Articles

Seismic Loading on a Retaining Wall

At JCSE, we continue to receive questions whether the 2007 California Building Code (CBC) requires seismic loads induced by retained soils be accounted for in the design and construction of retaining walls. The short answer is yes.

There is little evidence that indicated retaining walls had failed due to seismically induced loads. Thus, the CBC didn’t prescribe a seismic force be accounted for. How the CBC has been modified is to simply not be silent on the topic. The CBC (ASCE 7-05 11.8.3) requires a Geotechnical Engineer consider what the seismically induced loading on a retaining wall is. The Geotechnical Engineer could of course, determine that force to be zero if the conditions and soil type of the site are such.

The simplified Mononobe-Okabe formulation (initially postulated in the 1920s) shown below captures the seismically induced loading on a retaining wall reasonably well:

Pae = (3/8)kjh^2

Where j is the weight of the soil and H is the height of the retained soil against the wall. k is where the crux lies. It can range from 0 all the way up to the site’s peak ground acceleration as determined by Sds/2.5. This can lead to values in excess of 1g depending on the site. Good judgment in the selection of kh must be used. Simply using the site’s peak ground acceleration will lead to an unduly conservative retaining wall design with a corresponding construction cost premium.

Structural vs. Professional Engineers

All licensed Structural Engineers (S.E.s) are also licensed Professional Engineers (P.E.s). All Professional Engineers, however, are not licensed Structural Engineers. In fact, only a small fraction of Professional Engineers go on to pass the State requirements that allow Professional Engineers to be licensed Structural Engineers.

A practicing engineer is initially required to pass an examination to be conferred the title of Professional Engineer. This is a title that allows the engineer to practice in the varied branches of Civil Engineering such as Structural Engineering, Transportation Engineering, and Geotechnical Engineering to name a few. Only after serving an additional four year apprenticeship under a licensed Structural Engineer, and passing a much more rigorous series of examinations for the state licensing board, is a Professional Engineer conferred the title of Structural Engineer.

A good analogy that serves to illustrate the difference between a P.E. and an S.E. is that of a medical General Practitioner versus a specialist such as a Cardiologist. While a General Practitioner could see a patient for a heart problem, one would likely prefer to be seen by a Cardiologist. Much the same relationship exists in the engineering field.

In addition, the experience we have gained providing litigation support has shown us that when a project or evaluation is brought into question and the engineer who performed it is a Professional Engineer and not a Structural Engineer, the party who retained the engineer has opened themselves to scrutiny. Again using the medical analogy, it is much less likely for the Cardiologist to make a mistaken diagnosis as it relates to a heart condition than a General Practitioner – and if a mistake is made, at least the decision by the client cannot be brought into question since they did rely on the appropriate expert for the field.

Seismic Evaluation of Existing Buildings

The American Society of Civil Engineers (ASCE) has developed a national standard entitled the Seismic Evaluation of Existing Buildings (ASCE 31-03). The value this standard provides is that a building need not be evaluated by the more stringent current building standards but rather for structural deficiencies that through the course of time have proven to be the cause of structural failures.

The evaluation begins with a checklist that gets completed for the particular type of building being evaluated. This ascertains if known structural deficiencies for the type of structure in question are present in the subject structure. If none are encountered, the structure is deemed to be adequate by the provisions of this national standard. No further evaluation or strengthening is required.

If the structure has components on the checklist that are deemed to be initially deficient, there are two possible courses of action: perform a more refined evaluation that may show that the component with the perceived deficiency is actually adequate, or do no further evaluation and strengthen the component.

If you need more information on this issue, please contact us and we will be glad to assist you.

Checking for Termite Infestation / Dry Rot Damage

Dry rot and pest damage can have a substantial adverse effect on a wood-framed structure’s response to earthquakes. Damage to structural elements may not always be as readily apparent as in the photo above from one of our litigation support cases. In some cases, fungus or termite infestation will be found only inside the wood element involved and will progress outward. By the time it is noticeable on the surface as staining or softening, the wood’s strength may already be significantly degraded. Therefore, if there is doubt, structural wood elements should be thoroughly reviewed to ensure they are in good condition.

Fungus and termite growth can occur where food (wood) and moisture are present. The source of the moisture may be a leaking pipe within a wall or a tear in the stucco backing paper. It is important to find the cause of the water intrusion, repair it and allow the remaining wood to dry.

Damage in structural elements can be detected by probing the wood with a sharp object, like a knife or an awl. This can be done by the homeowner. Try it on a large sample of wood so that you get comfortable with the feel of good wood. You will then be more apt to notice elements where the probe easily penetrates the surface. A structural element easily penetrated by the probe is likely to be infested, which leads to a reduction in its effective cross-section – thereby reducing its structural integrity. Depending on the structural significance of the elements affected, it may become necessary to retain a Structural Engineer to assess the structural impact of the infestation and determine if remedial steps are warranted before the damage, and the cost of reconstruction, escalate as in the case illustrated in the photo below.

Tilt-Up Building Retrofits

Background

Older tilt-up concrete wall buildings have been categorized, based on past earthquakes, as being potentially hazardous and prone to significant damage, including possible collapse, in a moderate to major earthquake. It is highly recommended, and depending on the jurisdiction, mandatory, for such buildings designed in accordance to building codes in effect prior to 1976 to be retrofitted. Jurisdictions with such mandatory ordinances are City of Los Angeles (Division 91), County of Los Angeles (Chapter 95), and City of Burbank (Ordinance 3512). The purpose of the implementation of these ordinances is to reduce the risk of death or injury that may result from the effects of earthquakes.

Tilt-up structures tend to have very light plywood sheathed roofs in comparison to the much heavier concrete wall panels that support them. When exposed to moderate seismic excitation, the nail connections between the plywood sheathing and the concrete walls are susceptible to failure. In this type of failure mode, the concrete walls are free to separate from the roof sheathing causing the roof to collapse. Subsequently, the walls will collapse, as the roof is no longer there to stabilize the walls.

 

Retrofit Mechanics

There are four major aspects to the retrofit of a tilt-up concrete wall building:

 

Sub-purlin to Wall Anchorage

Sub-purlins are the light, repetitive, 2x members at closely spaced intervals that support the roof. Steel brackets are used to provide a positive connection between the concrete walls and the sub-purlins that reduce the risk of the walls separating from the plywood sheathing. See typical detail shown below.

 

Purlin to Wall Anchorage

Purlins are the heavy 4x members typically spaced 8 feet apart that support the sub-purlins. Steel brackets are used to provide a positive connection between the concrete walls and the purlins that reduce the risk of the walls separating from the plywood sheathing. See typical detail shown below.

 

Glu-Lam Beam to Wall Anchorage

Glu-Lam beams are the main beams that support the purlins and are in turn supported by concrete pilasters or posts. Steel brackets are used to provide a positive connection between the concrete walls and the glu-lam beams that reduce the risk of the walls separating from the plywood sheathing. See typical detail shown below.

 

Continuity Ties

Continuity ties are achieved by adding steel brackets at interior points in the roof framing so as to tie a concrete wall with another concrete wall parallel to it. The concept here is to inhibit the ability of the wall panel to pull away from the roof by tying it back to the opposite side wall. See typical detail shown below.

Eccentric vs. Concentric Continuous Footings

There are two types of continuous footings that can be used to support exterior walls in residential construction.

The eccentric continuous footing,

 

and the concentric continuous footing.

 

However, after conducting many post-earthquake damage assessments, we believe the eccentric continuous footing is susceptible to cracking in the slab.

When the walls rack in an earthquake, a downward force is exerted on the footing by the wall. Because the eccentric continuous footing is not centered under the wall, a rotational force is introduced into the footing. If this force is great enough, the slab will crack to relieve the induced stresses. This can be seen in Figure 3. It is our opinion that the concentric continuous footing is structurally more sound than the eccentric continuous footing.

Therefore, a better product is delivered to the owner when exterior walls are supported by concentric continuous footings.