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  • Re-roof Dead Load




    I. Purpose of Investigation: 2 of 8

    Structural evaluation of existing 80’ x 140’ building with 36’ x 40’ and 25’ x 33’ lean-tos to determine suitability for supporting the additional roof dead weight of a re-roof system to be installed.

    II. Description of Building:

    The building in question consists of three adjoining pre-engineered metal buildings. The location of the building is in MI.

    III. History:

    Our best information is that the main building was constructed in 1976. The facility is currently in operation as a truck maintenance building.

    Roof leak problems have created the need to repair the roof, and the re-roofing system described below is under consideration for this purpose.

    IV. Referenced Documents:

    5/111/07 sketch by Contractor. 9/17/07 re-roof material description and weight, by Manufacturer.

    V. Field Observations and Notes:

    Field inspections took place on 9/14/07, and 10/4/07 for the purpose of reviewing the intended building modifications, and measuring the existing steel framing so that a structural analysis could be carried out.

    VI. Investigation Results:

    MBC Sec. 3402.2 allows for increases in structural member forces of up to 5 percent, when making alterations or repairs to a structure.

    The proposed re-roofing system has a self weight of 1.25 psf. If the original building was designed for at least 25 psf gravity roof load, then the re-roofing materials can be installed, without any structural reinforcement necessary.

    Analysis of the building roof purlins and primary framing has been performed (see appendix), and the results are clear that the building was designed for roof loads greater than or equal to 25 psf. Therefore, the proposed re-roof may be installed without reinforcement being required, generally throughout the structure.

    3 of 8 Light gauge members (roof purlins and endwall framing), typically are manufactured from 50 or 55 ksi steel. 50 ksi was used in this evaluation. Testing a sample of the light gauge material for yield point is recommended. There is some question as to whether it is mandatory, as we are basing this evaluation on the 5% load increase limit, as opposed to a complete structural analysis. There is ample evidence that the re-roof loads are not in excess of this 5%. Also, the comparatively small load increase reduces the necessity of material testing, in the author’s opinion.

    MBC 3402.2 also calls for any structural elements which are found, in the course of altering or repairing, to be deficient, be reinforced, or otherwise made to meet current Code requirements. The jib crane columns and endwall columns described above, must be addressed, in order to meet this Code provision.

    Line 2 endwall posts:

    When the west addition was built, the girts and sheeting were removed from the endwall at line 2. A masonry wall was built approximately 5 ft. to the west of the endwall framing. Purlins were added in this end bay, extending over to the new masonry wall, to handle the snow drift loads caused by the taller building addition. The original purlins still bear upon the endwall rafter. The drift purlins also bear upon the rafter, though they would have support at the masonry wall, if the endwall framing was unable to resist their load, as they are attached to a ledger angle fastened to the masonry wall.

    Since the endwall posts are now unbraced laterally, their axial strength is significantly decreased. The easiest way to return these columns to proper strength, is to install struts at the original girt elevations, and run x-bracing from the struts to the column bases. See Fig. 4.

    Jib Cranes:

    Two jib cranes have been installed in the building. A 1 ton jib at column 4-B, and a ½ ton jib at column 4-B. At 4-D, a heavier (W12) column was installed, replacing the original W8. At 4-B, the jib was mounted to the inside flange of the tapered sidewall column, and steel angle kickers were installed, running over to the adjacent fixed-base portal column.

    The issue here is not the additional axial stress in these columns due to re-roof loads, as these stresses are miniscule compared to the stresses due to crane forces. Based upon past experience, jib cranes nearly always require a reinforcing channel to be installed on one or both flanges of a building column, as well as special connection details at the column top and bottom support points.

    4 of 8 If there is no documentation of these jib crane support columns being designed by a qualified professional, they should be considered potentially dangerous, regardless of any re-roofing being undertaken. These cranes should not be operated during times when significant snow (or other) load exists on the roof. It is recommended that the Owner search for any information available, in regard to the installation of these cranes, and either verify the design this way, or retain a qualified professional to conduct an analysis.


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