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  • Sample report of a structural investigation - crane system


    Building III: Underhung Crane Investigation

    Project Location: Anytown



    II. Background & History

    III. Crane Loads: Original & Proposed Changes

    IV. Existing Frame Checks & Results

    V. Proposed Frame Modifications

    Option 1: Column & Rafter Coverplating

    Option 2: New Support Column Addition

    Option 3: Add 3 to Hoist to Existing

    VI. Conclusions

    VIl. References

    VIlI. Appendix

AAA A preliminary engineering investigation has been undertaken to address the Building III proposed changes for two underhung cranes as follows: Remove the two 60' wide three ton underhung cranes in the 70' module and re-install these two cranes to run on two new bridge spans of 30' in the same 70' module. (The existing bridge giDrers are to be shortened). No new cranes are added. The two existing side crane supports will remain and function as the side supports for the two new locations. Two new supports are required in the middle portion of the 70' frame span. Also will consider using one 60' bridge, with 2 - 3 ton hoists. The other 60' bridge to be removed. NOTE: All engineered materials and sizes shown in this report are preliminary and are not intended as a final design. Specific sizes and quantities called out in this report are intended for cost estimating, and to give an idea of what modifications will be required.

Objectives: To determine the extent of the structural impact and effects on the metal building system due to the change in support locations of two three ton existing underhung DeMag cranes. To determine the required structural modifications to the existing structure so that it can properly support the new loadings.

II. Background & History: The building in question is a pre-engineered metal building manufactured by an AISC MB certified producer, designed to Metal Building Manufacturers Association specification. It is designated as Building III and is a single slope structure measuring 120' wide by 110' long having a low eave height of 22' and reaching 25' at the high eave, resulting in a roof slope of 1/3:12. The primary frames are modular, single slope rigid frames with spans of 70' and 50\' and are spaced at 22' on center. Building purlins are 8" deep cold-formed zees at spacings of 5' -0" on center. The interior frame columns are built-up columns, with pinned top and bottom connections. The original rigid frame was designed for two three-ton underhung cranes to operate in the 70' aisle in the same bay at the same time. Please see the attached copy of manufacturer crane data which pertains to the existing cranes. Thomas R. Price P.E., of Structural Solutions LLC met with the owner at the job site and conducted a visual inspection of the building on. The purpose of this visit was to obtain an overall perspective of the existing and proposed crane changes and to verify randomly selected sections of the rigid frame. Along with the field visit, additional information was obtained from the original crane supplier. The crane supplier was contacted in order to thoroughly understand the role of the crane supplier with regaDrs to the proposed changes. They provided crane layout drawings and details of the planned changes, confirming the proposed 30' and 25' new bridge spans, the reuse of the existing crane supports, and the locations of the two new mid-span supports.

III. Original Crane Loads & Proposed Changes The following sketch and the information on the following page represent the pre-engineered metal building system manufacturer's crane design information for the original underhung crane systems, showing the crane loads and load locations. Maximum crane loads, p1 = 17.19 kips without impact, as per manufacturer drawings and calculations which were provided. The load p2 at the minimum is estimated to equal approximately 7 kips, which is half the weight of the bridge plus crane rails, plus a portion of the trolleys These loads represents two cranes in the same bay at the same time with either both cranes on the left side or both cranes on the right side of the frame.

The following load cases were considered for the new layout for the two cranes in the same bay at the same time: Where p1 = 9.5 kips, p2 = 3 .0 kips The total crane lateral load is 1.6 kips and was distributed evenly to each support, equivalent to 0.40 kips per support.

IV. Existing Frame Checks & Results The pre-engineered metal building system manufacturer's frame was modeled and the new crane loadings and locations were applied. Members 9-12 represent the 70' -0 frame span which supports the two cranes. Members 13-17 represent the 50'- 0 frame span between columns 2 and 3. Refer to the following page for a graphic representation of the model. The results clearly indicate that the new crane loading primarily overstress all of the rafter members. This was expected due to the magnitude of the new loads in combination with their mid-span locations. It is to be noted that column 2 is also overstressed. The maximum overstress occurred in member 10 having almost 100% overstress. In addition to the adverse effects on the members, member connections including bolts, plates, and welds will experience design overloading. Frame and rafter deflections and serviceablity requirements are also pertinent and minimum allowable values must be met. StandaDr metal building rafter deflections are held to a value equivalent to: L/180, where L=span in inches. This L/180 deflection represents dead load and live loads and is not applied to the crane loads. In this case, the maximum allowable frame deflection which was held was less than or equal to 1.50 inches at middle of the 70' -0 span. L/180 is 4.67 inches for the 70' -0 module. The new crane loadings in addition to the code snow loads resulted in a rafter deflection of 6.5 inches, greatly exceeding the allowable deflection. Crane load deflection was also excessive, reaching 2.8 inches.

V. Proposed Frame Modifications:

Option 1: Build up the frame members by coverplating the deficient columns and rafters to improve and substantially remake their section properties including moments of inertia, for deflection considerations, their section modulus along with the compression flange area to accommodate the significantly increased bending moments, and web shear stiffeners to deal with the added shear forces.

Option 2: Introduce a new interior frame column at a location between the two new crane supports, stiffen the existing rafter at the new support location, and add a foundation to support the new column loads.

Option 3: Utilizing one of the existing 60' bridges, install a second 3 ton hoist on the bridge.

Option 1: Column and Rafter coverplating: In order to meet required frame deflections and stresses the existing rafters will need the following field-installed materials: NOTE: These sizes for estimating only - not final. Two 50 ksi hot rolled angles field welded to the top of the rafters. One -WT8X38.5 50 ksi hot-rolled tee section field welded to the bottom flange of the rafters. Interior column two will need more cross sectional area to support the added axial loads. An estimated 20 pounds per lineal foot of reinforcement will be required. Field welding will be needed in this operation equivalent to approximately 48 inches of a 3/16" fillet weld per linear foot of column. Connection reinforcements will be required as follows: - Column1-Rafter 1 knee area will need field installed diagonal stiffeners. All of the rafter 8-bolt connection plates will need additional field welds, plate stiffeners and the 3/4 inch A-325 bolts replaced with A-490 bolts except for the Rafter1-Rafter 2 connection which will require major modifications which include plate and bolt changes. Existing foundations will not have any significant load increase.

Option 2: Add new Support Column This option represents a significant contrast to Option 1 and can be summarized as follows: Add an interior column between the two interior crane supports at in the 70' -0 span: TS 6x6x1/4 tube steel, or W8x31 Wide Flange. Reinforce the rafter at this connection by adding bearing stiffeners and additional field welds either side of the support. Reinforce member 15 in the 50' -0 span with an 8x3/8 bottom flange cover plate and two hot rolled angles. Construct new concrete footer for the new column: 5'-0 x 5'-0 x 1'-6. (Assumes allowable soil bearing to be 3000 psf.). Add new column here.

Option 3: Add 3 ton hoist to existing 60' bridge: For this option, a second 3 ton hoist is added to one of the existing 60' bridges. The other existing 60' bridge is to be removed from service. Provided that the bridge end truck wheel load is 10,570# or less, the existing building frames would see no more load than was considered in the original building design. Therefore, no building modifications would be required. The actual wheel load should be very close to this limit. It would be the role of the crane supplier to verify this new wheel load. The existing W16X57 runway beams must be checked with the new crane data. It is assumed that this would be up to the crane supplier, since the beams are part of their system. 2 - 3 TON HOISTS (ONE BRIDGE)

Vl. Conclusions: Option 1 (2 bridges, no new column) will require extensive reinforcment of the building frames, and may be cost-prohibitave. Each production line will have its own independant hoist.

Option 2 (2 bridges, add column) will provide the same crane system as option 1, but with the inconvenience of a new line of columns in the building. Cost would likely be less than option 1. Each production line will have its own independant hoist.

Option 3 (1 bridge, add hoist) will be the simplest of the three to achieve, but does not provide an independant hoist to both of the production lines.

VIl. Referenced Documents: 1) Mfg. calculations. 2) Mfg. drawings.

More information: Structural Engineering books

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