RETAINING WALLS

[EBAT75]

This is in relation to Andy Lin's post on his site.

It says Bearing pressure and other service related design aspects are typically done with un-factored loads under LRFD.

I am not clear on the part on unfactored loads under LRFD. Isn't LRFD itself based on using factored loads as the Load and Resistance Factor implies? Do you mean use LRFD equations instead of ASD; use a LF of 1.0 in them. What would then be the Resistance Factor to use; are the Factors of Safety different for Sliding, Overturning etc under LRFD?

I realize concrete elements design (stem, base etc) has to be on Factored Loads. Don't we do the stability checks under service levels and then factor the loads on the elements such as stems?

Andy or any Member, please shed light on this. Thank you in anticipation.

 

[MA_PE]

Look in any foundations book. Bearing loads are evaluated under service load conditions

 

[piscescon]

Because soil capacity uses Safety Factor.
Safety Factor = ASD.
LRFD = Yield Design.
Soil won't yield so it's ASD.
Wood won't yield so it's ASD as well.

 

[Reverse Polish]

This is in relation to Andy Lin's post on his site.

It says Bearing pressure and other service related design aspects are typically done with un-factored loads under LRFD.

I am not clear on the part on unfactored loads under LRFD. Isn't LRFD itself based on using factored loads as the Load and Resistance Factor implies? Do you mean use LRFD equations instead of ASD; use a LF of 1.0 in them. What would then be the Resistance Factor to use; are the Factors of Safety different for Sliding, Overturning etc under LRFD?

I realize concrete elements design (stem, base etc) has to be on Factored Loads. Don't we do the stability checks under service levels and then factor the loads on the elements such as stems?

Andy or any Member, please shed light on this. Thank you in anticipation.

Not to get too far into semantics, but using a literal interpretation of ASCE-7, service-level loads are factored too. I encourage my colleagues to use the terms "service" and "strength" to distinguish the load combinations for Allowable Stress and Strength Design, respectively. This may seem trivial, but designing a footing for uplift due to overturning requires use of the 0.6D+0.6W and 0.6D +0.7E load combinations, which are hardly "unfactored". Retaining wall footings usually retain enough soil that uplift isn't a concern, but don't put it beyond the exam writers to have you evaluate a "flagpole" or screen wall footing. They could do that just to see if you know how to apply load combinations (vis-a-vis using load factors of 1.0).

Allowable bearing capacities of soil already incorporate healthy factors of safety, so service loads are used for stability design, as stated above.

 

[EBAT75]

Any equation can then be interpreted as having a “factor “ of 1.

The W and E cases in ASD are applied 0.6 and 0.7 respectively, not as load factors but as load conversions. E was always strength level. W has joined its rank now. They are both just Converted to service level not “factored”.

If a conventional soil retaining concrete wall & footing were to have net uplift, it is already unstable and would need a deep foundation such as piles. It would then be a piled foundation holding up an earth retaining structure, not a retaining wall per se.



There is one small correction. That wood design is ASD only is not true in recent code cycles. With a Lambda multiplier wood can also be designed to Strength design/LRFD. Among the four main materials in the exam, Concrete is the only one requiring Strength design as ACI considered concrete has ductility needs that can be met with more reliability under Strength design.

 

[dlegofan]

LRFD = Yield Design.

I completely disagree with this. LRFD = load resistance factor design. It has nothing to do with yielding other than calibrating the resistance factors. Since LRFD is based in statistics, it absolutely should be applied to timber design as well, and it is.

 

[piscescon]

I completely disagree with this. LRFD = load resistance factor design. It has nothing to do with yielding other than calibrating the resistance factors. Since LRFD is based in statistics, it absolutely should be applied to timber design as well, and it is.

I don’t disagree with you because lots of countries only accept LRFD. My 2 cents is to easily remember where to use ASD only. They all are equations. No difference at all.

 

[piscescon]

When a yield-able member reaches its yield strength, it’s still safe and both LRFD & ASD equations are ok. When a non-yield-able member reaches its ultimate strength, it’s not safe and a reduced strength must be used in design. That is allowable strength design. If you use LRFD with 1.2/1.6 factors applied for soil, that reaches soil’s ultimate strength. Theoretically it’s ok as long as you add a safety factor such as 1.5 to reduce the ultimate strength to allowable strength. That way you use both LRFD & ASD equations, which is too complicated. The easier way is to use ASD equations only with a larger safety factor greater than 2.0. That’s my 2 cents regarding to the codes making.

 

[EBAT75]

LRFD is the acronym for Load AND Resistance Factor Design. Load Factor was in earlier codes. The factors are based on the variability of different types of loads. It is a bell curve. It provided a basis for design but did not provide a way to more reliably predict failure rates.

Different types of structures require different levels of reliability. e.g. a nuclear plant or a high traffic carrying bridge vs a single family residence. That is where Resistance Factor comes in. Just as loads are variable, so is resistance provided by different resistance elements, say flexural, compressive, shear, torsional etc. They also form a bell curve for each element and material.

Where the two bell curves overlap is the failure zone. The codes for each material or type of structure can then be made with a predetermined level of reliability.

 

[Titleistguy]

This is a rabbit hole if I’ve ever seen one

 

[EBAT75]

Hope it is not right next to the 18th.

 

[Titleistguy]

Side bar here something that always helped me clarify the difference was ultimate pressure, design pressure .... are factored bc those pressures are what you design with on your footing and so forth.

Bearing is a serviceability thing, the same way you wouldn’t factor a live load when checking beam deflection.

 

[EBAT75]

Agreed, if they are specifying ultimate bearing pressure. But if they give only the allowable pressure, as they do sometimes, then you need 2 steps - one for stability using ASD combos, another for design of Concrete elements using LRFD.

If only ultimate bearing pressure is given, do we reduce that by dividing say with 1.6 to get the allowable bearing pressure and go from there if a section is given and stability needs to be checked first before designing the concrete elements?

I saw this before the exam but it will come in useful for future reference.

The question can wait for any answers. I am also brain-drained.