The present invention relates to a top piece for a manhole structure, comprising an access part and a cone which connects to the access part and can be placed on a manhole structure shaft with a peripheral edge on the side of the cone opposite the access part, the cone having a substantially outward curved basic shape.

Such a top piece is known in practice. Owing to the outward curved shape, the top piece in general has good resistance to pressure forces which are exerted on the top piece by the ground surrounding the manhole structure. The disadvantage of the known top piece is, however, that the region of the transition from the cone to the access part is more readily deformed than the cone or the access part. An extreme pressure load, as the result of the mutually amplifying radially inward directed components of forces exerted on the transition, can cause the top piece to bend inwards at that position.

The object of the invention is to provide a top piece which does not have this disadvantage. To this end, the top piece according to the invention is characterized in that in the region of the transition from the access part to the cone, a rein¬ forcement rib, which is substantially parallel to the plane in which the peripheral edge is situated, is provided on the outer surface of the cone, extending over the entire periphery of the cone.

The provision of this reinforcement rib gives the cone in the region of the transition to the access part greater resistance to radially inward directed forces than is the case without said rib.

Further preferred embodiments of a top piece according to the invention are described in subclaims 2 - 13.

The invention will be explained in greater detail below with reference to the appended drawing, in which: Fig. 1 is a view, partially in section, of a

manhole structure which is provided with a top piece according to the invention;

Fig. 2 is a top view of a top piece according to the invention. The manhole structure 1 shown in Fig. 1 is substantially cylindrical and comprises a bottom 2 on which a manhole structure shaft 3 is placed. The manhole structure 1 further comprises a top piece 4 which is placed on the manhole structure shaft 3. The top piece 4 consists substantially of two parts, a cone 5 and an access part 6. There is a sharp transition 21 between the cone 5 and the access part 6. Fitted around the access part 6 is a concrete ring 7, which can slide up and down over the access part 6. The concrete ring 7 serves as a support for a manhole cover edge 8. A manhole cover (not shown) of standardized dimensions, which shuts off the manhole formed by the access part, can be placed in this manhole cover edge 8. The top piece 4 can be placed on manhole structures for pipe systems, for example a sewer system. A number of connecting sockets 9 are fitted in the bottom 2 of the manhole structure l, for making the connection of the manhole structure l to the pipe system.

For the connection between the bottom 2 and the manhole structure shaft 3, on the one hand, and the top piece 4, on the other hand, the manhole structure shaft 3 and the top piece 4 are provided with shaft sockets 10, 11. The parts of the bottom 2 and the manhole structure shaft 3 projecting into the shaft sockets 10, 11 respectively are reinforced by means of peripheral ribs 14, 15. The seal between the bottom 2 and the shaft socket 10 and between the manhole structure shaft 3 and the shaft socket 11 is provided by sealing rings 12, 13.

The access part 6 is provided with reinforcement ribs 16 on the outside. The dimensions of the concrete ring 7 are standardized.

The external diameter of the access part 6 is thus determined by the internal diameter of the concrete ring 7. Owing to the large number of reinforcement ribs 16, the reinforcement ribs 16 can be small, and the internal

diameter/external diameter ratio is as large as possible.

The cone 5 has a substantially outward curved basic shape and at its underside defines a peripheral edge 17. The cone 5 comprises a first part 18 whose cross- sections, which are parallel to the peripheral edge 17, are substantially circular, but have different diameters, and all intersect one straight line which is substantially parallel to the axis of the access part 6. The first part 18 consequently has one straight generating line 20. The cone 5 further comprises a second part 19 which is situated between the first part 18 and the access part 6, and whose cross-sections parallel to the peripheral edge 17 are likewise circular, but do not intersect a straight line. The cross-sections of the first part 18 and the second part 19 of the cone parallel to the peripheral edge 17 can also be of a different shape, for example an elliptical shape.

The shaft socket 11 increases the resistance of the peripheral edge 17 to forces in the radial direction, and thus to deformation of the peripheral edge inwards and outwards in the radial direction. When pressure forces are exerted by the surrounding ground on the cone 5 and the access part 6, the curved generating lines of the cone 5 will tend to stretch. Since the movement of the peripheral edge 17 is limited in the horizontal direction by the shaft socket 11 and in the vertical direction by the manhole structure shaft 3, this stretching will be evident particularly by a movement of the transition 21 radially inwards.

In order to reinforce the transition 21, a reinforcement rib 22 is provided on the outer surface of the second part 19 in the region of the transition 21. Said reinforcement rib 22 forms a continuous closed ring. A number of ribs 23 are also provided parallel to the peripheral edge 17. It can be seen in Fig. 2 that each rib 23 extends over an equal distance from the straight generating line 20 to both sides of the cone 5.

These ribs 23 also increase the resistance of the

cone 5 to compression.

A number of slanting ribs 24, placed at an angle relative to the plane in which the peripheral edge 17 is situated, are provided in the region of the outer surface of the cone 5 lying diametrically opposite the straight generating line 20. This means that the slanting ribs 24 are substantially at right angles to the outer surface of the cone 5, which has a beneficial effect on the resistance of the cone 5 to pressure forces. The ribs 23 parallel to the peripheral edge 17 merge into the slanting ribs 24, each rib 23, with the exception of the two ribs 23 lying closest to the periph¬ eral edge 17, continuing to the next slanting rib 24.

The two ribs 23 situated closest to the peripheral edge 17 run on and form a continuous closed ring.

This produces a zigzag structure on the outside surface of the cone 5 in the region of the transition from the ribs 23 into the ribs 24, which has been found to have a beneficial effect on the resistance of the cone 5 to compression.

In the plane through the axis of the cone 5, a rib 25 is provided on the outer surface of the cone 5. Said rib 25 also increases the resistance of the cone 5 to compression. Moreover, said rib 25 prevents bending of the slanting ribs 24 as a result of the vertical component of forces exerted on the slanting ribs 24.

In another possible embodiment (not shown) of the top piece 4 according to the invention, the ribs 23 pass smoothly into the slanting ribs 24. In this case the ribs 23 do not extend to the next slanting ribs 24.

In yet another possible embodiment (not shown) of the top piece 4 according to the invention, all slanting ribs 24 extend from the peripheral edge 17, by way of the region which is situated diametrically opposite the straight generating line 20, to the peripheral edge 17 at the other side of the top piece 4. The ribs 23 extend from the straight generating line 20 to the first slanting rib 24. Therefore, for all ribs 23 this is the same slanting rib 24.

In yet another possible embodiment (not shown) of the top piece 4 according to the invention, only ribs 23 which are parallel to the peripheral edge 17 are provided on the outer surface of the cone 5. In the case of all embodiments of the top piece 4 the access part 6 can be placed either concentrically or eccentrically on the cone 5.

In the case of all embodiments of the top piece 4 two ribs which are parallel to the axis of the access part 6 can be provided on the inside of the cone 5. These ribs give additional reinforcement of the outer surface of the top piece 4.

Rungs can be fixed between the ribs, thereby forming a ladder part, in order to facilitate access to the manhole structure 1.

The bottom 2, the manhole structure shaft 3 and the top piece 4 are in practice preferably made of synthetic material.