Historic Mill Building Beam

[bouk715]

I'm hoping someone here can shed some light on a framing system I saw in a mill building recently. The mill is located in the Northeast. Bascially, the beam/girder consists of two 3x or 4x members with a tie-rod in the middle. The tie rod projects well below the wood members (say up to 2' in some cases where it is a girder) and has a turnbuckle in the middle. Steel posts attach the rods to the underside of the wood members. I have attached a photo. This particular mill had these members acting as beams in some cases and beams/girders in others. Anyone know what this framing system is called and where I can get more information on it?

 

[MiketheEngineer]

Can't believe you have never seen this before - but maybe you are young. Very common in old structures. Basically an inverted truss. The tie rod is in tension - the beam in compression. Some even had adjustable tie-rods that could be tightened to reduce deflection.

With today's more modern materials and practices - these have gone by the way side.

Just analyze it as a truss - inverted. Close enough. Knowing the wood and steel values is however somewhat hard to figure. I would give the wood about 1000 psi Fb and the steel about 30K tension

 

[OldPaperMaker]

bouk715,
The official name is a "Queen-Post Truss". If it had one post in the middle it would be a King-Post truss. Early (100 years ago) all railroad cars were constructed with wood floor beams and iron or steel queen post trusses.

 

[bouk715]

Thanks guys for the help, much appreciated. I suppose I'd be described on the younger side...I've been in a few mill buildings before (heavy timber), but hadn't seen anything like this until now! Thanks again!

 

[dhengr]

For that era, that was a very efficient use of the two materials to their best advantage. The steel tie rod or bottom chord in tension and the wooden top chords in compression and for bearing and simple attachment of framing from above. The turnbuckle in the bottom rod allowed for some adjustment for loading and deflection. Given the state of the art of Structural Engineering at the time, those people had a fairly good handle on designing simple trusses like this. Note that the bottom tie rod goes up btwn. the two top chord members and probably attaches (in bearing, thru a corner bracket) at the upper corners of the top chord members right over the beam bearing points; and that the queen posts would be in compression and also go up btwn. the top chord members, as a spacer, with a bearing seat for the top chord members (again, basically a bearing connection). Then with a few through bolts at these connection points the entire member was held together with little shear loading on the bolts or wood. You might look at this design concept much like we now look at a harped tendon in a prestressed beam. The top chord or compression block takes the compression, while the bottom chord, prestressing tendon takes the tension; and at the harping points (the queen posts) the upward loading counteracts the moment diagram of the downward gravity loads on the beam. Superimposing these shear and moment diagrams is part of the design process.

 

[chicopee]

It appears that same concept was adopted by using cables to restore the camber of the Hartford Civic Center space frame roof before it collapsed.

 

[RHTPE]

Didn't much help the Hartford Civic Center - kinda like putting a bandaid on a gunshot wound.

This is an early example of post-tensioning a structural member - intuitively it makes sense, though I'm not convinced every application was thoroughly analyzed - it was an effective way to mitigate deflection.

The Bissell Bridge over the Connecticut River had a similar technique applied after the DOT realized the existing prestressed girders were deteriorating. A short-term fix for poor quality control. The bridge has since be replaced.


Ralph
Structures Consulting
Northeast USA

 

[hokie66]

chicopee,
In the interest of completeness, do you have a reference stating that cables were installed to reduce the deflection in the Hartford Civic Center? Having read all about that collapse, I don't recall that they tried such a scheme.

 

[chicopee]

It was stated in newspaper articles back then.

 

[Splitrings]

I spent a few minutes this past fall looking at those same members, if it is the building I am thinking it is. Located in Essex? Looks like the building has seen several modifications over the years.