Why Do Rigid Frame Structures Better Support Loads?

Large fabric structures are available with two framing types: rigid frames and open web trusses. While there are pros and cons to each system, one advantage of rigid frames is that they may be engineered to support greater loads than open web truss systems, including loads from collateral systems such as conveyors.

Collateral systems such as conveyors and bridge cranes simplify the process of moving inventory in, out and around the building. Without these systems, it is a time-consuming, manual process to move material. Keeping these systems attached to the building frame keeps them out of the way and maximizes the available floor space inside the building.

Rigid Frames vs. Open Web Trusses

The constant motion of collateral systems, as well as environmental loads such as snow, wind and seismic forces, cause fatigue on any type of frame. The stresses on the frame are in turn transferred to the frame connections. In open web truss buildings, the connections are a series of members welded together to create the truss shape – resulting in multiple connections transferring forces. In contrast, rigid steel frames use full height splices at the haunch and at the rafter, joining two solid steel I-beams with a continuous weld.

Rafter and column splices are more significant in transferring forces than truss joints, and there are far fewer connections. Splices efficiently transfer shear, axial force and bending moments. Truss web to chord connections transfer shear and axial forces but are much more limited in transferring bending forces.

Trusses are also subject to chord plastification, wherein tension or compression on the web starts to distort the shape of the chord. This is a phenomenon that isn’t adequately addressed by many truss suppliers and often isn’t properly accounted for in the building engineering.

Structural Support for Free Standing Cranes

Using a free standing crane instead? Even independently supported systems which include columns that run from the floor to the underside of the crane runway beam can be attached to a rigid frame steel building. Using the strength of a solid frame building will provide the lateral stability the crane requires. You can even directly support the crane with the rigid frame building’s columns by way of crane brackets, which then maximizes the clear area inside the building and reduces the cost of the crane.

Plan for Collateral Loads Before Design Begins

When designing any type of conveyor, be sure the building engineer knows what type of hanging load will be added to the structure and the weight of the load. Loading from a conveyor will consist of live and dead loads.  Dead loads include the fixed weight of the conveyor and any support steel that hangs from the frame to support the conveyor. Conveyor live loads include product or people loads, which will be designed for a worst case maximum. Once these loads are determined they can easily be grouped with code-enforced load combinations in the design of the rigid steel frame.

Using proper engineering practices, including accounting for additional loads before design begins, ensures that a fabric structure will provide decades of safe operation. Determining the best frame type for your application is part of ensuring that long-term safety and operation of your structure.

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