CEE498
Lisheng Yang
Constituent Components
Girder Bridge
1) Truss Girder
A typical gantry crane is made up of one major bridge and two supporting legs. The bridge is placed on top of the legs and can be cantilevered at either or both sides. The hoist is installed on the bridge, and it can be mounted on a rail supported by the bridge or directly suspended under the bridge, or placed between the beams in the case of double-beam bridge. The bridge can be single or double depending on needs. And there can be one or two hoists installed running along the horizontal direction of the bridge. If the crane has two hoists, they can be configured in parallel either vertically or horizontally. For the first instance, one hoist can be placed on top of the bridge while another suspended at bottom; for the second instance, two hoists can be configured separately on two movable beams. Two hoists can provide more flexibility for some complex lifting operations.
The legs of the gantry crane are generally not fixed but movable along a track, which provides it more access to different areas of the site. There are even specific types having rubber tire installed at the bottom of the legs.
There are three major structural forms of bridge girder. The first one is truss form, which is welded together by angle steels and “I” beams. This type of bridge has the advantage of lower cost and weight, and has good resistance against wind pressure, however the deflection of truss structure is relatively large. In addition, the welding points need frequent checking to assure the structure performs reliably, therefore, this type of bridge is more suitable in relatively light lifting operation, typically no more than 100 tons.
2) Box Girder
The second type is the box form, which is welded together by steel plates. This type of bridge has complementary characteristics compared with truss bridge. It has high rigidity and heavy self-weight, and costs much more than truss bridges. The box bridge provides enough strength for the gantry crane to lift giant components weighing more than 1000 tons, but its shortcoming lies in weak wind force resistance.
3) Castellated Girder
The third type of bridge is a trade-off between the previous two types. It is called castellated beam, which has a section of triangle. The two symmetrical sides of the triangle are made up of two cellular steel plates, while the top and bottom portions are linked by structural chords. This form maintains good rigidity and flexibility simultaneously, but it is less common in practice because of the patent.
Supporting Legs
"C-Shape" Legs
"L-Shape" Legs
As the vertical supporting members of the gantry, the legs generally takes a shape of “A” with two bracing columns crossing at the bridge cradle. However, there are also a lot of other shapes widely adopted. For example, the “L” shape is mainly applied in cantilevered gantry cranes to provide the space needed for trolleys travelling to cantilevered bridge portion. An improved shape based on “L” shape can be analogous to a “C” shape which has an outward curve in the vertical columns to provide wider space for larger components to travel through. Both of these two shapes need to adopt rigid box-form steel members, because in the case of “L” shape, the compression force can almost triple compared to that of “A” shape, and in the “C” shape legs, the steel structure needs to resist both high compression and high bending force. Accordingly, the cost of “L” and “C” shape legs is higher than that of “A” shape.
Besides, if the bridge consists of double beams with relatively large intervals, there can be two parallel legs perpendicularly connected with each beam and braced together with “X” bracing.
Hoists and Other Accessories
The hoists used in the gantry crane trolleys are electric-driven and mostly adopt a drum and wire gearing system. Here are two important accessories called bumpers and brakes which are very important to the safe and normal operation of the the gantry crane. The bumper is an energy absorbing device for reducing impact when a moving crane or trolley reaches the end of its permitted travel; or when two moving cranes or trolleys come in contact. They can be divided into gantry bumpers and trolley bumpers. The gantry bumper shall be able to stop the crane at an average rate of deceleration not to exceed 3 ft/s when traveling in either direction at 20% of the rated load speed. The bumpers shall be mounted on the legs and cause no direct shear on bolts, and its design shall also minimize parts falling from the crane in case of breakage.
Brakes are crucial in controling the lifting operations. Usually the gantry crane will apply both drag brakes and holding brakes besides the normal brakes used for gantry and trolly. The drag brakes make sure the speed of the crane won't exceed the safety limit, it can provide retarding force without external control. And the holding brakes will automatically stop the motion when power is off.