An Interesting Roof... What is it?

Replies to This Discussion

Permalink Reply by Anthony Avillo on April 19, 2008 at 9:40pm

It appears to be an arch with a lower support acting as a bottom chord in a truss-like fashion
As it is under construction, this steel might be temporary until something more permanenet is put into place. No matter what, it still appears to be unprotected steel so it must not be trusted. Is that wood on the roof -- it looks like wood planking, maybe tongue and groove. Is this area going to be covered up or left open? I'd make sure all members are aware of this construction feature

Permalink Reply by Matt Thomson on April 20, 2008 at 4:15am

Yes that is wood underneath the roof. I'll find out if that particular area is going to be enclosed.

Permalink Reply by Jay Michael on April 20, 2008 at 3:53pm

The smaller steel members are absolutely functional. The arch it is attached to contains 4 members that come together in the middle connected by a pin, thus making a hinge. The vertical member in the center effectively holds the hinge up by supporting it on the horizontal members which are attached about 1/3 back on the main arch. All of the attachment points of the smaller members have a steel member going to the single arch next to it (next one over). I would guess if you went inside the structure you would see this same series repeated for the depth of the building. In answer to your question, it is an arch. I am not certain it can be called a truss as the smaller steel members are not at the bottom, but it works the same way. Glenn Corbett may have answer to that. The smaller steel members in horizontal position are under tension and the vertical member is under compression. Fire in the structure, heat will be applied to the tensioned pieces first, causing them to lengthen and not supporting the members in compression, eventually leading to failure of the roof assembly. Heat applied to structural steel that is under tension is very dangerous. In this roof enough heat will probably cause the two arches that do not have the smaller pieces to twist causing even more problems. It would depend a lot on the fire load in the structure. A crew with a line would probably have a tough time reaching the ceiling with a hose stream to cool the steel as it looks to be about 10 feet of brick and the scaffold is at least 8 feet above the brick and still doesn’t reach the steel members. This would be a lot like the problems encountered when battling a church fire in the sanctuary; the ceiling is too high for us to be effective in interior operations.

Permalink Reply by Gregory Havel on April 21, 2008 at 8:54am

This looks like an arch reinforced by an inverted king post, rather than by the traditional "bow-string". Compare it to the drawing of the Inverted King Post truss on page 519 of Brannigan's 3rd edition (I couldn't find it in the 4th edition), or on page 41 of IFSTA's "Building Construction Related to the Fire Service", 2nd edition. The king post and its tension members are permanent parts of this truss assembly.

The roof deck appears to be of wood plank, probably 2x6 or 2x8 with interlocking tongue-and-groove edges. The inside will probably be stained and finished with lacquer or varnish. The outside will probably be covered with several layers of extruded foam insulation board and a waterproof membrane. EPDM and other synthetic rubber materials are most common.

The final product will be a combustible roof supported by a steel truss.

Permalink Reply by Matt Thomson on April 23, 2008 at 6:21pm

I'm glad you mentioned the inverted king post. I was thinking it was some variation of that concept. My real question here is arch vs truss. If we use Brannigan's definition the difference would be that an arch will have horizontal forces at it's base and needs to be either tied by a member in tension holding the ends together or by attaching the ends to whatever they connect to (wall or floor). The attachments would have to be pretty substantial while the tie could have much less mass.

A truss, by Brannigan's definition, excerts it's force directly perpendicular to the ground, there would be no need to tie the ends together if it were a true truss.

Knowing that steel in tension will require less mass than steel in compression for the same load, it makes sense that the horizontal smaller steel members in this picture are in tension. Perhaps the architect needed the open area of the arch to remain open and couldn't place a tie at the bottom of the arch. The inverted king post, which we know to be used in truss construction could be used here to function as the tie in the arch, mid-way down instead of at the bottom of the arch. If that's the case, is this still an arch?

Thoughts?

Permalink Reply by Gregory Havel on April 23, 2008 at 7:07pm

This looks like a hybrid, a three-dimensional trussed arch.

Each of the arches is made up of an I-beam, with the flanges balancing tension and compression and the web providing enough depth for strength as the roof load tries to straighten the curve. The truss elements (the inverted king-post assembly) prevent the arch from straightening, and allow the span to be longer than it could be without a tension member connecting the two ends, or without buttresses to brace the wall. It is further complicated by the pairs of arches crossing at the connection to the king-post.

Most trusses are two-dimensional: they have length and depth, but comparatively little width. These assemblies of two crossed arches and inverted king-post truss elements have length, depth, and significant width.

Something like this would be a lot more difficult to design using pencil, paper, and slide rule, than it would be using a computer design and drawing software.

Perhaps Mr. Corbett can provide insight on exactly how this roof reacts to loads and other stress?

Permalink Reply by David W. Daniel on April 25, 2008 at 8:22pm

I will make a wild guess at this system. It appears that the king post is the predominant supporting piece of this roof system. It ties the top of the arches together at a single focal point allowing the forces of compression to transfer through it. It then appears that the bottom of the king post is maintaing the tension of the bottom half of the arches thus keeping the walls and roof system in place. I think that exposure to heat and flame at the area of the king post will pose a challenge for the building to maintain its integrity. I have not seen this type of construction in our AHJ. A question I do have is this building equiped with a sprinkler system?