Design of Long Span Steel Structures and Hangars [With Eave Bracing Concept]

Amit Bharana, President, ERA BUILDSYS Ltd.

The majority of industrial buildings are made out of steel for obvious reasons of high strength/weight ratio. A study, on the Efficient Design of Large span hangars/structures, ensuring lighter foundations is presented.

Introduction
Structure with Span larger than 40 m can be regarded as long span structures and need to be carefully designed keeping a balance of all the aspects like its weight, deflections (sway) and foundation forces. There are many combinations of designing large spans, like conventional truss & RCC column combination, truss & steel columns, Pre-engineered building (PEB) etc.


These days with the concept of PEB, the major advantage we get is the use of high strength steel plates (usually Fe 350), lighter but high strength cold form purlins, and 550 Mpa Galvalume profiled sheets. The use of PEB not only reduces the weight of the structure because high tensile steel grades are used but also ensures quality control of the structure. In the following study, we have designed a hangar using this modern concept of PEB.

Design Philosphy
The design under discussion is a 42 meter clear span hangar for aircrafts maintenance. We have designed this Hangar in 3D on STAAD software, for proper simulation of the load distribution uniformly in three co-ordinates system i.e. X, Y and Z. All the Basic loads i.e. Dead, Live, Wind, Temperature, seismic etc have been taken into consideration for designing of the frames. The structure has been designed under enclosed as well as open condition for application of wind loads, because of the opening & closing of the large sized Hangar Door.


The basic philosophy of rigid frame design is by adopting 'FIXED' or 'PINNED' column base conditions. A fixed column base is always a sturdy frame and helps in controlling allowable deflection (side sway) in the frames. Steel designers always prefer fixed base to pinned base frames. On the contrary, for foundation designers the design of foundations becomes a nightmare particularly in large span buildings. In fixed base design, the frame is rigid, but transfers heavy moments to the foundations. On weak soil, designing foundations becomes tedious task. Likewise for pinned support, the frame does not transfer any moment to the foundation and only vertical & horizontal reactions affect the design of foundation. It looks simple but in case of large spans, controlling deflections of frame in pinned base condition is a challenging task.


To control this deflection, the simplest way is to increase the Geometrical properties/sectional sizes of frame, but it is not advisable as it adds to the tonnage of the whole building, adding not only to the seismic forces but also adding to the cost subsequently. We need a solution wherein the sway of the frame can be controlled and the section sizes are also not increased.

The best way we could find is to 'BRACE' the frame to control the excess deflection. In the present case we have provided bracing at eave level (braced eave) on both sides of the structure along the length for this purpose. Span of this Eave bracing is taken approximately L / 10 of each side. We can observe in the following example that eave bracing is of a great help in controlling Horizontal deflections and leading to lighter foundation design.