Help Converting Static Bending Properties to Fb ?

[figurnitout]

Hello,

I am performing a design check on timber crane mats that will be used in a bending application. In the literature provided by the material supplier (attached), Fb is not given.

The properties given are Static Bending Properties, ie, Modulus of Rupture, Modulus of Elasticity, Work to Maximum Load, and Impact Bending.

Is there any way to convert these properties into an allowable bending stress? Or is there any way to use these values to evaluate the timbers in bending?

I am familiar with NDS, but didn't trust those values for this application because the allowables differ greatly between NDS and what the supplier says the mats are made of.

Any thoughts? Suggestions?

Thanks for your help!

 

[dhengr]

figurnitout:

I’m curious to know if you understand your total problem sufficiently when you say, “I am performing a design check on timber crane mats that will be used in a bending application.” Is that a simple beam with uniform load and two end reactions?

What is loading the mats? Dimensions, weights and C.G’s., bearing areas on the mats, loads and load radius, etc. What are these mats supported by, and its capacity? Where are the reaction points? How many mats (as the one shown) will be ganged together under your loading. Draw us (yourself) a picture of what the beam actually looks like; its loads and reactions and their locations on the mats.

Who are you working for on this and what is their risk aversion level? In other words, you must determine who’s ‘seat-of-the-pants’ you are going to use for this design, NDS does not apply, but many of its principles do. What are the conditions of the mats, new or at the end of their life and only 8" thick in the middle 20'? Your mat supplier should be able to be a bit more specific if they want your business. Did they give you all three pieces of info.? Where did chapter 4 come from, I’ve seen the other two. Did they bracket the species used in the Forest Products Lab. tabulation? What Factor or Safety do you intend to use, is this a stability problem? Where does OSHA fit in the picture? The exact bending stress may be the least of your considerations.

 

[figurnitout]

Great questions. Attached is sketch. Concrete trucks will drive across this temporary bridge. The liability is mine.

I am aware that I could easily double the mats, or place timbers (or even steel plates) longitudinally under the wheel paths. The cost of the materials gets very high very quickly.

Mats are new, 12" thick, rough cut timbers. Yes, Chapter 4 is from the Forest Products Lab. Supplier says the mats are 95% oak of varying species.

Do you have any other (economical or otherwise) suggestions? I would surely pay for a consultation.

Thanks

 

[dhengr]

How long are the steel bridge members? Are the fabed yet or could you reduce the 12.167' dim.? You won’t get four times the WF moment cap’y. with the arrangement you show. (not 25% of load to each of the four beams) send me your e dress. You will tend to lift or unload the outer WF’s and add this load to the inner WF’s. (thus, 15-20% to outer WF’s and 35-30% of the moving load to the inner WF’s) Now you are forcing me to think, rather than be glib, as in my first post. How do you tie adjacent 4' mats together longitudinally? I would like 12x12 curbs on this bridge and they would tie things together lengthwise too. With your layout horiz. shear may be as big a problem as bending. Am not real sure about your cross framing either.

 

[oldrunner]

You should also include an impact factor for the truck loads. Load spreading of the tires can be found in AASHTO's code for bridges considering the mats as laminate members.

You can obtain the impact factor from AASHTO also. If this becomes a problem, you can set a speed limit on the bridge to a crawling pace.

Someplace in the mat suppliers literature there should be a call-out for the lumber specie and grade. If not, request the information.

 

[figurnitout]

More great questions. The bridge was originally designed to handle a Link-Belt 218 crawler crane (see attached drawings). The contractor now wants to deliver concrete via the bridge. The beam spacing was originally designed to accommodate the crane, therefore the beam spacing needs to remain as-is.

The mats are new 1' x 4' x 24' and are tied together with 5 bolts (min. 1" diameter A-36). There is no way of knowing exactly which species the mats are. They are hard woods made predominately from oak.

As far as speed, the trucks will be at a crawl.

I know there are many very simple, but not cost effective, ways to solve the problem, ie, add a 5th beam line at centerline of the bridge, double up the mats, add longitudinal timber runners to spread out the load, place steel trench plates logitudinally to distribute the load.

Thanks again for your efforts in helping me with this design.

 

[oldrunner]

Refer to Table 4A of the current NDS Design Values for Wood Construction. There are a number of types of Oak. You might compare the weakest #2 Oak and see if that is adequate.

Crawler cranes spread their loads out while wheel loads have limited spread in comparison. You should be able to work backwards the allowable stress from the bending properties and also the modulus of elasticity.

Also check to see if the 12x12's are actually 12 x 12.