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Curved glulam

Non-specialists often imagine that like bent-wood furniture, for instance, curved glulam is produced by heating or steaming the laminations.  This is not the case as it is always manufactured using cold timber that is simply curved around formers and held in place until the adhesive is sufficiently cured for the member to be released.  At this stage, a certain amount of spring-back occurs, which manufacturers are able to predict when aiming for the intended radius.

Compared with straight or only lightly pre-cambered members, it is necessary to reduce the thickness of the individual laminations for curved work.  A rule of thumb based on BS 5268 is that for members with constant cross-section, the ratio of the radius of curvature, r, to the lamination thickness, t, should be greater than the mean modulus of elasticity (N/mm2) divided by 70. As a guide, this may still be used with BS EN 1995-1-1, where the mean modulus of elasticity may be obtained from BS EN 1194 for softwood glulam or for hardwoods from BS EN 338.  In the latter case, it is necessary to use the species/grade combination to look up the property for the strength class of the individual laminations.

As an example, the empirical formula cited above suggests the following minimum curvature radii for GL28 strength class, with four common lamination thicknesses:

Thickness and radius:

t mm

r mm

 

45

8100

 

33.3

6000

 

28

5040

 

20

3600

Some past empirical tests and published guidance have suggested tighter limits, with a minimum ratio of 150 for softwoods and even tighter for certain hardwoods such as American white oak and African mahogany.  The grade of the laminations has an influence, together with the bending strength of the finger joints in the laminations. An expression in BS EN 386 also affects the required thickness for a given radius of curvature, and this depends in part upon the strength of the finger joints.

An option for preliminary designs may be simply to stipulate the desired radius and request manufacturers to propose solutions. To provide an effective response to preliminary proposals, general structural concepts should be developed in terms of pitch, frame spacings and spanning capacity.  Assisting in this, design manuals and texts are becoming available referring to the latest Eurocodes and accompanying standards.

Curved glulam is inevitably to some degree more expensive than straight stock.   The reasons include the necessity for thinner laminations, with each one being thicknessed and end-jointed.  There is also more adhesive spread in total.  The labour costs are higher, because curved glulam is more hand-made.  Formers and jigs have to be set up in the first place and after removal and curing the curved components usually require further machining to finish the shapes to their final profiles.

Four alternatives for glulam portals
Four alternatives for glulam portals – Front to Rear: Traditional type with curved haunches – requires thinner laminations, good aesthetics, corners may occupy useful space; Paired tapered stanchions sandwiching single tapered rafters - fair aesthetics; Haunches using large finger joints - good aesthetics, avoids losing space at corners; Strutted stanchions – probably least expensive option for moderate spans if appearance and space permit.

However the designer may offset some of these manufacturer’s costs by aiming for a degree of replication of shapes rather than including a great variety in a single structure.  Lightly cambered beams may be little more expensive than straight ones, but as shown above, very tight radii involve thin laminations. Traditional portal haunches commonly require quite tight radii and therefore thin laminations, but nowadays there are several alternatives, including “large finger joints.”

There is a wide variety of regularly manufactured prefabricated curved components for which design procedures are well established.  These include curved and pitched cambered beams whose design is covered in BS EN 1995-1-1.  Since these are likely to be more economical than totally unique curved forms, such “semi-standard” types should be considered whenever it is inessential for the architecture to be totally unique!

When such forms are subjected to normal vertical downwards loading, tensile stresses parallel to the inner curved edges occur.  Hence the design code includes recommended expressions to accommodate such effects. 

Other examples of well-established shaped components include single- and double-tapered roof beams, complete portal frames and propped half-arch types; also various well-known forms of arch – round, elliptical and parabolic.  These may be two- or three-pinned and are sometimes also tied.  There are numerous built examples of such forms that may be taken as references.

Laminated rings have been included in built designs, and these are either horizontally or vertically laminated.  Curved column head assemblies have appealed to some architects in order to attain a “tree-like” aesthetic.  Load-bearing joinery is sometimes even fabricated using twisted laminations to obtain three-dimensional curvature.  In all such cases however, the above warnings concerning cost should be noted.

For all curved and shaped glulam components, guidelines on delivery and transportation should particularly be borne in mind. Delivery costs may be higher for shapes that cannot be closely stacked and bundled or that become a tall or wide load. Site-assembly nodes that are partially prefabricated before delivery may ease such difficulties.  This is another example of the wisdom of submitting preliminary designs for advice before completing the scheme.

 

 

 

 

 

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Glulam in a jig
Curved glulam being pressed, after thickness planing, finger jointing and gluing. Note thinner laminations than for straight beams. The radius of the jig may be adjusted.


laminated compression rings
Laminated compression rings included in feature trusses.


curved glulam
A “roof tree” from curved glulam, culminating in a support ring at the roof-light.