The invention relates to pre-stressed beams having compact cross-sectional areas where the structural elements comprises two bent,-pre-stressed, symmetrical or near symmetrical outer elements which are joined together in the bent, pre-stressed condition.

The invention also relates to a method for the manufacture of such pre-stressed beams.

By "outer elements" in this connection is understood long, compact elements of compact material such as woodwork or composite materials having a shape and properties making them useful as supporting beams.

It is common to manufacture beams and floors with cross-sectional shapes that imply good material exploitation, such as the H-profile and the I-profile. It is also common to make beams with super elevation in order to obtain a reduction in the resulting deflection where the deformation requirements are decisive for the dimensioning.

In the production of concrete is known to cast beams and supporting elements with pre-stressed armouring steel rods in order to obtain super elevation and increased supporting ability, e.g. hollow core floorings.

Of earlier patented solutions we refer to US patent No 4 754 718, US patent No. 2 039 398 disclosing a pre-stressed compact beam according to the preamble of claim 1, DE 2 335 998, GB patent No. 1 305 645 and Norwegian patent No. 162124.

The main objective of the present invention is to provide a method for pre-stressing and joining of compact beams for use in building constructions and plants, providing an optimal material exploitation as well as versatility in the choice of cross-sectional shape, so that available raw materials can be better exploited.

It is furthermore a significant object to be able to proportion the pre-stressing so that the initial shape (i.e. the super elevation) can be adapted to the relevant functional requirements with regard to user needs, span, load, governmental regulations etc.

According to the present invention the above objectives can be achieved by the principle solution defined by claim 1, in which two outer beam elements are joined by means of a center element subsequent to bending and mutual sliding displacement of the elements.

According to another aspect the present invention concerns the method for the manufacture of such a beam as defined by claim 4.

By a "center element" in this connection is understood a shear transmitting element having shape and properties which allows it to hold the outer elements together in a pre-stressed, rigid structure.

The present solution is distinguished over the prior art by providing new possible cross-sectional shapes, versatility with regard to method of joining and a higher load capacity without increasing the material consumption.

The center element can be manufactured in the same material as the outer elements or by a material having a higher shear capacity. The joining is made mechanically by arranging the outer elements and the center element to be "locked" together at a certain pre-bending.

The finished, shear pre-stressed beam has in unloaded condition a tension(stress) reserve and super elevation which can be proportioned according to predetermined functional requirements. As an example the pre-stressing can be dimensioned such that a resulting deflection at normally occurring loads on the beam is minimal or near zero. With regard to beams for which requirements of deformation is decisive for the dimensioning, e.g. beams in wooden supporting elements, the method allows much larger spans than traditional methods.

The invention is illustrated in the accompanying drawings, where:

• Figure 1A is a schematic side view of assembled beam elements in non-loaded condition.
• Figure 1B shows the elements of Figure 1A after pre-bending, mutual sliding, joining and unloading.
• Figure 1C shows principle-detail of center element made in same material as outer elements and made with a shape providing mechanical force transmission.
• Figure 1D shows principle-detail of center element made in a material of higher shear capacity than outer elements, made in a shape providing mechanical force transmission.
• Figure 1E shows the joined beam of Figure 1B with an evenly distributed outer load.
• Figure 1F shows principle-diagrams for bending and normal stresses in the joined beam of Figure 1E.
• Figure 1G shows principle-diagrams for shear stresses in the joined beam of Figure 1E.
• Figures 2A-B show examples of relevant cross-sections of pre-stressed compact supporting beams.
• Figures 3A-B are perspectival views of relevant compact supporting beams.
• Figures 4A-C show examples of supporting (floor?) elements in which the supporting beam of
• Figure 2A is used.

• Z₁ = neutral axis lower element, Z₂ = neutral axis upper element, Z₀ = neutral axis joined beam,
• a = distance between Z₁ and Z₂, t = net thickness center element, f = bending and normal stress, and τ= shear stress.
• Neutral axis refers to the position in a beam having zero length change when exposed to an elastic bending moment.

• Figure 1. Principle drawing
  • Figure 1A shows schematically a side view of the beam elements 10, 11 and 12 in non-loaded condition.
    The outer elements 10 and 12 are symmetrical or near symmetrical about axis Z₀. This ensures a favourable stress distribution across the cross-section and correspondingly good material exploitation. The elements can e.g. be made in woodwork, composite or other elastic material. The center element 11 can be made in same type of material as outer elements (Figure 1C) or in a material having a higher shear capacity (Figure 1D).
  • Figure 1B shows elements of Figure 1A after having been pre-bent, joined, and unloaded.
    The pre-bending, which is performed with a hydraulic press or the like, known equipment, implies a sliding motion in the contacting surfaces of the elements. At the joining and unloading a shear flow occurs between the center element 11 and outer elements 10 and 12, as indicated with arrows in Figure 1B. The shear transmission is made mechanically with an adapted surface structure.
  • Figures 1C-D show principle-details of center element, designed with a surface structure which implies that the elements 10, 11, and 12 are locked together in a pre-stressed, rigid structure at a certain degree of pre-bending.
    In addition to shear transmitting function (pre-stressing) the center element provides an increase of the beam's load capacity by increasing the distance between the neutral axises Z₁ og Z₂. For some types of beams and dimensions this capacity increase amounts to more than 50%, see the enclosed calculation example.
    If the center element is made in a sufficiently hard material, the shear transmitting (pre-stressing) can be obtained by pressing the elements together in a pre-bent condition. The beam thereby may be produced without milling.
    The center element can be made in short lengths and e.g. be casted in moulds. It is assumed that the design allows a sliding motion between the elements when the pre-bending takes place.
    For beams with a "narrowed" central section, the elements can be made with longitudinal grooves to obtain a sufficient contact area.
  • Figure 1E shows the joined beam when an external load has been applied.
  • Figure 1F shows principle-diagrams for bending and normal stresses from:
    1. (1) Pre-bending/ unloading, (2) stresses from external load, and (3) resulting stress.
    
    At high pre-stressing symmetrical cross-sections can achieve a substantially plastic stress distribution when an evenly distributed load is applied - without plastification of the material. This allows a maximum exploitation of the beam's cross-sectional area with regard to load capacity. In practice a reduced super elevation will be more relevant and the deformation requirements will be decisive for the choice of pre-stress.
  • Figure 1G shows principle-diagram for shear stresses from:
    1. (1) pre-bending/ unloading, (2) stress from external load, and (3) resulting stress.
    
    By the principle diagram 1F is shown that the beam bending stress reserve corresponds to the stress level after pre-bending and unloading. The beam must therefore be dimensioned for the sum of shear stress from pre-bending/ unloading and from external loads. For relevant beam types and spans this reduction has little practical significance.
• Figure 2. Example of beam cross-sections
  • In Figure 2A a compact rectangular beam comprising outer elements 21 and 23 and center element 22 is shown. The beam is joined during pre-bending by means of adapted surface structures/ pressing.
  • In Figure 2B a compact beam with substantially circularly cross-section (e.g. round timber) comprising outer elements 24 and 26 and center element 25, is shown. The beam is joined during pre-bending by means of adapted surface structures / pressing.
• Figure 3. Perspectival views of beams
  • Figures 3A to 3BC show examples of compact wooden beams.
• Figure 4. Supporting beam in floor elements
  Examples of floor elements based on the current principle as illustrated by Figure 2A.
  • Figure 4A shows an example of a "rib floor", Figure 4B shows an example of a "beam floor" while
  • Figure 4C shows an example of a "compact floor".

Freely suspended beam with evenly distributed load and span L = 6 m.

Raw material:

Round timber (i.e. limited availability at increasing dimensions).

Outer elements:

Split round timber with 250 mm diameter.

Center element:

Rectangular, net thickness = t

Cross-section:

See Fig. 2B.

Basic value:

Load capacity calculated for center element t=0 and pre-stressing corresponding to a super elevation of L/100, i.e. 6 cm after unloading.

Capacity increase for various thicknesses of center element:

• Thickness t=30 mm: about 40 % increase
• Thickness t=40 mm: about 55 % increase
• Thickness t=50 mm: about 70 % increase

The beam as described above is distinguished from prior art techniques and patented solutions by:

1. 1. Allowing compact beams to be produced with desired pre-stressing without adhering/ gluing, ref. Figs. 2A-B.
2. 2. Allowing use of new pre-stressed cross-sectional shapes, see, Figs. 2C and 3C.
3. 3. The method is well suited for beams in floor elements of increased spans, see Figs. 4A-C.
4. 4. The method provides possibility of improved utilization of the basic material, see calculation example.