'97 UBC Fig. 18-I-1 Setback Dimensions

[rlflower]

I have posted this on the Civil section as well...

This figure defines the value "H" as the overall height of the slope, and "H/3" as the required distance to daylight.

I need to clarify the term "H". If it is to be defined as the total height of the slope from the bottom of the creek bed to the top of the ridge, then the "H/3" requirement would often be far too large to be acceptable. However, if the term "H" is defined as the height between two relatively level areas, then I would need clarification on just how large these level areas have to be in order to be considered effective.

My clients are anxious; the way this is interpeted has a very large effect on the cost of the project. (you know how it goes).

-Richard L. Flower, P. E.

 

[ERV]

"H" is defined as the top of the pad (cut building pad. "H-2" is the setback requirement from the structure to start of the upslope if the structure is at the bottom of the hill. "H-3" is the distance from the FOUNDATION to the downslope if the structure is at the top of the hill.

Note that H = the height of the building pad if on a slope.

H-2 = Horizontal distance

H-3 = Horizontal distance

 

[ERV]

rlflower,

Additional comments:

Figure 18-I-1 is defined in 1806.5.3 as:

Footing on or adjacent to slope surfaces shall be founded in firm material with an embedment and setback from the slope surface sufficient to provide vertical and lateral support for the footing without detrimental settlement. Except as provided for in section 1806.5.6 and figure 18-I-1, the following setbck is deemed adequate to meet the criteria. Where the slope is steeper than 1 unit vertical in 1 unit horizontal (100% slpoe), the required setback shall be measured from an imaginary plane 45 degrees to the horizontal, projected upward from the toe of the slope.

NOTE: H-2 need not exceed 15 ft. and H-3 need not exceed 40 ft.

1806.5.6 Alternate setback and clearance. The building official may approve alternate setbacks and clearances. The building official may require an investigation and recommendation of a qualified engineer to demonstrate that the intent of this section has been satisfied. Such an investigation shall include consideration of material, height of slope, slope gradient, load intensity and erosion characteristics of the slope material.

Special foundation design can decrease the setback if the structure is located on the top of the hill. A slope of less than 1:1 will also require less setback.

 

[rlflower]

ERV:

Thank you for your prompt response; let's continue this discussion. The problem I have is that the uniform building code simply does not define the term "H" indicated in figure 18-I-1. I maintain that the simplified graphical description of this term offered in this figure is not adequate.

The question is simple: where is the top of the slope and the toe of the slope in un-graded natural terrain? There are no guidelines offered in the code for this condition. How flat does the natural terrain have to be at the toe and at the top for these locations to be considered the toe and top of the slope? Natural terrain almost never has flat surfaces adjacent to slopes. And, for what distance must this "flat" surface extend to be considered as the toe or top of slope?

Would it be reasonable to assume the "flat" surface must extend at least "H/3" distance from the toe and top of the slope, and that "H" is the distance between that toe and top?

Now, for slopes greater than 1:1 (which applies in some cases to the project I am currently working on), I am trying to understand what the code is saying in section 1806.5.3. Is it saying that I measure the "H/3" distance along a 45 degree line from the toe of the slope?

Thanks for your help. It is greatly appreciated.

-Richard L. Flower, P. E.

 

[ERV]

rlflower,

Let me see if I can be of help, I have designed several residential structures on cut pads on hillsides, seismic zone 4.

Let me I ask you this:

1. Is this a residential structure?
2. Is this structure at the bottom of the slope, top of the slope or somewhere in the middle?
3. I assume a building pad will be cut, or has been cut.
4. Seismic zone?
5. Any other special conditions?
6. What is the elevation of the building site above the lowest point (approximately)?

 

[rlflower]

On this project, I have six residential structures. The site is flanked with a one-lane road below and a one-lane road above. The whole site slopes between these two roads with a widely varying gradient from nearly flat to almost 1:1 near the upper road. The soils report indicates that the upper road was cut some time ago into material designated as "Hg" which is what we are to use as "bedrock".

Let's say for now that three of these structures are somewhere in the middle of the slope, and the other three are at the bottom of the slope. There will be cuts for the lower three sites. A retaining wall will be placed at the toe of the slope to allow the required 15 foot setback from the lower three structures. The upper three structures are on the slope below the road cut -- these structures will depend upon a system of piles for support.

This is seismic zone 4, in the City of Los Angeles. The soils report indicates favorable conditions. The Hg material is close to the surface.

You are asking for specific elevations; well, it is a collage of conditions. Since I have written my last comment, I have worked out a tenative plan that I will have to review with the soils engineer. He will have to confirm for me that I am properly interpeting the code. For example, I am presently thinking that the top of the retaining wall we will place at the bottom of the slope will define the "toe" of the slope, in keeping with the description given in UBC section 1806.5.2. This a complicated mess and so much is riding on just how to interpet the code; I appreciate your help, but I think I should dialouge at this point with the soils engineer.

If there is anything of general interest to add to this string after I meet with the soils engineer, then I will add it to this post.

Thanks again.

-Richard L. Flower, P. E.

 

[ERV]

rlflower,

That is the best course of action. The City of Los Angeles can be tough to deal with. Glad I am no longer in that area. I did the structural design for a hillside residence in Chatsworth in 1978 (approximately 200 ft elevation above the road), when the pad was cut, a natural spring was opened up on the upslope. What a nightmare trying to deal with the building official! The project was successfully finished in late 1979, but not without major headaches and cost overuns.