2016_SourdoughCampgroundBridge_LoadRating.pdf

Sourdough Campground Bridge Glennallen Field Office FAMS Location #: L59815 Load Rating Date: 1/12/2016

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Bridge Description:

Sourdough Campground Bridge is a single span, two lane bridge. The bridge was constructed in its current location in 1991, but the Glulam girders are from a previous bridge that was built in 1952. The design plans are on file at the NOC. The span length for the bridge is 48’-10” and the out to out width is 26’-0”. The superstructure consists of four glulam girders that are 35 ¾” deep. The two interior girders are 13” wide and the two exterior girders are 11” wide. The girder spacing is 6’-8”. There are timber diaphragms between each girder located at the one third span length locations. The deck consists of 6- ¾” thick glulam deck panels. For the wearing surface, there are four sets of three 3x12” longitudinal running planks. There is a timber curb and railing system. The substructure consists of a 2’-0” thick concrete cap beam that is supported by four 2’-0” by 2’-0” concrete columns that tie into a 1’-6” thick concrete spread footing that is 7’-0” wide.

Load Rating Assumptions:

The biggest assumption in this load rating was what value to use for the reference design value of the timber girders in flexure (Fbo). The timber glulam girders were constructed in 1952. Historically, the structural properties of timber have decreased over time due to the decrease of old growth timber available in the timber industry. In general, timber from 1952 should have higher structural properties compared to timber harvested today. For Sourdough Campground Bridge, the original glulam girder drawings from 1952 are on file at the NOC. However, the drawings do not specify the grade of the glulam girders. The drawings just reference the WCLA Specs. from 1951. This specification book has not been found so the original timber grade is unknown. In 1987, Keith Christiansen, Structural Engineer, at the NOC began designing the existing Sourdough Campground Bridge. In Keith’s design calculations for the timber glulam girders, he assumed a timber flexural design value of 2,200 psi. Keith did not use any timber adjustment factors so the adjusted timber flexural design value (Fb) was 2,200 psi. In 1995, the Alaska DOT performed a load rating of the bridge. The load rating engineer assumed an Fbo of 2,400 psi. Then the reference design value was modified using timber adjustment factors.

Originally, the engineer modified the bending value by assuming that the beam was wet which reduced the bending strength by 20% (Cm = 0.8). Then the engineer changed the assumption and assumed that the beam was dry which increased the Cm factor to 1.0. The engineer also used a flat use factor which increased the bending strength by 15% (Cfu = 1.15). In today’s code, the flat use factor is only applied for bending in the Y-Y axis or lateral loads. Lateral loads were not analyzed in this load rating so the flat use factor does not apply. A volume factor (Cv) of 0.76 was also used. The adjusted bending strength (Fb) that was used in the Alaska DOT load rating was 2,108 psi. In addition, a permit load rating was performed by Keith Christiansen in 1998. He again used an Fb of 2,200 psi.

With these previous designs and load ratings in mind, a reference design value, Fbo, of 2,400 psi was used in the load rating. This load rating used Load Resistance Factor Rating (LRFR). Some of the timber adjustment factors in LRFR are different than Allowable Stress Rating (ASR). However, the Cm, Cfu, and Cv factors that were discussed previously are the same. In the 2016 load rating, it was assumed that the girders were wet since moisture can likely seep through the timber deck. This is a conservative assumption which lowers the bending strength by 20% (Cm = 0.8). As discussed previously, the flat use factor does not apply for conventional load ratings about the X-X axis so the flat use factor (Cfu) was taken as 1.0. The Cv factor was approximately equal to the 1995 load rating. A Cv factor of 0.75 was used. For comparison, the adjusted bending strength using only these three timber adjustment factors was 1,440 psi. This is a conservative bending strength assumption compared to the values that were used in previous calculations. The actual adjusted timber bending strength (Fb) that was used in the load rating was higher due to differences between LRFR and ASR.

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An assumption on the girder size also needed to be made. The original plans from 1952 for the glulam girders call out the interior girders as 13” wide by 35.75” deep. The design calculations from 1987 and the plans from 1990 call out the interior girders as 13.5” wide by 35.75” deep. To be conservative, the smaller girder size of 13” wide by 35.75” deep was used for the interior girders in the load rating. This ended up being a non-issue since the exterior girders control the load rating. The exterior girders are shown as 11” wide by 35.75” deep on all references.

In addition, there is some discrepancy on the condition of the girders. The current data in the BLM National Bridge Inventory (NBI) database shows the superstructure in Good Condition (7). However, on the 2014 inspection report under the superstructure components, the girders were rated in Fair Condition (5). A rating for the overall superstructure is not shown on the 2014 inspection report. In the superstructure section, the report says “Girders are over 60 years old. Weathering and loss of preservative treatment is evident. Checks are evident although minor and shallow. Due to age, bridge is load rated and posted.” To be conservative, the girders were assumed to be in Fair Condition. This means that a condition factor (φc) of 0.95 needs to be applied to the timber bending strength to reduce the bending strength.

Sourdough Campground Bridge has timber running planks for the wearing surface. The approaches to the bridge are graveled and it’s possible to have some gravel accumulation on the bridge deck. This would increase the wearing surface dead load. However, the previous inspection photos and conversations with BLM AK personnel show that the bridge deck is kept clean with minimal gravel accumulation. Only the weight of the timber running planks was used for the wearing surface dead load.

The Average Daily Truck Traffic (ADTT) has not been measured at the bridge. However, the Average Daily Traffic (ADT) is listed as 50 in the BLM NBI database. It was assumed that the ADTT was less than 100. This means that a 10% reduction could be applied to the Multiple Presence Factor. It also means that the legal live load factor (γLL) can be taken as 1.30.

Load Rating Results:

Sourdough Campground Bridge was load rated using a variety of methods. The primary method was using hand calculations with LRFR. However, the live load maximum moments for each vehicle were calculated using Bridge Rating Software. Bridge Rating Software cannot load rate timber members so a notional bridge was created with the same span length and live load distribution factors. The maximum live load moments were taken from this model and used in the hand calculations. In addition, the bridge deck was checked using TBAR software to ensure that the timber glulam deck did not control the load rating. The load rating calculations are attached to this report. The TBAR load rating results show a load rating for the timber girders as well. However, this load rating is inaccurate due to software limitation in TBAR. Only the timber deck load rating from the TBAR results was used. The controlling condition for the load rating is timber flexure in the exterior girder at midspan. The HL-93 Design-Load Rating Inventory Rating Factor (RF) and Operating RF are 0.443 and 0.575 respectively.

The Design-Load Rating Operating RF was less than 1.0 which means that the legal loads need to be calculated. The RFs for the three AASHTO trucks (Type 3, Type 3S2, and Type 3-3) were all greater than 1.0. However, the RFs for the four Special Hauling Vehicles (SHVs) were all less than 1.0 which means that the bridge needs to be posted. The Sourdough Campground Bridge Rating Factors are shown in Table 1. The load rating data to be submitted to the NBI is shown in Table 2.

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The bridge is currently posted with a sign that reads “Weight Limit (Type 3 Diagram) 17T, (Type 3S2 Diagram) 27T, (Type 3-3 Diagram) 33T.” There is also a sign underneath that reads “Weight Limit Tandem Group 16,000 LBS.” It is unknown how these current posting values were determined. This sign should be removed.

There are a few different options for posting the bridge for weight limit:

Posting Option 1: Alaska DOT was contacted to see how they post bridges that do not have capacity for SHVs. Alaska posts all bridges at the design inventory level (HS-20 Inventory or HL-93 Inventory).

This posting threshold encompasses the SHV vehicles meaning that Alaska DOT does not need to load rate for SHV because their posting requirements are more conservative. For Sourdough Campground Bridge, the bridge was load rated at the design level using the HL-93 vehicle. HL-93 is not a realistic truck since it is a worst case combination of the design truck plus the design lane uniform load or the design tandem plus the design lane uniform load. However, the 36 Ton design truck controlled the design load rating for Sourdough Campground Bridge so the approximate Inventory posting value for the HL-93 load would be 15 Tons. Posting Option 1 is to post the bridge with a sign that reads “Weight Limit 15 Tons.” The sign should follow the Manual for Uniform Traffic Control (MUTCD) specifications for sign R12-1.

Posting Option 2: The AASHTO Manual for Bridge Evaluation (MBE) has a recommended Safe Posting Load equation. This equation was used for each of the four SHVs that have RFs less than 1.0. The lowest Safe Posting Load value was for the SU7 truck with a posting value of 21 Tons. Posting Option 2 is to post the bridge with a sign that reads “Weight Limit 21 Tons.” The sign should follow the MUTCD specification for sign R12-1.

Posting Option 3: Load rating SHVs is a relatively new requirement for bridge owners. State DOTs across America are still trying to figure out the best way to post for SHVs. The problem with single weight limit signs shown in Option 1 and 2 is that they can be too restrictive for long and heavy trucks.

For example, the AASHTO Type 3-3 truck is 54 ft. long and weighs 40 Tons. The RF for this truck is greater 1.0 which means that the truck is safe to cross Sourdough Campground Bridge. However, the single weight limit signs in Options 1 and 2 would prevent this type of truck from crossing the bridge.

One way to prevent being too restrictive is to post specifically for the single unit SHVs. Posting Option 3 is to post the bridge with the following sign which comes from Oregon DOT requirements (the Safe Posting Equation was not used to reduce the posting values):

Weight Limit Reduced Single Unit Vehicles

4 Axles 24 T 5 Axles 25 T 6 Axles 26 T

7+ Axles 26 T

In our opinion, the posting value for Option 1 is overly conservative and the posting sign for Option 3 could lead to confusion among truck drivers. We recommend using the sign from Posting Option 2 which is simple and conservative. BLM AK is the bridge owner and can make the final determination on how to post the bridge. One sign should be posted at each approach to the bridge (two signs total).

The bridge needs to be posted within 30 days of receiving this load rating report.

This load rating is highly dependent on existing conditions at the bridge. If any conditions such as truck

5 | P a g e traffic, dead load, or member condition change, then the bridge should be load rated again based on the new conditions.

Table 1: Sourdough Campground Bridge Rating Factors

Design Rating Inventory Rating Operating Rating

HL-93 0.443 0.575

AASHTO Legal Rating

Rating Factor

Type 3 1.03 Type 3S2 1.12 Type 3-3 1.24

SU4 0.897

SU5 0.831

SU6 0.747

SU7 0.693

Table 2: Sourdough Campground Bridge Load Rating NBI Data

NBI Item # Value Units/Description 63 – Operating Rating Method 8 LRFR rating reported by RF 64 – Operating Rating 058 RF 65 – Inventory Rating Method 8 LRFR rating reported by RF 66 – Inventory Rating 044 RF

Inputs Rating Method: Load and Resistance Factor Design Deck Type: Glulam Panels, Interconnected Stringer Type: Glulam Number of Lanes: 2 Number of Stringers: 4 Stringer Span: 48.83 (ft) E of Stringer: 1,350.000 (ksi) Stringer Fb: 1.980 (ksi)

Stringer FV: 0.149 (ksi)

Deck Skew: 0.00 (deg) Min. Panel Width: 312.00 (in) Decking Fb: 1.800 (ksi)

Decking FV: 0.145 (ksi)

Superimposed Dead Load: 225.00 (plf) Wearing Surface Dead Load: 840.00 (plf) Wheel/Track Gage: 6.00 (ft) Effective Length of Deck Resisting Wheel Load: 57.00 (in) Exterior Stringer Live Load Distribution Factor: 0.61 (wheel loads) Interior Stringer Live Load Distribution Factor: 0.68 (wheel loads) Stringer Width: 11.00 (in) Stringer Depth: 35.75 (in) C-C Ext. Stringers: 20.00 (ft) Deck Thickness: 6.75 (in) Deck Travel Clear Width: 24.00 (ft) Total Deck Width: 26.00 (ft) Component Dead Load Factor, γDL: 1.25

Wearing Surface Dead Load Factor, γWS: 1.50

Legal Live Load Factor, γL: 1.30

Inventory γLL 1.75 (for design loads)

Operating γLL 1.35 (for design loads)

Stringer Moment Resistance Factor, Φ: 0.85 Stringer Shear Resistance Factor, Φ: 0.75 Timber Deck Moment Resistance Factor, Φ: 0.85 Timber Deck Shear Resistance Factor, Φ: 0.75

Page 1 of 3Sourdough Campground Bridge - Results

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HL-93 LOADING - Live Loading Results Span Length: 48.83 (ft) Maximum Moment: 797.72 (kip-ft) Maximum Moment Location From Left End: 23.25 (ft) Maximum Shear (at "3d" or "L/4"): 58.78 (kip) Max. Bridge Live Load Deflection: 9.81 (in) L / 60 Maximum Reaction: 73.86 (kip)

Load Info Controlling Moment Load: Design Truck and Lane Controlling Shear Load: Design Truck and Lane Controlling Reaction Load: Design Truck and Lane

HL-93 LOADING - Stresses and Ratios Dead Load Moment and Shear

Stringer Deck Moment (DL + SDL) 111.95 (kip-ft) 3.69 (kip-ft) Moment (WS) 62.59 (kip-ft) 3.24 (kip-ft) Shear (DL + SDL) 8.05 (kip) 2.43 (kip) Shear (WS) 4.50 (kip) 2.14 (kip) Distributed Live Load Moment and Shear

Interior Exterior Shear Bending Shear Bending

Deck 12.40 (kip) 17.20 (kip-ft) Stringer 37.47 (kip) 538.46 (kip-ft) 35.62 (kip) 488.20 (kip-ft) Inventory Rating Ratio

Interior Exterior Shear Moment Shear Moment

Deck 2.08 3.87 Stringer 0.51 0.25 0.53 0.28 Operating Rating Ratio

Interior Exterior Shear Moment Shear Moment

Deck 2.69 5.01 Stringer 0.66 0.33 0.69 0.36 Additional Information

Inventory GVW N/A See Appendix C Operating GVW N/A See Appendix C Inventory LL 1.75

Operating LL 1.35

Multi-Presence Factor (Interior) 0.90 Multi-Presence Factor (Exterior) 0.90

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The timber deck HL-93 Inventory and Operating Rating Factors of 2.08 and 2.69 are much greater than the exterior girder HL-93 Inventory and Operating Rating Factors of 0.443 and 0.575 shown in the hand calculations. Therefore, the exterior girder load rating controls.

Nominal Resistance Stringer Deck

FV Tab. Stress 0.250 (ksi) 0.243 (ksi)

Fb Tab. Stress 2.844 (ksi) 3.726 (ksi)

Nominal Capacity Vn 65.56 (kip) 62.42 (kip)

Nominal Capacity Mn 555.37 (kip-ft) 134.38 (kip-ft)

Shear Capacity Stringer Deck

Φ * Vn 46.71 (kip) 46.82 (kip)

Resistance Factor (Φ) 0.71 0.75 Moment Capacity

Stringer Deck Φ * Mn 448.46 (kip-ft) 114.23 (kip-ft)

Resistance Factor (Φ) 0.81 0.85 Section Properties

Stringer Deck Area 393.25 (in2) 384.75 (in2) MOI X=41,883.17 (in4) Y=3,965.27 (in4) 1,460.85 (in4) Section Modulus X=2,343.11 (in3) Y=720.96 (in3) 432.84 (in3) Q 1,757.34 (in3) N/A

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