Structural systems application and selection practice questions.

A 2.4 m run of stockroom shelving carries 40 kg of stock per metre. The board supplier states that at this load the shelf reaches its 3 mm sag limit at a support spacing of 600 mm. The board, its thickness and the run length are all fixed. How many brackets should be specified for the run?

• Five brackets, giving four spans of 600 mm
• Four brackets, giving three equal spans of 800 mm
• Three brackets, giving two spans of 1200 mm
• Two brackets, one at each end of the run

Answer: Five brackets, giving four spans of 600 mm

The data sheet fixes the largest span that still meets the sag limit at 600 mm, so a 2.4 m run needs four spans and therefore five supports. Under a load spread along the shelf, sag grows with the fourth power of the span, so widening the spacing to 1200 mm would multiply the sag about sixteenfold rather than doubling it.

A steel safety cage for a roof-access ladder is delivered in three sections that have to be joined on site. The sections must be taken apart every few years so that they can be sent away for regalvanising, and no welding supply can be taken onto the roof. Which connection should be specified for the site joints?

• A fillet weld all round, because a welded joint is stiffer than any other
• Solid rivets, because a riveted joint cannot loosen under vibration
• An adhesive lap joint, because it spreads the load over a wide area
• Bolts and locking nuts, because the joint can be undone on site

Answer: Bolts and locking nuts, because the joint can be undone on site

The stated requirements are a joint that can be undone repeatedly and one that can be made without welding equipment, so the connection has to be mechanical and reversible. Welds, rivets and structural adhesive all give permanent joints that would have to be cut or drilled out before every regalvanising.

A climbing anchor and a static shelf bracket are made from the same steel, yet the anchor is sized with a factor of safety of 8 and the bracket with a factor of safety of 2. Which statement best explains the difference?

• The anchor is made from steel with a lower ultimate tensile stress
• The anchor is too small for its working stress to be measured directly
• The anchor is used outdoors, so the Young's modulus of its steel falls
• The anchor is shock-loaded and its failure would be life-threatening

Answer: The anchor is shock-loaded and its failure would be life-threatening

Factor of safety is raised where the loading is uncertain or dynamic and where failure has severe consequences, not because the material behaves differently. The same steel has the same ultimate stress in both products; only the working stress the designer allows changes.

A market stall canopy must span 3 m and be carried and erected by one person, so every aluminium frame member must be as light as possible while resisting bending. The frame is currently made from 25 mm solid round bar, and the four replacement sections offered all have about the same mass per metre. Which one should be selected?

• A 60 x 8 mm flat strip laid on its flat face
• A 50 x 50 x 2.5 mm square tube
• A 22 mm solid square bar
• A 60 x 8 mm flat strip laid on edge

Answer: A 50 x 50 x 2.5 mm square tube

For a fixed mass per metre the deeper thin-walled tube is much the stiffer in bending, because its material sits far from the neutral axis where it does the most work. The strip on edge is stiff about one axis only and very weak about the other, and the flat strip laid down is the worst of the four.

A square aluminium portal frame holds the banner of an exhibition stand. It is bolted together on site each week by one person, it racks sideways when pushed at the top, and a diagonal brace cannot be fitted because it would cross the banner opening. Which change makes the frame resist racking?

• Bolt a gusset plate across each corner joint
• Increase the length of the two vertical members
• Use a heavier section for the horizontal top member
• Add a second bolt at the mid-point of each member

Answer: Bolt a gusset plate across each corner joint

A four-bar frame with single-bolt corners behaves as a mechanism and changes shape sideways; where triangulation is ruled out, the corners themselves must resist rotation, and a bolted gusset plate turns each pin joint into a rigid one while still coming apart for transport. Longer or heavier members add mass without stopping the corner angles from changing.

A machine guard has a slender hanger that carries a steady pull. The workshop proposes to remake the hanger in cast iron, because cast iron is the hardest metal it stocks. Which single fact makes cast iron the wrong choice here?

• Cast iron is brittle and weak in tension, and the part is pulled.
• Cast iron cannot be cut or drilled once it has been cast to shape.
• Cast iron is a non-ferrous metal and will corrode inside the guard.
• Cast iron has too low a melting point and would soften beside a motor.

Answer: Cast iron is brittle and weak in tension, and the part is pulled.

Hardness is the resistance to indentation or scratching and is the right criterion where surfaces rub or are struck; it says nothing about how a part behaves when it is pulled. A member in tension is judged on tensile strength and ductility, and cast iron is ferrous, machinable and high-melting but fails suddenly at a low tensile stress.