Structural Engineers Learning the Hard Way

Transcription

1 Lessons Learned from Structural Engineers Learning the Hard Way for SEAoT State Conference September 28, 2012 Presented by Davy Beicker, PE, RMT

2 Lesson 1: Prefabricated Wood Trusses Project Description: Roof framing using prefabricated 4x2 wood trusses 24 inches on center.

3 Lesson 1: Prefabricated Wood Trusses Structural Engineer of Record Involvement: Provided design requirements for trusses Provided layout of trusses Checked specialty engineer s design calculations Checked to see that Fabricator was certified by TPI Visited the site to check construction of the trusses

4 Lesson 1: Prefabricated Wood Trusses Specialty Engineer Involvement: Provided truss design calculations Provided layout of trusses Fabricator was certified by TPI Provided the first set of truss repair plans

5 Lesson 1: Prefabricated Wood Trusses What was Discovered? During a visit to the site, the Structural Engineer noticed some truss members were damaged and he thought needed repair. SE requested another engineer review the structural condition of the trusses and design a repair for each damaged d condition.

6 Lesson 1: Prefabricated Wood Trusses What was Discovered? At first, the fabricator s engineer designed some repairs but then would not come to the site to check if the repairs were performed. (He was the out-of town expert) A local engineer was then hired to inspect the repairs.

7 Lesson 1: Prefabricated Wood Trusses What Else was Discovered? Truss metal connection plates were poorly installed on the trusses. Some (around 500) missed the truss member.

8 Lesson 1: Prefabricated Wood Trusses Plates missed panel points Knot good

9 Lesson 1: Prefabricated Wood Trusses What Else was Discovered? Broken and Knotty boards were used.

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11 Lesson 1: Prefabricated Wood Trusses

12 Lesson 1: Prefabricated Wood Trusses What Else was Discovered? Heavy sprinkler pp pipes were hung from the wrong place of the truss.

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14 Lesson 1: Prefabricated Wood Trusses What Else was Discovered? A roof hatch opening was not framed out.

15 Lesson 1: Prefabricated Wood Trusses What Did We Learn? SER site observations are important. SER needs to know what he is looking for. Special inspections would not have discovered any of this. End of Lesson 1

16 Lesson 1: Prefabricated Wood Trusses Questions?

17 Olmos Dam 1998 Take unexpected loads seriously

18 Lesson 2: Built-up Wood Trusses Project Description: The homeowner wanted an unusual shape truss he found in a magazine. The truss builder insisted that the truss and connections be designed d by an engineer due to the lack of a continuous bottom chord.

19 Lesson 2: Built-up Wood Trusses

20 Lesson 2: Built-up Wood Trusses The Engineer designed the 40 ft trusses using RISA with concealed steel plates between the wood members. The truss builder built them in his shop and then sent them to the site.

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22 Lesson 2: Built-up Wood Trusses After the trusses were erected, the erecter noticed about a 1 inch sag in all the trusses. The engineer visited the site (120 miles round trip). Someone had added a notch in the top chord of the trusses. The engineer believed the reduced size of the compression top chord accounted for some of the unexpected amount of deflection.

23 Lesson 2: Built-up Wood Trusses

24 Lesson 2: Built-up Wood Trusses The roof joists were installed and the trusses sagged an additional inch. The engineer visited the site again and took more measurements and reviewed the truss construction, erection and his design closer. He hired his engineer buddy to check his design. (because that is what an engineer buddy does)

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26 Lesson 2: Built-up Wood Trusses His buddy engineer confirmed that the design was not the problem. Something was apparently not built right. The engineer prepared plans to repair the truss by adding new exterior steel plates, additional bolts and epoxy filling bolt holes and ridge beam gaps.

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28 Lesson 2: Built-up Wood Trusses The new plates were fabricated and sent to the site. The engineer visited the site again to review the repairs. At this site visit he learned that all the bolt holes were overbored 1/8 inch bigger than the bolt diameter in both the wood members and the steel plates rather than 1/16. The schedule was getting behind and the homeowner was getting more and more upset.

29 Lesson 2: Built-up Wood Trusses The engineer also discovered that the bolts used did not have enough thread and couldn t be tightened. The bolts would spin in the holes. Since the plates added an additional ½ inch of thickness they could be reused.

30 Lesson 2: Built-up Wood Trusses The bolts were removed, the new plates were added, the trusses were shored into position, the holes were epoxy- filled, the same bolts were tightened because the new side plates helped the thread problem. Shores were removed and the trusses held in place.

31 Lesson 2: Built-up Wood Trusses The homeowner was not thrilled about the exterior plates but decided they were acceptable. The home builder, the engineer and truss builder were lucky there was not a lawsuit. The homeowner was an attorney but diligent and proactive work by everyone to get the problem resolved prevented the situation from escalating.

32 Lesson 2: Built-up Wood Trusses Some Lessons Learned: Look at all possibilities that could have an impact on your design. Don t forget about things like a ridge beam. Think about adding camber in the design. Find out if your owner is a snake lawyer.

33 Lesson 2: Built-up Wood Trusses The Bigger Lesson Learned: Be diligent when there is a problem and work to solve it first, then worry about getting paid if it is not due to your errors. Even if you are right, everyone gets named in a lawsuit and the only winners are the lawyers. The owner did not pay any for all the extra pain and suffering by the engineer or the builder. The home builder is still the engineer s client.

34 Lesson 2: Built-up Wood Trusses Special Inspections may have avoided the problem with the oversize holes and wrong length bolts.

35 Lesson 2: Built-up Wood Trusses SEAoT provides a great resource to meet other engineers. An outside design check by an engineer you trust and you know is competent can be beneficial if everything goes South. If there is a problem with your design, you have an ally in being pro-active on a repair. The engineer let everyone know that he retained another engineer to review his design. This kept others from finding an engineer that may not be interested in resolving the situation. End of Lesson 2

36 Lesson 2: Built-up Wood Trusses Questions?

37 Ray Breslin (Stallone) is a structural t engineer who is wrongly convicted of a crime. Breslin designed a variety of high tech buildings and was a top structural engineer. After Breslin was wrongly convicted, cted, he was sent to serve e his sentence in a maximum security prison of his own design. During his time in prison, Breslin has analyzed every high security prison and gained significant survival skills, but faces his biggest challenge in trying to break out of an ultimate high-tech prison that he himself designed to be escape-proof.

38 Lesson 3: Tilt Wall Panel Embed Plates Embed plates have to fit in the wall panel along with all the reinforcing steel.

39 Lesson 3: Tilt Wall Panel Embed Plates

40 Lesson 3: Tilt Wall Panel Embed Plates

41 Lesson 3: Tilt Wall Panel Embed Plates Special Inspectors won t know how to direct the contractor how to fit everything in the panel without structural details.

42 Lesson 4: Plumbing in Beams The plumber will always figure out a way to put his pipes right in the middle of your grade beam.

43 Lesson 5: Quality Control vs. Quality Assurance Are Special Inspections considered as quality assurance or quality control? Special Inspections are paid by the Owner. Contractor is responsible for his own quality control. Quality assurance (special inspections) spot check the material and workmanship that the contractor should already be sure of meeting project requirements. If the contractor relies on the SI for his quality control, he may not know his material is unacceptable until too late. Project delays may be blamed on the innocent.

44 Lesson 5: Quality Control vs. Quality Assurance Project: Concrete structure with reinforcing steel.

45 Lesson 5: Quality Control vs. Quality Assurance Scenario: Reinforcing steel shop drawings reviewed Mill certifications reviewed During fabrication, the fabricator accidentally used foreign steel instead of specified domestic Reinforcing delivered and placed. Special inspector discovered wrong steel after it was all placed. Penalty exists for time overage.

46 Lesson 5: Quality Control vs. Quality Assurance Contractor Point of View: Contractor was relying on SI to review reinforcing to catch mistakes. Contractor asks for additional time because SI should have caught mistake much sooner. Should he receive additional days?

47 Lesson 5: Quality Control vs. Quality Assurance Questions?

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49 Lesson 6: Testing of Non-Shrink Grout Project Description: 7 story structural steel frame building. The Structural Engineer specified non-shrink, nonmetallic, high-strength bagged product grout mixed onsite and hand packed under the steel column base plates. Contractor asked the Owner if he should test the grout strength. The engineer did not specify any testing.

50 Lesson 6: Testing of Non-Shrink Grout

51 Lesson 6: Testing of Non-Shrink Grout Grout Testing Criteria : 8,000 psi minimum at 28 days Compressive strength test using 2x2x2 inch cubes Test according to ASTM C109 Modified & C1107

52 Lesson 6: Testing of Non-Shrink Grout After several sets of cubes were made, and several 28 day results were obtained; only 1 of the 11 sets (covering 85 columns) reached the minimum compressive strength of 8,000 psi.. Houston, we have a problem. First item of action: INFORM EVERYONE (with options to fix the problem)

53 Lesson 6: Testing of Non-Shrink Grout Engineer had to review his calculations and loads and determine which column loads exceeded the weak grout strength. Engineer determined that 59 of the 85 columns were acceptable. 26 were overstressed.

54 Lesson 6: Testing of Non-Shrink Grout Reasons determined to be the cause of the low breaks: Plastic molds instead of brass molds were used No cover plate on the molds Uneven faces of cubes Non-shrink grout doesn t shrink, but it does expand. Fingers pointed at the testing lab

55 Lesson 6: Testing of Non-Shrink Grout More testing: Removed sections of the grout that were not directly below the column plates for testing. Cubes were cut from the sample and tested for compressive strength.

56 Lesson 6: Testing of Non-Shrink Grout Strengths passed. Since the samples were obtained from an unconfined area, testing lab concluded that the material below the base plates would result in a higher compressive strength than the unconfined grout retested. 2 ½ month process No delays or changes in schedule

57 Lesson 6: Testing of Non-Shrink Grout Some lessons learned: Be careful what you ask for. This entire issue may have been avoided d if the contractor t could have asked the structural engineer about testing instead of having to navigate questions through all the proper p channels. End of Lesson 6

58 Lesson 6: Testing of Non-Shrink Grout Questions?

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60 Lesson 7: Construction ti Live Load During Concrete Placement on Metal Deck IBC requires a minimum of 20 psf construction live load during concrete placement on metal deck.

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