Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: asd vs lrfd

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Per Roger Turk: 
 
> One example that I like to give is in the design of a concrete footing  
> subject to overturning and partial uplift.  Unfactored loads have to  
> be used to size the footing and get the soil pressures.  But, when  
> load factors are applied to the loads, uplift on the footing will  
> frequently disappear!  This, in my opinion, does not result in a safe  
> design. 
 
Are you considering both load cases per ACI 318 Section 9.2.2 (i.e. a reduced 
dead load with wind or earthquake)?  I am surprised that uplift disappears if 
both load cases are considered. 
 
Nevertheless, I agree that load factors can create bizarre results in 
foundation design.  I generally only use working stress design for foundation 
design and for retaining wall design - it saves having 3 sets of design loads: 
one at service loads for soil stresses and stability; and two sets of load 
factors as noted above to determine worst case ultimate strength conditions.   
 
I feel comfortable with ultimate strength design for concrete frame design but 
I am more comfortable with ASD for steel frame design.  This is partially due 
to an unfamiliarity with LRFD steel design, but is also due to differences in 
material properties and behavior.  Structural steel has a well defined yield 
point and an elastic design is easier to have a feel for how a structure 
should behave (especially with pinned end connections).  I don't feel that 
comfortable with the idea that some portions of a steel structure may yield 
under service load conditions, so I want to know service load stresses.  Since 
concrete has a less defined yield point and since cast-in-place concrete 
structures are generally continuous and monolithic, stress distribution and 
some local yielding and cracking are likely - thus an ultimate strength 
criteria makes more sense to me.