8. Frames

The whole main structure of the Humber Bridge is load bearing and is the essence of the load bearing definition, pairs of columns with members spanning between, spaced apart to enclose volume of building. The two towers at either side of the bridge are the columns, with the main cable spanning between, supporting the bridge deck as the member enclosing the building. The advantages of using a framed construction for the Humber bridge is primarily functional in that it allows the functional part of the bridge, the road, to have maximum space without compromising with internal supports which would have created a lot of problems when installing the roadway and for people using the bridge. The roadway sections can also, if necessary, be removed and replaced due to them not having any dead load bearing capacity which can allow for simple maintenance. A final advantage of using this design is a reduction in the overall dead weight of the building, lowering cost and maintenance.

The functional requirement of the Humber Bridge’s frame is to create a strong, stable structure to allow the passage of vehicles from one side of the Humber estuary to the other. It does this by using materials such as concrete and steel in a simple, effective design using tensional steel main cables holding the steel bridge deck, engineered to be stable in wind, in place with bracing cables secured at either end of the bridge by concrete anchorage. The cables are then supported over two concrete towers to hold the bridge road deck in place at the correct height. All together this makes a structure in tension and compression, creating a semi-ridged structure allowing slight movement from live loads.

The reason concrete was used in the Humber Bridge was mainly due to its flexible range of properties. Reinforced concrete performs well in compression, allowing the towers to support the dead loads from the cables and providing a solid point of contact. Also, the fact that concrete can be formed into many shapes allows the towers to be the height they need to be whilst maintaining their strength and slender appearance. It would be extremely difficult or almost impossible to retain the combination of height and strength if other materials such as masonry or wood were used for the towers without a considerable amount of bracing which wouldn’t allow the slender appearance the concrete towers give. Also, using concrete for the bridge anchorage means the cost was kept lower than using a steel anchorage as the 490,000 tonnes of material used in the anchorage at either side would have cost considerably more in other materials with lower performance properties than concrete.

Finally, the layout of the Humber bridge frame allows the dead and live loads to be transmitted evenly along the bridge. The way the bridge is designed, with the deck hung from the cables, creates an even distribution of the loads, easing the stress on the structure at any one point. This means that the bridge can last longer without the need for maintenance in any one particular area over another.