youngstructural
Structural
There have been some great posts skirting around this subject, however I really think we need a Tips & Tricks FAQ. With that goal in mind, I thought it would be good if we all posted the routine rules of thumb we use in our daily work, in order to perhaps learn some useful shorthand and tips or tricks.
Structural Engineering Rules of Thumb:
1. Beams need to have shear effects considered specifically (increased deflection, possibly 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 Moment/Plastic Section Modulus is a good guage 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. 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.
6. When in doubt, add confinement to concrete. 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.
7. 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.
8. 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 (such as the ATC guideline or CISC Guide 11). 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.
9. For good ground checks in the field, get a metal or timber block made up which should impose the Ultimate Bearing Pressure required of the soil. Stand on this for a count of five anywhere you have some doubt over good ground. An indentation of anything more than a mark (so say greater than 1mm) is considered a failure. Example: I weight 100kg, my typical "Good Ground" value is 300 kPa Ultimate (rupture) bearing pressure, thus my block is roughly 57mm by 57mm.
I'll keep posting as I think of more.
Cheers all,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
Structural Engineering Rules of Thumb:
1. Beams need to have shear effects considered specifically (increased deflection, possibly 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 Moment/Plastic Section Modulus is a good guage 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. 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.
6. When in doubt, add confinement to concrete. 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.
7. 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.
8. 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 (such as the ATC guideline or CISC Guide 11). 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.
9. For good ground checks in the field, get a metal or timber block made up which should impose the Ultimate Bearing Pressure required of the soil. Stand on this for a count of five anywhere you have some doubt over good ground. An indentation of anything more than a mark (so say greater than 1mm) is considered a failure. Example: I weight 100kg, my typical "Good Ground" value is 300 kPa Ultimate (rupture) bearing pressure, thus my block is roughly 57mm by 57mm.
I'll keep posting as I think of more.
Cheers all,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
