The Things That Go Bump In The Night

A popular title for the fearful things that can take place during the nighttime. But do you ever worry about your building falling over during the night. I think structural engineers do; it may be their worst nightmare.

So, what holds a building up? What gives the building a sound and solid footing on mother earth?

The foot of the building is like the feet of the building’s structural skeleton. All the weight of the structure comes down to the point or points where the building touches the ground. The size of the building’s foot is based on the characteristics of the ground, the soil, and what is beneath the surface. Different soils have different bearing capacities or in other words the capacity to hold something up. The ultimate bearing material is solid rock. But not every site has that luxury in addition to it being at the right depth. Sometimes it’s down 60 feet or conversely it is close to the surface and may involve costly removal.

So how do you know what’s down there?

To really discover what the soil conditions are below the building footprint, a subsurface soils exploration is conducted. This task is performed by a geotechnical engineer and culminates in a geotechnical report. The geotechnical firm begins with taking borings at the site at the locations identified by the structural engineer. Borings are taken by a drilling rig that drills a vertical hole to capture core samples. The cores are taken to a lab and samples are selected from the cores for testing to determine soil types and characteristics and the depth at which different materials are found. The FSB structural engineer identified 5 boring locations that included 4 deep borings at the building footprint and 1 shallow boring in the parking area. From the identification and analysis of the soils, the geotechnical engineer makes a recommendation for the type of foundation and stabilization of slabs on grade for further design by the structural engineer. The deep borings will provide information for building foundation systems and the shallow boring will provide information for the paving system (parking lot).

You invariably get an interesting geological lesson in the process. This is a paraphrased excerpt from the report; The bedrock stratum surrounding the site may be classified as Vanoss Unit based on Engineering Classification of Geological Materials for Murray County. The unit consists of alternating moderately soft to moderately hard sandstone, conglomerates, shales and few thin limestones. The shales are multicolored and resemble those of the underlying Ada Unit. In the outcrop area between the Arbuckle Mountains (yes we have mountains in Oklahoma, ha) and the middle of Seminole County, the sandstones and conglomerates are thicker and so arkosic that they might visually be mistaken for true granites. Near the Arbuckle’s, the total thickness of the Vanoss Unit is 1550 feet and northward it thins to 650 feet and eventually to 250 feet near Konowa. The outcrop varies from a 2 to 10 mile wide band.

Well, after all the core analysis and geology lessons we get down to the business of what type of foundation the geotechnical engineer is going to recommend starting with some observations. There has been previous fill material on the site indicating past construction. Boring B2 revealed a thin layer of limestone classified as medium strong rock which will cause difficult excavation for the building pad. Because of the thin inconsistent limestone beds interbedded with soft lean clay and highly weathered shale, drilled shaft construction (piers) is not recommended. So the recommended foundation systems are spread footings for building columns and continuous footings for walls. The footings should be placed on 2 feet of properly prepared structural fill (which includes compaction). The bearing capacity recommended is 2000 pounds per square foot (psf) for the spread footings and 1400 psf for the continuous footings.

So we just need to figure how much this bad boy Visitor Center weighs and calculate how big the feet of the building need to be. I’m guessing size 24.

The engineered footings are detailed in the structural section of the construction documents. The spread footings are 2’ deep and vary from 3’ by 3’ to 6’ by 6’ depending on the load (weight) coming down the building column. The typical continuous wall footing is 2’ wide and 2’ deep with some areas of greater weight concentration due to column loads being as wide as 6’-6”.

Following the general excavation and grading of the site, the building’s foundation, i.e. footings, will be one of the early activities of construction. The day for the contractor to show up at the job site is growing near.

FSB Lead Structural Engineer: Tom Bush, P.E., Ph.D.

Geotechnical Report: Burgess Engineering and Testing, Inc.

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Filed under Construction, Design, Materials

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