Tirey & Associates, P.C.

Consulting Structural Engineers

Project: Wausau Papers #6 Paper Machine

Wet End Roof Renovations

Location: Groveton, New Hampshire

Completion Date: June, 2006

 

Due to deterioration of structural metal roof deck above the wet end (starting point) of an operating paper machine, it became necessary to replace the roof deck to prevent damage to product being manufactured. In conjunction with replacing the roof deck, a new roof membrane and more insulation was added to the roof to limit condensation which might form and drip on manufactured product. The addition of more insulation triggered the need to evaluate the load capacity of the roof trusses spanning the paper machine operating room.

 

The original roof deck for this section of the building had been wood decking supported by wood beams spanning between the steel trusses, with the beams framing into the trusses at the top chord panel points only. When the wood roof deck was replaced, both the deck and the beams were removed and replaced with a long span structural metal roof deck supported directly by the truss top chords. While easier to construct when replacing the wood roof deck, the long span metal roof deck added significant bending forces which had not previously existed into the truss top chord, thereby reducing the overall load capacity of the roof trusses.

 

An analysis of the existing roof trusses showed that by removing the uniform load applied to the truss top chord by the long span metal deck and reverting back to roof beams transferring loads to the truss at top chord panel points, the bending forces in the truss top chords were reduced to the point that minimal or no additional reinforcing of the trusses was required, even with the improved thermal performance of the new roofing system. The new structural metal deck spans parallel to the trusses, between the new roof beams. Stainless steel deck was used over the very high humidity, wet end of the paper machine to avoid future corrosion of the deck.

 

By changing the method of load application to the trusses, strength and load capacity was recaptured and labor intensive structural upgrade work to the steel trusses was avoided. Since the majority of roof work needed to be completed within a narrow plant shutdown window, this approach solved the roofing maintenance problems within the timeframe the owners had available to perform the renovation work.