Footing overturning calculations 2

[jeffhed]

Is the required factor of safety = 1.5 for overturning and sliding in addition to the load combinations of 0.6D + W and 0.6D +0.7E? 1/0.6 = 1.67 factor of safety against overturning, if I try to 0.6D + W overtuning FS > 1.5, wouldn't this be more of a FS > 3?

 

[ITHW]

StructuralEIT:

You said:
""Required actual resisting moment capacity = 10 k-ft * 1.6 = 16 k-ft"

I agree with that. However, if you are looking at it from that perspective, then you needn't use the 0.6*D factor - you would use 1.0*D."

The key point is that what you call "1.0*D" is what the code calls 0.6*D. They use the 0.6*D to account for the fact that the actual in place dead load is almost always less than we design for. When you say 1.0*D you mean the actual in place dead load, which the code calculates as 0.6 * D.

 

[ITHW]

Ok, it appears I was at least partly (mostly) wrong. The 0.6 D is indeed meant to be the factor of safety. In all cases when 0.6 times the resisting moment (where resisting moment is dead load * moment arm), you'll be good to go. Your factor of safety is 1 / 0.6 = 1.67

However, where you have a service overturning moment, OM, greater than your 0.6 times your resisting moment, RM, you have to provide an uplift support. The equation will look like:

OM - 0.6 * RM = T * d
where T is the service uplift and d is the moment arm. You'll have to use a factor of safety on the uplift force. So if the numbers worked out as:

OM - 0.6 * RM = T * d
20 k-ft - 0.6 * 15 k-ft = T * 10 ft
T = 1.1 kips (Service uplift)
T = 1.1 kips * F.S.
T = 1.1 kips * 1.67 = 1.84 kips (Required uplift capacity)

Sorry for all the confusion.

 

[jeffhed]

I have been looking in some concrete and soils text books. In the sample calculations the author creates a table of loads (horizontal and vertical), calculates a resisting and oveturning moment and shows that the FS must be 1.5. This is all done without load combinations, i.e. full vertical loads. This happens in my boss's concrete text book (Copyright 1943!), Schuams Reinforced concrete design (1993), and Foundation Analysis and design (1996). Not one of these books uses load combinations for overturning and sliding calculations. Full gravity loads are used for all overturning and sliding. This makes it clear to me that if the load combinations are used, the factor of safety is built in. A quick calculation on a cantilvered column awning I am designing, FS of 1.5 and load combination would give me a FS of 2.79, while just the load combination would give me a FS of 1.52 for overturning. It doesn't make since to me to use the combination of both.

 

[mrMikee]

jeffhed,

The way your company designs a foundation IMO is the way it should be done in that critical calculations and the foundation design is done by qualified engineers familiar with local regulations. As you found out the technical help at some of the equipment suppliers is limited, and in fact even the D, L, W, and E loads should be of suspect.

aggman,

I like your way of tabulating foundation loads. About a month ago I started working on a new format for our loads drawing similar to what you are doing. As you said it can look like a lot of information in particular when there are moments on base plates. For bins and silos I don't consider 0.6D+0.7E because it seldom controls. That would be another 8 load combinations and I already have 30 or more. I suppose for lighter equipment like conveyors the dead load only might be relevant.

I agree that this is an interesting business being an equipment supplier and working with other suppliers, and after working in it for many years it is my opinion that from the technical standpoint it has gotten worse, not better. But that's probably a better topic for another discussion.

-Mike

 

[jeffhed]

mrMikee,
Just curious, but when you design the equipment for a project, does your company typically have a licensed engineer signing and stamping your plans? Our city requires all engineered plans to be signed and stamped. This asphalt plant supplier did not have an engineer on staff and was surprised that the city would ask for stamped and signed drawings and claimed that this never happens. They then sent their designs to a consulting firm they use and the consulting firm stamped the plans but they were not even licensed in our state. We had to review and approve their calcs and plans, which was a nightmare. I guess I was just suprises that an asphalt plant supplier did not have someone on staff or a retaininer that was licensed in this state. Off the subject I know, but I was just curious since you are involved on the other side of the problem from me.

 

[mrMikee]

jeffhed,

I've been in the concrete plant industry for almost fifteen years and have worked for four different companies. One of these companies was either number one or two in the industry for several years. In each case I was the only degreed structural engineer on staff and when I left I was not replaced by another structural engineer. It's my opinion that it is not common for companies of this type to do the engineering required for their products. While I have pushed for compliance with building codes and related design specs at companies I've worked at, it's usually been an unpopular viewpoint.

However, almost as strange to me is the fact that very few customers ever ask for design information or stamped drawings. When there is a request it is usually states on the east coast or west coast. Sometimes it's just to get the load breakdown so that the correct load factors can be applied, and sometimes it's to get design calcs or stamped drawings.

If I were a customer I would ask for the load components and do my own calculation of the min and max loads on the foundation based on my interpretation of the code load combinations. I'd also ask how the wind and seismic loads were calculated and which codes were used. You'd probably be surprised. Often overlooked in my opinion are the effects of quartering wind and orthogonal seismic loads.

Regards,
-Mike

 

[CJJS]

Thank you WillisV for the link. I stand corrected.

 

[ChipB]

JAE,
I'm pretty sure the vertical component of seismic (.2S_DSD)goes away when considering foundations.
Just so you know.
Chip

 

[JAE]

ChipB - can you please indicate a reference section in the IBC where this is stated?

As I see it, E is defined by 1617.1.1. In that definition, Q_E is defined as the effect of horizontal seismic forces - which for overturning is found in 1617.4.5.

I'm not aware that foundations are exempt from the 0.2S_DSD value.

 

[ChipB]

In IBC 2000 & 2003 Section 1616.1, last paragraph
In ASCE 7-05, Section 12.4.2.2, Exception 2

 

[JAE]

ChipB - thanks. I guess I have seen that before.