GB2105390A

GB2105390A - Box girder

Info

Publication number: GB2105390A
Application number: GB08126240A
Authority: GB
Inventors: Ronald Sydney Johnson
Original assignee: TRANSPORT SECRETARY FOR
Current assignee: TRANSPORT SECRETARY FOR
Priority date: 1981-08-27
Filing date: 1981-08-27
Publication date: 1983-03-23

Abstract

A box girder, having one use in the construction of bridge decks, comprises upper and lower flanges 2, 3 linked by a plurality of webs 4, one or more of the webs being constituted of separated sections 5, each section extending in the longitudinal direction of the girder. The web sections may vary in size and the sections of adjacent webs may be arranged in various mutual configurations. A preferred design is one where for a pre-determined spacing in the longitudinal direction the web sections of any one web have a shape determined by minimum weight and the ability to sustain the transfer of vertical load from upper to lower flanges without distortion. <IMAGE>

Description

SPECIFICATION Improvements in or relating to box girders This invention relates to improved box girders having a principal application in the construction of bridge decks.

With long-span bridges self-weight of the bridge decks becomes a dominant factor in the total load. Methods of construction in current use which ensure satisfactory structural behaviour but contribute to reducing the loading include the use of box girders, beams with curved soffits and prestressed concrete.

The present invention is directed to further reducing the self-weight of bridge decks and to the construction of improved box girders whereby the self-weight of structural members of this kind can be reduced to values less than that inherent in their conventional design.

Box girders may be constructed from concrete or steel. In present day bridge construction, however, it is becoming more usual to use prefabricated concrete units. The webs of box girders, whether of concrete or steel, must have a certain minimum thickness governed by (a) the limitations of fabricating or casting techniques (b) the need to avoid buckling, resulting mainly from the transfer of vertical load from upper and lower flanges and (c) the consideration that it is generally not practicable for thin concrete webs to contain any form of reinforcing tendon for technical reasons.

In concrete constructions, where the webs are cast, there is a minimum practicable casting thickness which is of the order of 160 mm, resulting in a working range of 1 60-250 mm, and it is the constraint imposed by the casting technique in determining web thickness that is one factor determining the minimum overall weight of conventional concrete box girders.

Accordingly the present invention in recognising the constraints imposed in making the webs of concrete box girders has one objective in providing an alternative web design whereby the weight of girders is reduced.

According to the present invention a box girder comprises upper and lower flanges linked by a plurality of webs wherein at least one of the webs is constituted of separated sections each section extending in the longitudinal direction of the girder.

In one preferred form of box girder having 3 or more webs the relative disposition of web sections transverse to the girder is such that the web sections of adjacent webs are in mutually staggered relation and the web sections of alternate webs are in mutual alignment.

Box girder constructions including box girders according to the invention as illustrated, by way of example only, in the accompanying drawings wherein: FIG. 1 a is a longitudinal elevation of a bridge deck incorporating a box girder design.

FIG. 1 b is a sectional view on the line 1 b-i b of FIG. la.

FIG. 1 c is a sectional view on the line 1 c--l c of Fig. la.

FIG. 2 is a sectional plan view of part of one form of box girder according to the invention, being a section through a girder below the upper flange.

FIG. 3 is a sectional plan view of part of a preferred form of box girder according to the invention likewise, being a section through a girder below the upper flange.

FIG. 4a, FIG. 4b, FIG. 4c show in side elevation three webs each web having a web section design suitable for use in box girders according to the invention.

FIG. 5a, FIG. 5b show cross-sectional views of further known box girder designs capable of improvement according to the present invention.

In reference to FIGS. 1 a 1 b I c a box girder, indicated generally by 1, and being the substructure of a bridge deck indicated generally by 7, comprises upper and lower flanges 2, 3 linked by a plurality of webs 4.

The present invention resides in features illustrated in FIGS. 2 and 3. FIG. 2 is a sectional plan view of one form of box girder according to the invention having webs which are discontinuous and constituted of separated sections 5. In this embodiment only the inner webs are constituted of sections the outer webs 6 being continuous. FIG. 3 similarly is a sectional plan view of a box girder according to the invention wherein both inner and outer webs are discontinuous and constituted of separated sections 5. In this preferred embodiment it is to be noted that the relative disposition of web sections transverse the girder is such that the web sections 5(i), 5(ii) of adjacent webs are in mutually staggered relation and the web sections 5(i), 5(iii) of alternate webs are in mutual alignment.

It will be evident that, although the web sections of adjacent webs of the box girder illustrated in FIG. 3 are staggered in true alternate relation and the web sections of alternate webs are in true alignment, there can be some deviation from such a symmetrical arrangement without losing the benefit of the invention.

It will also be evident to a person of skill in the art of girder design that the web sections may vary in size and may comprise both long and short sections relative to the longitudinal direction.

Similarly the spacing between sections of the same web may vary within limits imposed by the need to retain structural rigidity. Further although the secions of each web will normally be aligned in the longitudinal direction of the girder this need not necessarily be so.

Web sections may also vary in design.

Examples of suitable designs are illustrated in FIGS. 4a, 4b and 4c. FIG. 4a illustrates in side elevation an equi-spaced arrangement of web sections 5 having a parallel configuration, whereas the web sections 5 illustrated in FIG. 4b have a non-parallel tapered configuration. FIG. 4c illustrates a design directed to a box girder having reduced weight relative a box girder constructed with web sections 5 as illustrated in FIG. 4a. The requirement of any design is that in contributing to reduced weight, it should be capable of providing a box girder having structural rigidity.

It is to be understood that box girders are not limited to those having a cross-sectional design depicted by FIGS. 1 b and 1 c and FIGS. 5a and 5b illustrate alternative known designs. Box girders according to the designs of FIGS. 5a and 5b which, in established practice, are constructed with continuous webs 8, may be constructed using web sections according to the present invention i.e. with any or all of their webs 8 replaced by separate web sections as shown for example in FIGS. 2, 3, 4a, b or c.

Claims

1. A box girder comprises upper and lower flanges linked by a plurality of webs wherein at least one of the webs is constituted of separated sections each section extending in the longitudinal direction of the girder.

2. A box girder as claimed in claim 1 having three or more webs each web being constituted of separated sections the relative disposition of web sections transverse to the girder being such that the web sections of adjacent webs are in mutually staggered relation and the web sections of alternate webs are in mutual alignment.

3. A box girder as claimed in claim 1 having at least one continuous web and including two or more adjacent webs constituted of separated sections in which the web Sections are in mutual alignment transverse to the girder.

4. A box girder as claimed in any of claims 1 to 3 wherein the web sections vary in size in the longitudinal direction.

5. A box girder as claimed in any of claims 1 to 4 wherein for a predetermined spacing in the longitudinal direction the web sections have a shape the design of which is determined by integrating a capability to sustain the transfer of a pre-determined maximum vertical load without permanent distortion and minimal weight.

6. A box girder substantially as hereinbefore described with reference to FIG. 2 of the accompanying drawings.

7. A box girder substantially as hereinbefore described with reference to FIG. 3 of the accompanying drawings.

8. A box girder as claimed in claim 1 and as illustrated in cross-section by either FIG. 5a or FIG.

5b of the accompanying drawings.

9. A box girder substantially as hereinbefore described with reference to any one of PIGS. 4a, 4b and 4c of the accompanying drawings.

10. A bridge incorporating a box girder according to any one preceding claim.