Tension vs. Compression Splices 1

[Quade999]

Hi Everyone,

When using high strength steel reinforcing bars (100ksi yield strength), the compression splice length will actually be larger than the tension splice length. My question is, if I have a situation like shown below, where the region of a slab will cycle through positive and negative moment depending on where the moving load is positioned, is it ok to design the splice for the tension yield provided that any benefit of the reinforcement in compression is ignored in analysis? Normally when high strength steel is used (100ksi), we assume reinforcement is 60ksi for the design of all components, including splices and development. With 60ksi reinforcement, tension development length will almost always govern.

Thanks

 

[MIStructE_IRE]

If this isn’t already built then just go with the worst case.

If it is already built and you’re looking for some form of sanity check, I wouldn’t be concerned since you have ignored the benefit of the compression steel in any event. It will therefore be fine in my view as the full compression force is not mobilised in the bar, since it doesn’t need to be, as you’ve excluded its benefit when sizing the tension reinforcement.

 

[r13]

Agree with IRE. It seems ACI commentary has pointed out that the compression splice is more critical in bearing type situation, meaning compression bars in columns (?). Maybe someone can read over the commentary, and provide better explanation. (ACI318-14 C25.5.5.1)

 

[r13]

... when high strength steel is used (100ksi), we assume reinforcement is 60ksi for the design of all components, ...

Sorry for come back late on this, but I've to question/remind you on the use of lower yield strength (60 ksi) in place of the higher yield strength (100 ksi) for design. It defeats the purpose for selecting a higher grade steel, and most importantly, you've created the undesirable condition for brittle failure - tension capacity is much greater than the compressive strength, such that the steel will not yield when it is supposed to (@60 ksi), but the concrete has started to crush. Watch out.

 

[Quade999]

So as long as the splice isn't relied on in compression then there is no detrimental effect to it being relied upon in tension? My thought was if putting the splice under compression would effect its ability to resist the tensile load afterwards when the loading moves and the stress at the splice becomes tensile.

 

[r13]

My last comment wasn't concerned with the splice, but the fact that you have proportioned the ultimate bending strength using fy = 60 ksi, instead of the actual fy = 100 ksi. So if you determined that 1 in² of As is required for balanced failure, that is, Fcu_DGN = Ftu_DGN = Asfy_DGN = (1)(60) = 60 kips, but the beam wouldn't yield until Fcu_ACT = Ftu_ACT = Asfy_ACT = (1)(100) = 100 kips >> Fcu_DGN = 60 kips. So prior to steel yield, the concrete has already crushed under the design load.

The bottom line is, you should use the actual strength of steel, fy = 100 ksi, in moment capacity calculation, otherwise it is unsafe.

 

[BAretired]

The bottom line is, you should use the actual strength of steel, fy = 100 ksi, in moment capacity calculation, otherwise it is unsafe.

@r13,
I have never used 100 ksi reinforcement, but if any of my previous designs using 40 or 60 ksi steel were to magically become 100 ksi steel, are you suggesting that some may be unsafe? I find that difficult to believe.

Can you demonstrate how that might come about?

BA

 

[r13]

BA,

In your case, it is favorable, as the actual reinforcing steel "magically" grew stronger (40 or 60ksi became 100 ksi) than the material proper record indicated on the spec and the drawing (40 or 60 ksi). But, it will be a catastrophic failure waiting to occur, if the stronger material is used, with its property indicated on the record consistent with the furnished steel (100 ksi), but a lower strength (40 or 60 ksi) was used in the design to proportion the required area of reinforcement for the present loading conditions. Without going through the design calculation, what conclusion the next engineer will draw based on the available record?

From the philosophy of reinforced concrete design, your case is undesirable, because the beam will not give ample warning - excessive deflection under over load, prior to failure. Which is the reason I call this practice unsafe. Then, if you decide to list the lower strength on all the records, why pay more for the higher strength? I haven't looked the potential problem with the shorter splice length derived from using 60 ksi in lieu of 100 ksi, but it does not seem right at the first impression.

 

[r13]

Hope we all agree on that consistency is one of the most important factor in structural engineering, on route to achieve the goal of error free design.

 

[BAretired]

When congestion of reinforcement is considered (see below), it is understandable why engineers might consider using higher yield material. There are some dangers, however, as high strength steel, with our current technology, tends to be more brittle and does not have such a well defined yield point as lower grades. Bars are more likely to shatter when bent too sharply or too often. Also, it becomes more tempting to over reinforce a member with 100 ksi steel than it does with lower grades.

BA

 

[Quade999]

Normally it is a result of client design guidelines to assume all steel is 60ksi for design purposes when higher strength is used. Normally we will make the splices long enough for full tension yield of the higher strength and check that it should yield under ultimate loads.
We usually make all laps for tension as we don't take into account bars in compression unless they are needed.
I still go back to the original question where the splice length is set for tension, but not for compression (compression splice is longer when using high strength steel). As long as the splice isn't relied on in compression then there is no detrimental effect to it being relied upon in tension? My thought was if putting the splice under compression would effect its ability to resist the tensile load afterwards when the loading moves and the stress at the splice becomes tensile.

 

[r13]

Good picture

 

[r13]

It should be fine for YOU under the present design conditions, if you've consistently used the same yield strength in calculation. It is more crucial on how to pass the fact/make it known to the next person, who is to evaluate your work without your calculation.

 

[r13]

Quade,

This article addresses changes made in ACI318-19, that incorporating the use of yield 100 ksi steel (ASTM A1035). Though it does not answer your question/concern directly, yet it is a good reading material for your current practice. Link