Things need to consider in trusses design for civil and structural engineer!

Reading Level: Intermediate

Trusses are use as structural element for roofing works in any building. There are two types of material being used; timber and steel. Among these, timber is frequent use. It has been selected; because it is more economical if compare to steel trusses in country like Malaysia (Timber is one of resources of Malaysia).

One of the duties of consulting engineer is to design and supervises the construction of truss. Since timber trusses are common used in Malaysia, there is another group of people who start manufacturing timber truss which we call the prefabricate trusses. These groups of people will undertaking the task of design, fabricate and construction of trusses at site. It comes to a point that consulting engineer has to check and approve the design of trusses and supervision of trusses construction.

When trusses fabricator undertakes the task of design, consulting engineer has to check and verify the design of trusses is fit, safe and structurally sound. Now days, most of the design is carry out by computer software and the accuracy is achievable. Therefore, engineer has to work on another part by verifying certain items that needs to consider in design. Below will be some guidelines that need to consider when design of trusses.

First is the loading. What are the roof components use and their weights? As for example, timber roof tiles. Is comprises of roof tiles, timber battens, timber trusses and ceiling. Engineer has to identify that these material weights and has been incorporate into design.

Second is to identify that it comply with theory of trusses. Trusses are modeled to its theory. Else, it will cause stability of trusses.  

Third is the design condition. Normally trusses fabricator come into picture when the structural frame is under construction or has been completed. Therefore, trusses fabricator has to design trusses system to suit existing structure layout. NOT existing structure layout to suit trusses design. It is important because a number of trusses fabricator ignore about this matter. Inexperience trusses fabricator will ask to amend the existing structure. It will increase the cost of structural works and delay the completion of project.

Fourth is the supports location. Sometime, timber fabricator wants to simplify their fabrication works. They just select one type and design for all others. As for example, a building has at least two types of trusses. First type is say 10m length with three supports. Second type is 10m length with four supports. When fabricator uses them as one design, the behavior of trusses will affect. Load transfer will be differenced. Subsequently the load transfer at roof structure. As a result, it changes the original load transfer of overall building structure. It may causes certain area overloaded such as roof beam and foundation.

Fifth is the load transfer for combination of trusses. Whenever a truss is too huge and in order to ease of transportation, it is common that it will break into at least two trusses or more. If one truss is acted to another truss, engineer has to ensure that the loading from first truss is proper transfer to another truss.

Sixth is the additional load to trusses. In certain case, there will be some additional load acting to trusses. For instance, water tank on trusses. The trusses fabricator will not take into consideration in design of trusses. Engineer has to highlight it to trusses fabricator for gather into trusses design.

Seventh is additional requirement from engineer. Every engineer has his own design criteria. Therefore, it is important that to ensure the trusses fabricator knows about it. As for example; what is the imposed load use? What is the wind load? Any additional dead load?