Preparatory Seminar
for
STL Examination
By
Dr. James Lau, BBS JP.
The STL Examination
• The purpose of the STL examination is to test candidates on their
abilities to carry out structural design to the level of a professional
structural engineer.
• The test will be based on the candidates’ ability to understand
– The client’s requirements. This is very important. The candidates must
read and study the client’s requirements very carefully. Failure to adhere
to the client’s requirements will be a failure to the test. So make sure
that you understand fully the client’s requirements before you proceed to
carry out the design. My advice to the candidates is that you should check
your design against the client’s requirements at different stages of your
design. The client’s requirements include the requirements for clear span.
This is very important. A clear span must be a clear span. It is not the
span between the centre lines of the columns. It is the span between the
outer edges of the finishing. It is the clear space/span where machines can
be placed or vehicles can enter.
The STL Examination
– The clear height/minimum ceiling height. This is another important
requirement. A clear height must be a clear height. It should not be
obstructed by beams or services.
– Fire resistance. Check the requirements of fire resistance. This affects
your choice of concrete covers and member sizes.
– Top of foundations. If there is a requirements for the top of foundation,
stick to the requirements. Certain underground services may have to pass
over the foundation.
– The relevant Codes of Practice. This includes the wind codes, the
concrete codes, the steel codes and the codes on loadings etc. In the
examination, you should always quote the code you used.
The STL Examination
– Impose Loads. Use the imposed loads provided by the question.
– Site conditions. Check the site conditions. This has important impact on
the wind pressures you use in the structure.
– Wind loads and pressures. For light weight structure, for example a
large span steel structure, the wind effects can be a dominating factor in
the design. Check the uplift on the structure and the foundation. How are
you going to bring the wind shears from the top of the structure to the
foundations? How are you going to provide stability to the structure?
Bracings or frame actions. Specify this in your paper.
– Ground conditions. Check if there is compressible materials near the
base of your slabs. For example, the ground floor slabs. In that case, you
cannot put the slabs on grade. You have to design them as suspended
slabs. The on grade slabs may settle and cracks. Your design will be
marked down.
The STL Examination
– Look at the soil profiles and choose the appropriate type of foundation.
Make sure that you choose the right type of foundations. Do not use pile
foundations blindly. A more appropriate type of foundation, for example,
raft footings may be more appropriate.
– Designs to be omitted- The questions sometimes tell you that certain part
of the structure need not be designed. For example, the question tell you
that the staircase or core walls can be omitted from your design. In that
case, don’t do it. You just waste valuable time.
The actual design part. Read the questions carefully to understand what
are required from you.
The design part asks for two distinct and viable schemes that meet the
client’s requirements. You do not have much time. So provide enough
sketches in the form of framing plans to illustrate your schemes. There is
no need for you to do any design for your schemes. Just the framing and
some descriptions.
The STL Examination
– Let us look at the words, distinct and viable. The schemes must be
distinct. That means the framing must be different. The vertical and
horizontal load paths must be different. It is not just a change of material.
A good example is that the locations of the columns are different. One
scheme involves the floors to be suspended from a large truss from above.
The other scheme involves the floors to be supported from columns. If
you use the same framing and tell and the examiner that one scheme uses
reinforced concrete and the other structural steel. The schemes are not
distinct. The schemes must be viable. A scheme is viable if it can be
built and is stable. I shall talk about stability in more details in a later
section.
– After presenting the two distinct and viable schemes, you should choose
one of the schemes, give reasons for your choice and proceed to do detail
design for the chosen scheme.
The STL Examination
– The detail design. The candidate should prepare sufficient
calculations to establish the size of all the principal structural
elements including the foundations.
– The point the candidates want to ask is, “ What are sufficient
calculations?”
– Sufficient calculations mean enough structural details that one can
give an estimate of the quantity of structural materials involved put
a price on the completed structure.
– Remember, please read the question carefully.
– You may have to design the foundation as well.
– In the design of the foundations, please read the part concerning
the ground conditions.
The STL Examination
– In the choice of the right type of foundation system, do not blindly choose
pile foundation. You have to prove to the examiners that you are qualified
as a professional engineer, then act like one.
– The foundation system must be safe and economical.
– If a suitable foundation type is raft foundation, then adopts raft foundation.
– Try not to mix different type of foundation systems in the same
structure, unless the structures are separated and are independent
from each other. This is to prevent the onset of differential settlement
thus causing cracks to the completed structure.
– Going back to the question of sufficient calculations.
– We expect the candidates to prepare a design for all the principal structural
elements.
The STL Examination
– The question is. Will the candidates have sufficient time? We know that
you will not have sufficient time. So the technique is to choose the most
critical structural element, design it and assume that the other element will
have the same size and reinforcement details.
– Let me give you an example. In the scheme chosen by you, there are 40
columns. You choose the most critical column, design this column and
tell the examiner that the other 39 columns have the same size and same
number of reinforcing bars. Some candidates will design one interior and
one exterior columns and assume all interior and all exterior columns to be
the same. This is acceptable. But tell the examiner.
– Similarly for the slabs, you design for the most critical slab and assume
that the other slabs have the same thickness and reinforcing details.
– For beams, you design one primary and one secondary beam and assume
that the other primary and secondary beams will follow.
The STL Examination
– During the design process, do not forget that the most important part of
your design is the stability of the structure.
– You have to make sure that your structure is stable.
– How do you transfer the lateral loads from the top of the building to the
foundations.
– Do you use bracings?
– Do you use rigid frame actions for the transfer of the loads?
– Check for uplifts in the structure and the foundations.
– If there is uplift due to floatation, check the stability of the structure
taking into account of winds and floatation, using the ground water table
provided in the question.
The STL Examination
– The next question is the drawings. You have to prepare the
framing plans, sections and elevations for the adopted structural
scheme.
– You have to give dimensions, layout and disposition of the
structural elements.
– If the client requires a minimum headroom, show it.
– If the client requires a minimum clear span, show it.
– The next point is critical details.
– What are critical details. ?
– Critical details are not typical details.
– Critical details are details such as the connection between new and
existing structure.; the details of bracings, column and foundation
connections etc.
The STL Examination
– The question asks for a method statement.
– The idea is to test your ability to design and construct a safe structure.
– You may be able to design a safe structure, but we also want you to know
how to build a safe structure. This is the reason why we ask for a
method statement.
– Let me give you a simple example. You are required to construct a
football stadium. You have to decide whether to prefabricate the structure
and erect the prefabricated part on site. In that case, your method
statement must give an account of how you prefabricate the structure, how
you send the prefabricated part to the site, how you lift the structure to
position and how you fix the structure. What is the capacity of your crane
etc etc.
– The construction programme is another technique to show your site
experience.
The STL Examination
– Do you know how much time to fix the reinforcing steel for a 10,000
square metres floor slab, how much time to concrete the same slabs, where
do you put in a construction joints etc etc.
– We expect you to tell us that you do know about site matters.
– General matters.
– After talking about the question itself, I would also like to say something
about general matters.
– Please write properly so that the examiner can understand what you write.
– Please draw properly so that the examiner can understand your design.
– Freehand sketches are acceptable provided that they are approximately to
scale.
– Provide enough time for all parts of the question. Make sure that you
answer all parts of the question.
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The STL Examination
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The STL Examination