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HomeMy WebLinkAbout(5a) Water Filtration Updatecrrv n~ sir. x~~wH Council Agenda Item .5 a MEETING DATE: November 20, 2008 AGENDA ITEM: Water Treatment Plant -Update on Beams SUBMITTED BY: City Engineer BOARD/COMMISSION/COMMITTEE RECOMMENDATION: PREVIOUS COUNCIL ACTION: The City Counci{ hired a Mazda Consultants to prepare an independent evaluation of the "checking" at the Water Filtration Plant. Some of the support beams have checking and it was questioned if the support is structurally sound and if the weight loads were designed correctly. BACKGROUND INFORMATION: BUDGET/FISCAL IMPACT: ATTACHMENTS: Independent Review of Mazda Consultants REQUESTED COUNCIL ACTION: MAZDA CONSULTANTS, /NC. West 960, First National Bank Building 332 Minnesota Street St. Paul, MN 55101 Tel: (657) 292-8040 Fax: (651) 292-3383 November 3, 2008 Ms. Judy Weyrens City Administrator City of St. Joseph 25 College Avenue North PO Box 668 St. Joseph, MN 56374-0668 RE: City of St. Joseph- Water Facility Glue Laminated Beam Project No: 8135 By E-Mail: jweyrens@cityofstjoseph.com Dear Ms. Weyrens: Mazda Consultants was retained by the City of St. Joseph to inspect and analyze a Glue Laminated Beam at the Water Facility located at St. Joseph, MN. The beams support a walkway and a water pipe. The typical beams have developed cracks, which is of concern to the city. Executive Summary: 1. The stresses in the Glue laminated beams will meet the allowable design stress and safety factors provided that the pipe load is equally distributed at each hanger. The ratio of actual stress to allowable stress is tabulated in this report. 2. The stresses in the Glue Laminated beams can be substantially higher if the loads from the pipe are not equally distributed at each hanger. 3. The cracks at mid-depth of the beams could be from a combination of stresses, the beam material, original moisture content and drying process. Design Conditions: The Glue laminated beams support the following loads: An aluminum walkway along the north side of the beams. The walkway is 4'-2" wide at the west area and 6'-1 '/2" wide at the east area. The design live load for the walkway is 60 psf. 2. An 18" diameter class 53 water pipe oriented perpendicular to the beams, and located 4'- 6" from the north wall. Ms. Judy Weyrens Glue Laminated Beam Page 2 of 3 Glue Laminated Beam Data The Glue Laminated beams are 5 1/8" wide x 7'/2" deep. The beam length varies from 9'-4 '/2" at the west area and 11'-4" at the east area. 2. The stress class for the Glue Laminated beams is 16F-V3. The scope of the project is to analyze (1) 11'-4" long beam. This is the length subjected to the highest load at the mid-span. Conclusions: 1. Stress Analysis The Glue Laminated beam is adequate for the walkway and water pipe loads provided the water pipe load is equal at each hanger located at 6'-0" on centers. The stress values, for this loading configuration, are as follows: S 'an ' fb Fb' Ratio fv. Fv' Ratio (ft (si si si si 6'-0" 1, 594 1, 596 0.99 S5 195 °0.28 Based on the stiffness of the 18" water pipe and the small spacing between the hangers, the load at each hanger can vary substantially, depending on how and when each hanger was tightened and the deflection of the Glue Laminated beams due to other loads. In addition, differential settlement can also change the loads on the beams from the piping. The following table shows the deflection of the water pipe due to various spans: S an' Deflection, ft in 6'-0" 0.00021 12'-0" 0.0033 18'-0" 0.0168 24'-0" 0.0531 30'-0" 0.1297 The table indicates that the pipe can span 24 feet with less than 1/16" deflection. Hence up to (3) hangers within the 24 feet span can be carrying very small loads with the majority of load on end hangers only. Ms. Judy Weyrens Glue Laminated Beam Page 3 of 3 The following table shows the stress values based on various pipe spans, which are a multiple of hanger spacing. This assumes that the hangers within the span are ineffective. S an fb ' Fb' :Ratio! fv Fu' Ratio ft) si si (si si 12'-0" 2,325 1,596 1:46 83 195 0:43 18'-0" 3,114 1,596 1.95 110 195 0.56 24'-0" 3, 902 1, 596 2.44 ` 137 195 0.70 ~~ 30'-0" 4,691 1,596 2.94 165 195 0.85 The table above indicates that the beam allowable stresses can be exceeded substantially depending on the effectiveness of each hanger. The actual hanger load distribution can be determined in the field by strain gage measurements. The loads on each hanger can be adjusted using the same strain gages such that each hanger carries loads of the similar magnitude. 2. Mid-Death Cracks A number of the Glue laminated beams have horizontal cracks at about the mid depth of the beam. The bending stresses at the mid-depth are very low in magnitude, hence would not be the primary cause for the cracks.. This indicates that in addition to the stresses the beam material, original moisture content and drying process may also be contributing to the cracks. There is also a slight possibility that movement of the walls at each end could result in compression in the beams. Please call me if you have any questions. Sincerely, Jehangir (Rudy) E. Rudina P.E. 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