Multnomah Falls Benson Footbridge Investigation

About This Project

Carlson provided the specialty testing for the Multnomah Falls Benson Footbridge Investigation

On January 9, 2014 a boulder came off of the cliff above and smashed a three foot hole through the deck of the Benson Footbridge at Multnomah Falls. The Forest Service which manages the falls was tasked with repairing the hole. Carlson Testing was contacted to help by core sampling the concrete deck around the hole for compressive strength and possible petrography.

The compressive strength helps the bridge engineer determine how to replace the concrete and strengthen the deck with rebar. The petrography test looks at concrete under a microscope by a petrographer who is usually a geologist. They look for type of aggregates used, paste content of the cementitious part, water to cement ratio (an indicator of strength), air voids (for durability) and deleterious reactions that might weaken the concrete, among other things.

CTI used GPR to scan for rebar in order to avoid hitting the bar and get slab thickness but also to determine rebar patterns for the engineer to help design the fix. This information was important, because in 1914 when the bridge was built the documentation is not there to show what was done for the concrete as well as the rebar. Once this was complete and the bar locations marked on the deck, six locations were selected around the hole. We proceeded to drill and once through the deck our Forest Service rep would use a bucket to try and catch each core. We ended up with five cores as one was donated to the Multnomah Falls pool below which couldn’t be caught. Of these five cores, three were tested in our lab for compressive strength but the remaining cores were not used for petrographic analysis. The cores were intact and well consolidated with an average strength impressive for 100 year old concrete exposed to the wet environment (including freeze-thaw) at over 4,000 psi!

Subsequently we were requested to verify a coating that was applied to the deck in 1987 as to whether it was cementitious or an epoxy. Apparently an epoxy coating cannot be coated over and is not suitable for concrete applications in some cases because it will not breathe moisture. To accomplish this we made our own sample of a cementitious latex modified product that was specified to be used at the time. This reference sample and scrapings from the core tops were then sent to an analytical laboratory to perform an infrared spectroscopy test. Because the epoxy and latex portion are composed of carbon based organic molecules, once lit up will emit signature wavelengths that can be matched up to specific chemical makeups. It was found that the signatures matched up with the reference sample indicating that the product placed was that specified and not an epoxy! The Forest Service now has the information they need to come up with a fix

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Project Services

  • Ground Penetrating Radar (GPR)
  • Concrete Coring
  • Coating Verification

Fun Facts:

  • History - Built 1914
  • Builders - Karl P. Billner (Design Engineer), Pacific Bridge Co. of Oakland, California (Prime Contractor), Robert Lee Ringer (Sub-Contractor)
  • Recognition - Part of the Historic Columbia River Highway project was posted to the National Register of Historic Places on April 22, 1981
  • Tourism - Attracts approximately 2.5 million visitors per year, ranking Multnomah Falls as Oregon’s number 1 visited natural attraction.
  • Height - Nation’s fourth largest falls for all four drops. Multnomah Falls has a total drop of 620 feet.
  • Funding - Simon Benson, a philanthropist responsible for Portland’s “Benson Bubblers” and the Benson Hotel, wrote out a check on the site for the amount to build it. The resulting footbridge is a 45'-0" reinforced-concrete deck arch, 105' above the lower Multnomah Falls. The Balusters are precast concrete.
  • Secret Story - When the job was nearly completed, Simon Benson’s son allowed the subcontractor, Robert Lee Ringer, to put his name on the bridge in the concrete “as is done on famous projects”. But the bridge engineer was not happy. The builder had not yet been paid for his work and to appease the engineer he agreed "without any fussing to cement over the offensive letters," But, when he "was alone and attending to the last chores. [He] cleaned up the lettering and smeared it lightly with wet clay onto which cement will not adhere permanently and carefully troweled the place over to match the rest of the deck." Ringer did not again visit the footbridge for two years. "By that time the winter frosts had done their work and routed out the inscription, which reads ‘R. L. Ringer, 1914’."
  • Sources -,,