## Triggers Required for the use of Safety Factors

Structural Layout Vignette and Multiple Choice

### Triggers Required for the use of Safety Factors

Hey everyone this is my first post on the forum so I hope My question is clear.

When doing equations like Max moment or even the summation of live and dead loads, I have noticed that some questions require a safety factor applied and some questions do not. What triggers do I need to look for in order make sure the correct safety factor is applied and is their a location where all of the safety factors can be found in one place? Thanks for the assistance. I test tomorrow but understand if this question isn't answered before then, think im going to be taking this one again in two months
mbowman181

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Joined: Wed Feb 04, 2015 9:33 pm

### Re: Triggers Required for the use of Safety Factors

not sure if this is all the answer, but I just re-reviewed this watching Thaddeus:

1. the overturning moment for retaining walls needs to be multiplied by the safety factor of 1.5 (50%). Basically the SM (stabilizing moment) = 1.5 x OTM (overturning moment- which would be dead loads x the perpendicular distance to the pivot).
2. the overturning moment for building needs to be multiplied by the safety factor of 1.67 (67%). The SM= 1.67 X OTM (dead loads, live loads, etc x the perpendicular distance to the pivot).

The buildings have people in them, so the safety factor is larger. Hope this makes sense and answers your question. The safety factor deals with the stabilizing moment/ OTM equilibrium. I'm guessing these numbers are in the code. Just memorize them.

Good luck tomorrow. Let us know how it goes. No posts of the vignette?
rifkamattman

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Joined: Sun Nov 03, 2013 10:32 am

### Re: Triggers Required for the use of Safety Factors

No posts for the vignette. I think i have that down, simply from reviewing the comments of over thirty of the ones posted on the forum. I appreciate the OTM safety factors... completely forgot about those. The ones I was referencing in my question are in regards to the 1.2 and 1.6 of live and dead loads, as well as the factors involved with allowable bending stress calcs, and the max moment calcs in regards to concrete. Ive seen safety factors used and not used in different examples. Wasn't sure what was triggering the necessity of use.
mbowman181

Posts: 12
Joined: Wed Feb 04, 2015 9:33 pm

### Re: Triggers Required for the use of Safety Factors

you know, I just glanced the IBC chapter 16 and can't find where these numbers might be, but I am positive that this is what Thaddeus is teaching. Mike, someone, where are these safety factors published just for curiosity sake? I'm still talking about the 1.5 for retaining walls and 1.67 for buildings.
rifkamattman

Posts: 325
Joined: Sun Nov 03, 2013 10:32 am

### Re: Triggers Required for the use of Safety Factors

mbowman181 wrote:No posts for the vignette. I think i have that down, simply from reviewing the comments of over thirty of the ones posted on the forum. I appreciate the OTM safety factors... completely forgot about those. The ones I was referencing in my question are in regards to the 1.2 and 1.6 of live and dead loads, as well as the factors involved with allowable bending stress calcs, and the max moment calcs in regards to concrete. Ive seen safety factors used and not used in different examples. Wasn't sure what was triggering the necessity of use.

I'm not sure about these. I believe the 1.2 and 1.6 are part of formulas that are used and I see them in IBC chapter 16. They are under the 'Load Combinations' section, 1605.2.1 (IBC 2009). The descriptions says, 'Where strength design or load and resistance factor design is used, structures and portions thereof shall resist the most critical effects from the following combinations of factored load:

1.4 (D+F)
1.2 (D+F+T) + 1.6(L+H) + .5(Lr or S or R)
1.2D + 1.6 (Lr or S or R) + (f1L or .8W)
1.2D + 1.6W + f1L + .5(Lr or S or R)
1.2D + 1.0E + f1L + f2S
.9D + 1.6W + 1.6H
.9D + 1E + 1.6H

If I need to memorize these, then I am in trouble. Thaddeus doesn't mention these that I remember.

I'll have to get back to you regarding the factors involved with allowable bending stress calcs and the max moment calcs in regards to concrete. That most likely won't be researched by me before your test unfortunately. I'm not even sure which factors you are talking about for these. An example might be helpful, but it might not be helpful too.
rifkamattman

Posts: 325
Joined: Sun Nov 03, 2013 10:32 am

### Re: Triggers Required for the use of Safety Factors

I believe its just going to have to wait unfortunately. I will let you know how the test goes tomorrow and we will probably be comparing notes again soon.
mbowman181

Posts: 12
Joined: Wed Feb 04, 2015 9:33 pm

### Re: Triggers Required for the use of Safety Factors

sending positive vibes your way. hopefully we won't be comparing notes and you will just pass with flying colors. good luck. I think I am done studying for tonight. see ya.
rifkamattman

Posts: 325
Joined: Sun Nov 03, 2013 10:32 am

### Re: Triggers Required for the use of Safety Factors

He does mention in the Tributary Load Analysis video and I possibly in the concrete beam video. And isn't the whole point of the 1.2, 1.4, 1.6, etc factors of safety to design concrete so that it fails after the reinforcing steel has past its yield poin, which is a warning sign that the concrete is getting close to failure?
Arey

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Joined: Sun Mar 01, 2015 3:25 pm

### Re: Triggers Required for the use of Safety Factors

don't know if this would help... but this is what I found, from a while back
viewtopic.php?f=8&t=3410

Clover79 wrote:In the Archiflash cards, they state that "Model codes will require that stabilizing moment(dead load resisting moment) equals 1.5 x overturning moment" for the factor of safety. However, in Kaplan 2009, they state that the factor of safety is 1.67. In the section regarding retaining walls, the factor of safety mentioned is 1.5. Can someone clarify what the factor of safety is numerically and if it is different for building overturning vs. retaining wall overturning?

Mike-SE wrote:Yes, there is a difference. The IBC 2009 clarifies the situation. It is also discussed in my guides.
The retaining walls the factor of safety is 1.50
For buildings, the correct load combination is 06.0DL +/- WL. This is equivalent to a factor of safety of 1.67

-a
aiwan

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Joined: Sat Dec 27, 2014 9:22 pm

### Re: Triggers Required for the use of Safety Factors

rifkamattman wrote:you know, I just glanced the IBC chapter 16 and can't find where these numbers might be, but I am positive that this is what Thaddeus is teaching. Mike, someone, where are these safety factors published just for curiosity sake? I'm still talking about the 1.5 for retaining walls and 1.67 for buildings.

1.5 is in the foundation chapter of IBC. 1.67 = 1/0.60 and is found in the load combinations 0.6DL + WL See my 702 Guide for more information
The higher safety factor has nothing to do with the people in the building but rather with correlating ASD and USD design.

When to apply SF: the only time you need to consider SF is when checking stability of a structure. Note that stability is a serviceability issue not at strength issue. Now to address the original question in the thread:
When designing a member you first perform an analysis to determine the ACTUAL shears and moments in the members for each load case. Then you combine the loads using the appropriate load combinations from chapter 16, either ASD or LRFD. This provides the maximum shear and moment for the design of the member (think of this as "how fast is the car moving" not "how fast CAN the car go")
Finally you look at the capacity of the member "how fast can the car go". For ASD (wood) the code provides allowable stress for different grades of wood. THEY have incorporated the SF into the allow stress and no further SF is required.
For USD and LRFD the load combinations increase the load to ultimate strength, based on probability and statistics. The member capacity is shown as phi-Mn or phi-Vn. Mn and Vn represents failure points. The phi factor provides a slight reduction due to imperfections in the construction process but are not considered safety factors

I feel like I have answered a lot of questions but am not sure if I answered the ones that are asked. Let me know.
Mike-SE

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Joined: Fri May 31, 2013 7:10 pm

### Re: Triggers Required for the use of Safety Factors

rifkamattman wrote:If I need to memorize these, then I am in trouble. Thaddeus doesn't mention these that I remember.

If you were taking an engineering exam, perhaps, but the ARE requires minimal memorizing. But, as thaddeus says - you need to understand the whole reason for these factors, and how they apply, so when you see something like it on the exam you aren't thinking "wtf?"

jokermtb

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### Re: Triggers Required for the use of Safety Factors

Thanks everyone. Thanks joker for the reminder on understanding concepts. I am in the midst of Thaddeus a second time, so maybe it will make more sense. Mike, thanks for the answer regarding the 1.5 and 1.67 safety factors. Makes sense. The rest of your explanation is a bit overwhelming for me, but I am in the same room kind of. Not completely lost. Just a little. I am finishing reviewing ballast wood tonight.
rifkamattman

Posts: 325
Joined: Sun Nov 03, 2013 10:32 am

### Re: Triggers Required for the use of Safety Factors

rifkamattman wrote:Thanks everyone. Thanks joker for the reminder on understanding concepts. I am in the midst of Thaddeus a second time, so maybe it will make more sense. Mike, thanks for the answer regarding the 1.5 and 1.67 safety factors. Makes sense. The rest of your explanation is a bit overwhelming for me, but I am in the same room kind of. Not completely lost. Just a little. I am finishing reviewing ballast wood tonight.

After reading the part on 1.5 SF, Ignore the rest of the post.
Mike-SE

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Joined: Fri May 31, 2013 7:10 pm

### Re: Triggers Required for the use of Safety Factors

Arey wrote:He does mention in the Tributary Load Analysis video and I possibly in the concrete beam video. And isn't the whole point of the 1.2, 1.4, 1.6, etc factors of safety to design concrete so that it fails after the reinforcing steel has past its yield poin, which is a warning sign that the concrete is getting close to failure?

just getting to reviewing the Tributary Load Analysis video again and you are right, he goes into detail about the safety factors, etc. I am understanding now. I must have just glazed over this the first time around. Reviewing the Thaddeus again and re-reading this thread is making more sense to me. I am in the process of writing up a summary of what I think I am taking away from all this in a separate thread.
rifkamattman

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Joined: Sun Nov 03, 2013 10:32 am

mkloxsfpv659

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