Loading Platform Design

Two existing structures: one a 1950's constructed, multi-story, steel frame industrial structure with corrugated steel siding and metal pan, composite roof; the second, a multi-story, masonry block ash storage silo.

Problem

How to provide an access and loading platform connecting the two structures at an elevation of approximately eighty feet, with sufficient strength to handle a one ton load on pallet jack.

Additional Design Challenges :

• platform work surface to match the threshold elevation of existing silo exit door
• platform work surface to be accessible by pallet loader at the silo floor elevation
• minimum locations for attachment supports at the storage silo access door
• building frame top-of-steel does not match work platform surface elevation
• building roof parapet must be maintained
• building roof access door threshold elevation four feet above building parapet

Expert Solution

Provide a platform design consisting of two primary wide flange members with a steel grating work surface supported by welded channel cross members. Design end supports at the silo as an angled welded knee assembly, with bearing plates and attachment bolts at the silo access door threshold. Platform surface elevation to match silo door threshold and allows direct access for pallet jack. Design the platform attachments at the building end to interface with the steel frame using a sliding bolt interface. The platform safety rail to be designed as a by-section removable assembly, allowing access to load by crane. Provide a secondary access stairway, platform and safety rail on building roof to avoid interference with roof parapet and match elevation of roof access door.

Initial platform concept was developed using AutoCAD, with preliminary member sizing performed using hand calculations and engineering design experience. A detailed finite element model of the structure was developed with the MSC-MARC software and used to evaluate stress levels and deflections for all loading conditions. The model used for the analysis is illustrated in Figure No.1, which shows the overall platform and silo interface. Stress distributions shown in these figures are Von Mises stresses.

Attachment of the welded, angled platform support assembly to the existing silo structure was by field drilled, expoxy grouted anchor bolts- shown in Figure No. 2. In order to better evaluate the anchor bolt loads and bearing stresses on the silo structure, the platform and silo were modeled as individual contact bodies. The computer simulations provided contact stress and bolt load data, which was compared to allowable values for the silo materials and proposed anchor bolt sizes. Similarly, attachment stresses at the building interface is represented in Figure No. 3.

Figure No. 4 is a view of the bottom surface of the platform design showing the transverse channel support system for the steel grating work surface. This view provides detailed stress distributions in the assembly components.

Figure No. 5 illustrates the overall structure deflection distributions and the element mesh geometry used to perform the analysis.

Maximum vertical deflection predicted by the analysis model is -0.02 inches maximum applied loading. The peak equivalent stress predicted is approximately 8.2 Ksi at the bolt attachment points.

Figure No. 1 - Platform and Silo Model Interface

Figure No. 2 - Stress Distribution in Angled Knee Support

Figure No. 3 - Stress Distributions at Building Steel Frame Interface

Figure No. 4 - Details of Stress Distribution in Grating and Support Channels

Figure No. 5 - Vertical Displacement Distributions and Element Mesh

To see the resume of the expert associated with this case study, see the link below.