can anyone explain me what is wood armer method for
bridge slab design and what is the baisc of the
same.? under what condition this method if relevent
/irrelevent....?

thanks in advance

The Wood Armer method is a way of optimising reinforcement design in a reinforced concrete slab.

The original source is the paper "The reinforcement of slabs in accordance with a pre-determined field of moments", RH Wood, 'Concrete', February 1968, and a follow-on letter from GST Armer, 'Concrete', April 1968. Their method is summarised in the book 'Concrete Bridge Design to BS 5400' by LA Clarke.

Wood Armer is usable only for design, and only where the reinforcement design is relatively straightforward (e.g. ultimate limit state analysis) - it can't optimise for designs where the relationship between reinforcement and load effect is non-linear (as is the case for crack-width limits at the serviceability limit state, for example).

Wood Armer is useful because it allows moment triads from grillages (Mx, My, Mxy) to be transformed into simple bending moments in two directions for reinforcement design. This is important because the twisting moment Mxy can be significant.

An alternative to Wood Armer is the method of Denton and Burgoyne, see 'The assessment of reinforced concrete slabs', SR Denton and CJ Burgoyne, 'The Structural Engineer', Volume 74, Number 9, May 1996. This is useful where the reinforcement is already known e.g. in a existing bridge slab.

Modern software can automate the Wood Armer or Denton Burgoyne calculations, for example, SAM-LEAP5 is a program I use which can convert the grillage moments automatically.

Mr. Brian -Thank you very much for the information

i will tryout your references

can we summarise that this method is used when we
have Mx.My & Mxy is present together.
is this of any significance if Mxy is not present OR very small ?

thanks in advance

jay

It can still be useful if Mxy is not present or is low. It can be used to transform the directions of the moments, so if the reinforcement in the slab is not parallel to the members in the grillage model, these equations can be used to correctly derive moments perpendicular or parallel to the reinforcement.

The Wood Armer Method is used for Design and it is suggested that for existing slabs which are being assessed the method of Denton & Burgoyne should be used. However, I have noticed that many engineers have used Wood Armer for assessment and TAA's seem never to challenge the validity of their approach, contrary to the views of academics. What is the difference between the two methods and why is Wood Armer not appropriate for assessment of slabs ?

Wood Armer explicitly attempts to calculate "optimum" amounts of reinforcement in two directions - it doesn't explicitly use "real" moments of resistance in any of its terms. Denton and Burgoyne does precisely that - uses actual moments and moment resistances to check whether the combined moment triad exceeds the available capacity at any point on the slab.

Because Wood Armer is about calculating optimums, it contains an inherent assumption (that the cost of reinforcement is proportional to the capacity provided). This is pretty much valid for an under-reinforced slab at ultimate limit state, but is not valid for an over-reinforced slab, or for crack width limitations.

Clark's book explains on page 59 the approach in more detail, particularly that the entire method is about optimising so you have the smallest total amount of reinforcement (rebar in both perpendicular directions). For an existing bridge, this is irrelevant - the proportion in each direction is already known. As a result, Wood/Armer can give a *conservative* result for assessment for any situation where the rebar in each direction is not optimal i.e. most real situations.

Further information is available in, for example, Appendix F of the SAM-LEAP5 user manual (if you have access to that), or in Denton and Burgoyne's paper, available online at http://www-civ.eng.cam.ac.uk/cjb/papers/p33.pdf

you can do wood armer design assessment with LUSAS , send me a mail to tech@pro-sim.com I can get more details on this

Can u just explain me how wood-armer design assessment can be done by Lusas

You can see in tutorials (pdf), I don't know why in LUSAS we must define bars spacing first? somebody know? in other FE slabs based on W&A such as REAL3D is different (not define bar spacing)