Soldier Beam Design Methodology
Soldier Beam Design Methodology
The values used in the attached design include:
Surcharge - 72 PSF min (or actual surcharge if higher). 72 PSF is recommended by Caltrans Trenching & Shoring Manual
Active / Passive - as prescribed by geotech
Factor of Safety - 1.3 (recommended by Caltrans Trenching & Shoring Manual)
Neglected Depth - 1 ft (typically recommended by geotech)
Starting Passive Pressure - Passive Pressure x Neglected Depth to provide trapezoidal distribution
Effective Passive Factor - 2-3 as recommended by geotech
Effective Active Pressure - 1x pier diameter below dredge line
Cover on soldier beam - 1" min for temporary condition
These values often tend to produce 1.5:1 embed to retained height.
1) most of the examples for soldier beam calculations I've seen apply the active pressure the entire length of the pier including depth below the dredge line. I believe the thought is that to engage passive you need to have fairly significant movement so you will have active on the backside of the pier. Some calculations I've seen don't do this (Retain Pro module as an example) and instead utilize a form of the IBC cantilevered pole equation. Site soils will likely play into the appropriate approach, but what do you see as typical approach? When do you use one vs the other?
2) The cantilevered pole equation appear to have stemmed from Pole Building Design by Donald Patterson. In this assumptions, it looks like the moment arm couple to provide fixity for the pier has a longer lever than it assume with this approach which results in slightly shallower embedments (say 10-20%). Have you tried to use a similar philosophy for these embedments?
3) if you apply the active times the full depth of the pier, do you use the same effective width as used for the passive pressure or do you only use the width of the pier? Caltrans Trenching & Shoring Manual uses the same for both active and passive and so does other references (Army Corps I believe does as well based on f factor to convert sheet pile to soldier beam) but Civiltechs software manual appears to only use the pier diameter (I don't have the software just read the manual). Other shoring drawings I've seen from engineers also only use the pier diameter. The geotech I discussed this with didn't seem to agree with the concept of not using the same effective width for both active and passive.
4) Do you typically apply minimum surcharge for these conditions? The retaining side does not have traffic but is neighboring property where we cannot dictate to them not to do certain things in their yard during construction. Most similar shoring calculations in our area do not include any surcharge except where undermining existing structures.
5) What factor of safety do you use? I've seen other people use values as low as 1. 1.25-1.3 seems to be commonly recommended for temporary. Some geotechs include factors of safety in the passive values they give us. We try and parse our an ultimate gross passive value (some also provide net instead) for our design purposes.
6) What cover do you use on soldier beam for temporary conditions? Smaller cover allows us to provide deeper soldier beams with same size hole and therefore limit deflections a little with same steel tonnage.
7) Most geotechs require that we neglect the top 1'-0" in our design. While I understand the concept, it seems since we are so far below the natural ground surface this may be a bit conservative. Thoughts?