors to the northwest, which was added around 1898. Sagging and settlement are noticeable in several locations, particularly in the old wing. In a building of this age and history, such deformations are not uncommon. The following specific issues in the old wing were observed: 1)The brick pier in the basement appears to be located under the threshold of the entry into the Secretary’s Office. There was no timber beam on top of the pier, but rather only one joist. This pier is not labelled on the 1994 drawings, and so is presumed to pre-date them. The reason for the pier’s exact location is not clear, but it would seem that the weight of the wall above in the 1st floor is loading the joist through the door jambs on either side of the pier, with the pier under the middle of the door opening. 2)In the access entry corridor #100-C there appears to be some settlement in the surround of the ! 28 ALDRICH STREET ! NORTHAMPTON, MA 01060 ! ! VOICE 413-584-HLWG (4594) ! FAX 413-584-HLWFax (4593) ! Structural Assessment Dewey House Study Smith College January 11, 2012 new steel columns and the adjacent walls. This movement appears independent of the structural column. The exact cause is not known. 3)There is significant damage to the plaster and trim in the corner of conference room 105, opposite the exterior damage in the siding and trim. 4)There is significant sag in the 2 floor under the telephone booth. The exact cause is not known.nd In the 1994 renovation, some repairs were made to the existing roof structure in the old wing, including restoring some diagonal timbers and wooden pegs in the roof trusses and reinforcing some rafters; also some additional floor beams, columns and footings were installed in the 1898 wing. A stair tower was also built at the far side of the 1898 wing. The rafters in the old wing are 3x7 spaced at about two foot centers. They are supported by intermediate purlins which span between timber trusses. The rafters are flush with the purlins, being set into mortises, in a traditional timber-framed manner. In 1994 light-gauge metal joist hangers were added between the rafters and the purlins. The purlins are 8x8 running framing into the quarter points of each roof truss. The purlins are flush with the truss top chord, and steel plate saddle connectors were also added in 1994 to improve that connection. The timber trusses are also traditionally timber-framed, with a King + Queen Post profile. A copy of the drawing A-3.0 by Kraus-Fitch Architects, Inc., is attached to show the truss configuration. The following are of note in these trusses: the tops of the queen posts have diagonal braces that are parallel to the top chord; the 7x7 top chord is smaller than the 9x9 bottom chord. The rafters in the 1898 wing are full-sized 2 x 8 spaced at 19" to 20" on center. They bear on the exterior walls and on 6x10 timber purlins. The two lines of purlins are spaced 7 feet apart, and span approximately 17 feet between the endwalls and two roof trusses. The trusses have a simple queen-rod profile, spanning clear across the building. The truss members are 8 x 12 timber chords and 1¼" diameter vertical iron rods. All wood appears to be native Hemlock. The top chords support the ends of the purlins, and the bottom chords carry the 3 floor joists. The ends of the trusses appear to be mortisedrd into timber posts in the exterior walls of the building, although the specific details of the joint are not visible. Looseness was observed in one of the nuts on the iron pins that clamp the truss heel near these joints. A sketch of the truss and purlins is attached. Code Review - IEBC c.Chapter 3 - Prescriptive Compliance Method i.Section 303 - Alterations (1)303.3 Existing structural elements carrying gravity load. “Any existing gravity load-carrying structural element for which an alteration causes an increase in design gravity load of more than 5 percent shall be strengthened...as needed to carry the increased gravity load required by (the) code for new structures.” The increase in design snow load will be more than 5% for this project, so analysis of the roof will be required, and some reinforcement may be necessary. No increase in floor live load is anticipated in this project. Page 2 of 4 Structural Assessment Dewey House Study Smith College January 11, 2012 (2)303.4 Existing structural elements carrying lateral load. (a)Exception: “Any existing lateral load-carrying structural elements whose demand-capacity ratio with the alteration considered is no more than 10 percent greater than its demand-capacity ratio with the alteration ignored shall be permitted to remain unaltered.” There will be no increase in the lateral load demand-capacity ratios, so this exception applies to this project. ii.Section 308 - Historic Buildings (1)308.1 Historic buildings. The provisions of this code relating to the construction, repair, alteration, addition, restoration and movement of structures, and change of occupancy shall not be mandatory for historic buildings where such buildings are judged by the building official to not constitute a distinct life safety hazard. Structural Analysis Summary Since the floor live load and the use of the building will not change, the existing humps and dips in the floor will remain as they are, with the exception of the sagging floor in front of the Secretary’s Office. A slot in the ceiling in the basement under the existing floor joist will have to be removed so that the joist can be reinforced. Because of the increased insulation, the roof structure must comply with new Code loads. My preliminary stress analysis indicates that the rafters in both wings are adequate for current snow loads, assuming that they are at least a #1 grade, in good condition, with no rot or other damage. Where existing rafters have been cut, the adjacent rafters would be overstressed. The purlins in both wings would be overstressed under current snow loads. In wood structures, an overstress does not necessarily lead to collapse. Overstress is typically used to describe the condition where the standard allowable stresses would be surpassed. Wood is inherently strong and resilient, but allowable stresses are based on the presence of checks, splits, large knots, spiral grain and other strength- reducing defects, in addition to standard safety factors. Hand-hewn sides, paint, plaster and trim hide many of these defects. On-site grading of existing timbers generally results in more conservative assessments of the material capacity than grading of production lumber in the mill. The purlins in the old wing cannot be easily sistered, on account of the 1994 joist hangers. Reinforcement on the sides of the purlins requires cutting back the existing rafters, and would also conflict with the steel saddles that were added in 1994 (see photo). Reinforcement underneath the purlins is not feasible, since they are flush with the truss top chords. In the timber truss in the old wing the 9x9 bottom chord is just barely adequate, but the 7x7 top chord would be overstressed with the new snow loads. Reinforcing the top chords would require removal of the 1994 steel saddles at the purlins. The drawings for the 1994 work call for structural loads to follow the 5 edition of the Massachusettsth Page 3 of 4 Structural Assessment Dewey House Study Smith College January 11, 2012 State Building Code. It is presumed that in the new work, the beams, columns, etc. were designed in accordance with those loads. The snow load in the 5 edition was 35 psf. The current, 8 editionthth requires a 42 psf snow load when the roof is ventilated and well insulated, which is a 20% increase over 35 psf. However, the truss repair work appears to have been more of a restoration effort, rather than an upgrade to meet a specific load requirement. Some rafters were left cut at skylights and cross-braces. In the 1898 truss, the top chords are capable of supporting the snow load. The bottom chord has a high level of stress, but that is due primarily to the live loads on the 3 floor. No changes are proposed forrd the 3 floor that would affect the truss, and so it is below the 5% threshold of the IEBC. Both §303.3rd and §308.1 recognize that older buildings may not meet Code, but are not in danger of collapse. Recommendations If compliance with current Code is desired, much of the framing in the old roof would be affected: 1. The 1994 hangers and saddles would be removed 2. Some rafters cut at skylights and braces would be restored 3. The purlins would be sistered 4. The truss top chords would be sistered, including connection to the bottom chord at the heel joint In the 1898 roof, the only reinforcement necessary would be for the purlins. Some of the ceiling would have to be removed to provide access for this work. Loose nuts should be tightened. Of course the College could choose to reinforce the rest of this truss, and that also applies to the rest of the building. §308.1 allows for historic buildings to be exempt from the structural requirements of the IEBC, subject to approval of the Building Inspector. Thus the College could avoid the reinforcements proposed for the roof if the building qualifies for this exemption. This could apply to the roof structures in both the new and old wings. The loads on the trusses could be reduced by replacing the slate roofs with standing seam metal. A metal roof is much lighter than slate, and it tends to shed snow accumulations more quickly. The weight difference is about 8 psf, which is roughly equivalent to the increase in the Code snow loads. Upgrading the trusses in the old roof will be significant, intrusive, labor-intensive work. In my opinion, taking advantage of the historic building exemption will allow for the truss reinforcement work to be avoided, and installing metal roofs clearly results in a decrease of structural loads. Because the northwest corner of the building is in the Elm Street Historic District, and because the 1898 roof structure has a higher load capacity, slate could be kept on that roof. But the older trusses, which have a higher level of stress, would benefit greatly from a metal roof to offset the greater snow load retained as a result of the increased thermal insulation and air-sealing. Respectfully, Ryan S. Hellwig, PE Massachusetts Professional Engineer #37300 - STRUCTURAL Page 4 of 4 A-3.0DEWEY HALL SMITH COLLEGE NORTHAMPTON, MA KRAUS-FITCH ARCHITECTS, INC.HOME COMMUNITY PLANET110 Pulpit Hill Road Amherst, Ma 01002 413-549-5799 Fax: 413-549-7918 www.krausfitch.com JULY 26, 2011 NO T E : S E C T I O N I S A P P R O X I M A T E A N D N O T I N T E N D E D F O R CO N S T R U C T I O N . 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