Bridge load rating question

[bluetree]

Need some advice from any bridge engineers. Reviewing a bridge load rating report for a single span multi girder bridge for the std AASHTO truck loads. Bridge span is 26 ft and the width is 17 ft, single lane. Bridge consists of five W18x50 beams spaced at 3.96 ft, simple span. Deck consists of 8” th precast concrete deck planks. The ends of each plank are connected to only the exterior beams with shear studs. The planks only bear on the interior beams, there is no connection. There is no bracing from beam to beam, so the unbraced length of the interior beams is 26 ft, and they are the controlling member. The rating was performed using both the AASHTO Load Factor Design and Allowable Stress Design methods from the Std Specs for Highway Bridges manual. The report results using the LFD method give moment inventory and operating ratios of 0.4 to 0.7, so the bridge would have to be posted for lower loads. . The report results using the ASD method give moment inventory and operating ratios of 1.4 to 2.0, so the bridge would not have to be posted. The report summary concludes by ignoring the LFD results and using the ASD results, stating that the bridge does not need posting and has a capacity greater than the std AASHTO truck loading. There is no explanation on why engineer chose to ignore the LFD rating results.
The AASHTO ASD method seems very simplistic and does not address unbraced length with any degree of complexity. The LFD method seems to be a more extensive analysis method. The calcs appear to be correct and follow the AASHTO methods. Am I missing something here? Is one method better than the other? I am not experienced with bridge design and analysis. It makes no sense to me to have very divergent results, and then just go with the favorable result. Seems like poor engineering judgement at a minimum. Also, I did a capacity analysis of the bridge beams using the AISC ASD manual, and the results are similar to the AASHTO LFD results. Any suggestions?

 

[yakpol]

Bluetree,
For all means the interior beams shall be considered unbraced. I followed the formula for allowable stress in the table 10.32.1A of the code.
Fb = (50000/Sxc)(Iyc/L)*sqrt(0.772*J/Iyc + 9.87(d/l)^2)
for W18x50
Fb = (50000/ 89)*(20.0/ 312)*SQRT(0.772* 1.24/20.0+9.87*( 18/ 312)^2) = 10.25 ksi
Moment capacity M = 89(10.25)= 912 kip-in = 76 kip-ft
Rating factor RF = (76-37.7)/104.4 = 0.37
RF is lower than calculated by LFD, which is expected for ASD.

 

[miecz]

yakpol is right. My quick and dirty calculations were a little too quick.

 

[bluetree]

miecz
The formula for Sxc is based on the parallel axis theorems for moments of inertia. The term 1/12*b*d^3 is the moment of inertia of the flange about its own centroidal horizontal axis. The term a*D^2 is the moment of inertia of the flange area about horizontal axis of the W18 section.

 

[TXBRIDGEENG]

ASD IR H 28.0 & HS 27.1
OR H 42.9 & HS 40.1

LFD IR H 21.8 & HS 20.4
OR H 36.4 & HS 34.0

I did not know how old your bridge is. I assume fy=36ksi. If your bridge is between 1937 to 1963 then fy=33ksi.
If your bridge is between 1905 to 1936 then fy=30ksi.
In regards to bracing of the deck, read "Bracing Effects of Bridge Decks" Report No. 1239-4F, Center for Transportation Research, University of Texas at Austin, Otober 1993.

In Texas, your bridge would not require load posting. I do hundreds of load rating calculations a year.

 

[TXBRIDGEENG]

ASD IR H 28.0 & HS 27.1
OR H 42.9 & HS 40.1

LFD IR H 21.8 & HS 20.4
OR H 36.4 & HS 34.0

I did not know how old your bridge is. I assume fy=36ksi. If your bridge is between 1937 to 1963 then fy=33ksi.
If your bridge is between 1905 to 1936 then fy=30ksi.
In regards to bracing of the deck, read "Bracing Effects of Bridge Decks" Report No. 1239-4F, Center for Transportation Research, University of Texas at Austin, Otober 1993.

In Texas, your bridge would not require load posting. I do hundreds of load rating calculations a year.

 

[TXBRIDGEENG]

Ran some quick load ratings but I did not know about your misc dead loads like rails, diaframs, and etc. You did not indicate if you have an asphalt overlay so my numbers will probably be smaller.

 

[TXBRIDGEENG]

There was some research done that if you had large dead loads on the top flange, the top flange would not rotate.The weight of the 8" slab provides frictional resistance to movement of the top flange. See report referenced in my previous posting. Looking at the numbers they gave you, I suspect there might be an error in LFD calculations.

 

[TXBRIDGEENG]

Check to see if they used the proper section modulus for the Load Factor Method. The load factor design method requires the plastic modulus (Z) and not section modulus (S).

 

[bluetree]

TXBRIDG,
The bridge was built in 2001 and the steel is 50 ksi. The concrete deck is not monolithic, it consists of seven 4 ft wide precast deck planks. Scanned the report you mention and had some questions. So the design practice of the beam being considered braced at the wheel load point for short span steel bridges of my type, is this standard practice in a few, some, all state DOT's? Is this beam bracing under the wheel load accepted by AASHTO and is it stated in the AASHTO std specs for hwyway bridges manual? So for a bridge of my type, the beam unbraced length would constantly change as say the last HS20 truck axle load rolled across the bridge. And lastly, what value for Sxc did you use in your IR and OR calcs in the above thread?

 

[TXBRIDGEENG]

No, it is not stated in AASHTO manual, When doing load ratings on existing bridges, some engineering judgment is used. TxDOT has a bridge load rating manual that allows the top flange to be fully braced under certain condition such as the timber deck planks covered with over 6" of base and asphalt material. I ran your bridges above with two TxDOT programs. I used 36ksi for both programs. The ASD program used Sx=88.9 and the LFD program used Zx=101 since beam meet the conditions of AASHTO 10.48.1. If the beam did not meet the conditions of AASHTO 10.48.1 then the beam would have fall under AASHTO 10.48.2 and the LFD program would have used Sx=88.9
Research has shown that the beams does not have to be embedded in the deck nor does the deck have to be concrete to provide effective lateral bracing. The first 3 cases lised below may be analyzed as fully braced. In the last two cases the beam should be analyzed with an unbraced length equal to half the span length. In the last case, if the depth of the base is substantially greater tahn 6 inches, the beam may be analyzed as fully braced.
1. Concrete deck cast on top of the compression flange
2. Corrugated metal deck. The deck sections should be interconnected adequately to insure that the deck acts as a unit.
3. Lmainated timber deck
4. Timber planks deck with timber runner. Each of the two runners should be 2 feet wide and the individual runners planks should be securely attached to the deck.
5.Timber plank deck covered with at least about 6 inchges of base and asphalt overlay.

I am not sure if this is used in other DOT's.

The weight of your 8" slab provides enough dead load to provide frictional resistance to keep the top flange from moving laterally so it is considered fully braced regardless if the load is H 20 or HS 20.

 

[cap4000]

I have the 1960 Lateral Beam Bracing article by George Winter. He concludes for a 30 ft span and a uniform load of 2k/ft friction alone would in fact brace the beam. However he does NOT recommend using friction as a bracing mechanism.

 

[TXBRIDGEENG]

TxDOT has done extensive research in the effects of lateral bracing and the items listed above in my previous posting is straight from their bridge load rating manual based on their current research. You still have to look at the structure and use your engineering judgment to what you feel comfortable with. I have done thousands of load ratings and I know what I feel comfortable doing based on my 20 years of experience. I once had a boss long time ago that would say this when he was uncomfortable with something his advise was "When in doubt reinforce through out".

 

[TXBRIDGEENG]

Bluetree,
If you are with a city or county official, have your State DOT review the load rating. By the description of the bridge, I assume it is an off-system bridge (bridge owned by county or city or some other local government).State DOT's are given the responsibility to make sure that off-system bridges are inspected and load rate by the federal government.