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Structural engineering general discussion FAQ

Graduate manual design tips

where to look? by rowingengineer
Posted: 17 Aug 09 (Edited 13 Mar 10)

Design Considerations

1.    Beams need to have shear effects considered specifically (increased deflection, possibly higher reinforcement requirements in concrete, etc) when SPAN/DEPTH <=10
2.    Beam design is normally deflection governed when SPAN/DEPTH >=25.
3.    When checking drawings, looking at Section Modulus is a good gauge of a section's sizing, even if using Limit States Design.  Knowing the extreme fibre stress is a good "feel" for the beam size.
4.    Always consider a minimum accidental eccentricity of 100mm in your construction.  Increase this to 150mm in residential work
5.    Internal and External Spans, Because of the reduction of continuity at the outer column, moments in outer spans tend to be greater, and so, of course, are the deflections. It is therefore desirable that the external spans should be shorter than the internal ones. (Preferably about 10% to 20% shorter). If this is not possible, the slab depth may be increased in the outer spans and if a uniform slab depth is desired, in the internal spans as well. Alternatively a greater proportion of load can be balanced in the external span.
6.    Design shelf angles for the load at the very tip for strength (ULS), centre of bearing for serviceability (SLS).  This ensures that any rotation of the beam at the support does not lead to overstress in the fixing; Particularly for stiffened angles.
7.     When in doubt, add confinement to concrete.  
8.    Curtailment of reinforcing should occur at a distance of 130% development length past the point where strength is last required, or Ld+d from support, whichever is greater.
9.    To minimize the risk of timber floors (and all high frequency floors; Applies to Cold Formed Steel as well), check that the deflection is no greater than 1 to 2mm under a 1kN point load at centre.  Do not consider T-Beam stiffening effect for this check unless the plywood is glued and screwed; slip and fastener loosening may not permit adequate composite action otherwise.
10.    For steel and concrete beams, check the estimated natural frequency, equal to 18/SQRT(Total Deflection in mm), result in hertz (HZ).  Use anticipated actual loads in this check (thus typically 0.25kPa to 0.35 kPa) rather than full SLS loads.  A result of 15Hz or higher should be double checked with the point load check, a result between 8HZ and 15 HZ is likely okay, with likihood of difficulty increasing as the result decreases, and anything between 5HZ and 8HZ should be subject to a full accelerative methodology vibration check.  Picking the loading is very important, and entirely subjective; A good guide is to consider 30% of your floor load as the likely "routine" load.   That way you are basing the load used on the code's anticipated exposure loads for the floor type.  Remember that vibration problems normally happen under light loading.

11.    Use Preferred dimensions:
•    Offices & retail 6.0, 7.2, 9.0, 10.5, 12, 15m grids
•    Some retail outlets 5.5m or 11m grids (to suit shop units)
•    Car parks (6,7.5 or 7.2) (8.4m by 10.2m or 8.4m by 8.4m)

12.    Use load and resistance calculation techniques that have stood the test of time, but update as necessary possibly due to failures
13.    Use characteristic values e.g., 5% exclusion values
14.    Use prototypes where possible reduces the impact of contingency (prototype are not limited to cars ect an example would be, Pile-always try and get the first pile tested)
15.    Check designs and inspect construction quality control reduces human error see the checking questions below;
16.    Make appropriately conservative assumptions in analysis in complex analyses, this technique can sometimes be difficult, e.g., leaving out non-structural elements is not always conservative.
17.    Check complex analyses with more simple methods where possible reduces model uncertainty and human error;
18.    Develop and Use your own experience and heuristics
What is a Heuristic?
Recognize that heuristics are used everywhere in design and think about their limits.
Koen _2003_ has defined a heuristic as "anything that provides a plausible aid or direction in the solution of a problem but is in the final analysis unjustified, incapable of justification, and potentially fallible." Heuristics are techniques we as structural engineers use to help us solve problems and perform designs that would otherwise be intractable or too expensive. According to Koen, all parts of the design process are heuristics. At the limit, his thesis is likely to be true, but let's consider some heuristics that are a bit more obvious. As will become evident, most tools and ideas you use to design structures are heuristics. Consider a few common heuristics.
2.     The yield stress for high-strength steel is the 0.2% offset stress. This is a heuristic; it helps us solve problems using high strength steel.
3.     The dynamics of wind loads can be ignored in the design of most buildings. If you are designing a low-rise building, you use equivalent static wind loads. You do not include the dynamic effects of the wind directly.
4.     Occupancy live load can be modelled as a uniformly distributed static load. Look around you, the live load is not uniformly distributed and is not usually static
5.    As a final example, consider the determination of the effective flange width of reinforced concrete T-beams. The procedure used in codes, have been in use for over 90 years Chen et al. 2007. It is a crude but effective way to account for shear lag, a heuristic.
Heuristics are absolutely vital to our ability to design structures. We use them every day without thinking about them, and that is okay as long as we recognize the limits of our heuristics. When the Tacoma Narrows bridge was designed, the heuristic that was used was that wind load only needed to be examined to see how it deflected the bridge laterally Petrosky 1994. That heuristic had reached its limit of applicability for that bridge.
The belief, based on the heuristic that if a structure is still standing then it must be safe is a human error never fall into this trap

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