COMMON MISTAKES OF NEW ENGINEER (General for Beam, Slab and Column)

Staircase

The loading from the staircase landing stiffer/column is always missed out. Need to study the architectural drawings properly. The mid-landing always have mid landing beam supported by column stiffeners, which sit on beam at floor below. If the floor height is high, then there may be more than one landing.

For the first flight of the staircase, below the staircase, there is slab. If the floor is suspended, the loading to the beam need to be taken into account.

To take care of multiple flight loading if the floor height is high in relation to the length of the staircase.

Brickwall

The brickwall loading, many engineers try to be accurate and take the exact brickwalll height from beam soffit to floor. The typical floor design, the load of brickwall should be taken from floor to floor unless the roof beam profile is the same as typical floor. Otherwise, the highest typical floor may not be the same as the lower floor.

Water Tank Above Toilet

Even if this is indicated in Architectural Drawing, more often than not are without the size. This water tank is normally supported by individual timber or steel channel supported by Brickwall.

Edge Beam

• For area which need ceiling but without edge wall, the edge/perimeter beam may need to be deeper to accomodate.
• For beam above aluminium sliding door, the beam may need to be deepen to avoid the use of lintel which rum through form column to column.
• For roller shutter, the use of perpendicular beam of full depth may not be allowed.

Slab

The slab is often being designed for individual panel only. Due to this, if two adjacent panel is of different thickness, the heavy moment is designed using the thicker slab which is not correct.

If the detailing is done to 0.3 span, the distribution bar from main bar (on long span) becomes the main secondary bar. The main rebar for secondary panel may need to be increased proportional due to the reduction.

Calculation

The design calculation is a final document, not exercise or test or exam where you are allowed to make a 25% error and get an A for excellence. The calculation must be correct. If there is any thing unsure, you are supposed to ask. If you get deflection problem or maximum shear exceeds the required warning, solve the problem or ask senior for solution. Do not leave the wrong calculation to be check by Senior or the alertness  of Senior. One beam collapse out of few hundred cans sent someone and the engineer to hell.

If things is shown not okay in computer print out due to certain conservative assumption but you know this is not critical, write down the word “OK” and if possible put down the reason.

Window Bay

In first floor design for a double storey house, for window bay for room, the slab need to be designed as a cantilever slab with additional beam around the bay window. For roof area, additional facial and slab cover need to be provided. Unless the fascia beam is supported by stiffener/column to the floor slab, the roof beam need to check for torsion stress.

Required Accuracy

It is impossible to achieve a perfect calculation as there are amendment requirement to architectural function and details, M/E details, structural detailing and standardisation, etc. Theoretically brickwall on beam is affected by column width, the depth of beam on top, any airconditional hanging on it, window, etc. The concrete in the beam and slab intersection, beam intersection, etc. may be accounted twice.

It is practical for loading that is not critical, just be a bit conservative and live with it. Eg., when beam sizes varies, it is ok to take the larger one. To details make design life miserable and divert attention from more critical issue.

Overall a loading accuracy of 5 % higher should be consider good standard.

Tags : engineer, engineering, design, beam, beams, slab, column, drawings, mistakes, knowledge

This entry was posted on Monday, April 16th, 2007 at 10:00 am and is filed under Design. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.