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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.
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Preparatory Seminarfor 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.
