Bridge Design & Bolts - Bolt Failures - Torque Errors

Bridge design often relies on a bolted joint's tension, which relies on bolt tension, not bolt torque. But the bridge engineer has to specify bolt torque for the construction crew (not tension). Bolt tension errors (20% to 50%) often become the limiting factor for these bridge designs. There is a new ultrasonic technology that allows bridge designers to more reliably measure actual bolt tension, not just bolt torque.

NDT Update says:
" SureBolt outperformed the one-point bolt gauges on every bolt and every test in reliability and accuracy."

A bridge construction engineer may find the following information useful:

1. Bolts fail. What is the main reason, stated by structural engineers, for bolt failures? Improper pre-load tension. What are some of the factors that cause pre-load tension errors? This affects an engineer's bolted joint design.

2. Bolt tension, not bolt torque. Tension not torque. You want to put a known amount of tension into your nut and bolt combination, not just a known amount of torque. Your structural analysis often relies on proper pre-load tension.

Using only a torque wrench, on flange bolts, can give very large tension errors.

Torque pass, after pass, after pass. But look what happens --- >>>>>

Notice the variation in bolt tensions (stud stretch) on each of three passes with a torque wrench.

Credit Integra Technologies for this graph. They have taken data on 94 joints. They mislabeled the first pass (it should show 100 Pound-Feet instead of 200 Pound-Feet)

X-axis = bolt position. 16 different bolts
Y-axis = bolt stretch (tension).
Large tension variations are typical with torque control. Notice the > 250% variations.

3.   Measure Tension errors. How can a bridge construction crew measure torque wrench errors? Do these bolt tension errors matter? These errors often affect an engineer's assumptions.

4.   Verify using a bolt gage. Your joint's stress analysis can be verified, bolt by bolt, using a bolt gage. What is an ultrasonic bolt gage? See an animated demonstration of an ultrasonic echo inside of a bolt.  Structural engineering labs rarely have enough resources for each engineer to learn ultrasonics. Are bolt gages reliable? Are they easy to use? New patented technology has finally made ultrasonic bolt gages reliable and easy to use. Verify your bolted joint design.

5.   Friction. Bridge designers know that bolt and nut friction consume over 80% of the torque measurement. But, how does the actual tension vary bolt to bolt? What if you use the same type of bolt? This may affect an engineer's structural analysis.

6.   Stress, strain & yield. A stress strain curve can be plotted using SureBolt. Stress strain and yield. SureBolt can help an engineer verify his bolted joint design. Did his bolt reach yield?

7.   Flanges. Flange joints sometimes leak. Why does the tension in a flange bolt vary so much, bolt to bolt? How can you tighten each flange bolt to closer tolerances? How can you use SureBolt on a flange?

8.   SureBolt. How does SureBolt differ from any other ultrasonic bolt gauge? What is the difference between a one-point bolt gage and SureBolt's patented whole echo method? Are the prices similar? (Yes)

9.   Bridge Engineers. Some engineers use a strain gauge to measure bolt tension because they know a torque wrench is not directly measuring tension. Yet their structural analysis software may assume a limited amount of error (+- 25%) when calculating bolt tension from bolt torque. Calculating Torque? If you think calculating the correct torque is easy, just see either of these two web pages (no affiliation with ARVC). Nut factors or torque is easy? With SureBolt, instead of calculating the torque to get you the correct tension, you just measure the tension directly. You can even use SureBolt to determine the torque.

10. Tension errors. What are some of the tension errors associated with using a torque wrench? Does your stress analysis software let you enter the tension error ranges?

11.  Structural engineering. This site is a structural engineering resource for torque versus tension in bolted joints. SureBolt is a nondestructive testing - NDT - method of measuring bolt tension. Non destructive testing includes the use of ultrasonics.

SureBolt's Whole Echo patented technology greatly increases reliability by using the whole echo instead of just one point.

All other bolt gages use just "One Point" (one zero crossing), that leaves you susceptible to peak jumping (20% or more error). Surebolt is a more reliable bolt gage due to a new patented technology. See comparison table.

Bolt gages have been around for over 20 years. Their "Peak Jumping" problems have kept them from being widely used. When your bolted joint fails or flange leaks, then you find out your bolt gage may have jumped peaks.

With a lot of training and laboratory testing, NASA has used bolt gages for years. NASA physically takes Polaroid pictures of the echoes before and after tensioning. Then they use their trained human experience to determine if the echo has distorted "too-much". Even after all this, they have to throw away approximately one third of the ET measurements because they are not reliable. When NASA used the SureBolt prototype, no measurements had to be thrown away.

SureBolt gives you a 13.3" TFT, portable PC (Panasonic Toughbook Model 72). The ultrasonic pulser and digitizer are built into the CD ROM and PCMCIA slots. Click here to see the actual SureBolt screens. See the animated DEMO page to see how an ultrasonic bolt gage can measure tension. SureBolt has been used on special high strength titanium bolts to standard "off the shelf" bolts.

For more information, see the main SureBolt^TM navigation screen.