Questionnaire     
submitted by Howard Cook    

1)- As a FEMA earthquake damage inspector, I don't recall seeing a single instance where the cripple wall of a house was intact yet the house failed at the floor-to-topplate connection or floor-to-mudsill connection. However, the San Leandro prescriptive code and the L.A. prescriptive blueprint standard, both require framing anchors regardless of the number of stories. Division 92 of the L.A. code and Appendix Chapter 6 of the Uniform Code of Building Conservation require framing anchors only on multi-story buildings.

The questions are:

1) Are the framing anchors used to strengthen the floor framing-to-topplate connection necessary on single and multi-story buildings?

Ke, Ko, L, R, S, T ,B Yes  ___ No

2) Should framing anchors on the transverse walls be considered more important than on the longitudinal walls?

Ke,  Ko  L,  R,  T,  B Yes   S_ No

comment from Sonntag - They are of equal importance.

3) Since many of the nails are not visible in old pre-1950 houses, (such as in the joist blocking, rim joists, and end joists), what kind of nailing can we assume already exists.

comments from Roselund and Schmid - Transverse walls where end joist is parallel to foundation - barely any toenails.  Longitudinal walls where joists are perpindicualar to foundation - More toenails. Probably enough.

Roselund , Sonntag, and Schmid had actually observed damage from the Northridge earthquake and when asked "Have you ever seen any damage caused by the absence of these anchors?" 

They all answered "No, I have never seen any damage caused by their absence."

Schmidt answered "Once you add cripple wall sheathing and anchors, the force has to be transferred to the top-plate"

Relevance to the code: Even though Sonntag, Roselund, and Schmid never observed any damage caused by their absence, all engineers recommended their use. This is because they all believed it is important to maintain a good load path. From what I have seen, most existing retrofits do not have framing anchors on either the transverse or longitudinal walls. Based on their belief that there are probably few toenails in the joist-to-topplate connection on the transverse walls, Schmid and Roselund both disagree with the L.A. code which says the connection of the joist-to-topplate on the transverse walls need not be verified on one story buildings.

1a) While doing remodels of older homes I have noticed that on the transverse walls (where the end joist runs parallel to the foundation), there is usually only one toenail every six feet connecting the end joist to the cripple wall topplate.  The longitudinal walls (the joists run perpendicular to the foundation) usually have one toenail in each joist, as well as a few toenails in the rim joist or blocking. From what I have seen, therefore, this nailing provides a much stronger joist-to-topplate connection in the longitudinal walls than found in the transverse walls.

The questions are:

1) Should framing anchors on the transverse walls be considered more important than on the longitudinal walls?

  R,  B Yes ___  No  S, B, Ke, Ko, L, T Equal importance

comments from Ke, Ko, L, S, T - They are of equal importance. There is no way to tell how many toenails are actually present.  For this reason they should be installed regardless of whether the wall is transverse or longitudinal.

comment from Roselund - I agree with the statement above.  They are relatively cheap and installing them should not substantially increase the cost of a retrofit The longitudinal walls where joists run perpendicular to the foundation normally have many more toenails.   I would say that on most existing houses the connection of the floor framing to the topplate on the longitudinal sides is probably sufficient.

2) Are framing anchors necessary to attach the end or rim joist to the subfloor necessary along the transverse or longitudinal walls in either single story or multi-story buildings?  These are installed by screwing or palm nailing them into the subfloor and then nailing them into the side of the joist.  Please see the San Leandro planset.

 _____Yes      Ke, Ke, L, R, S, T ,B No

RELEVANCE TO CODE: It is currently being debated by the code committee whether or not framing anchors should be installed to attach the tops of the rim joists or blocking to the sole plate of the first story wall.  The engineers who took part in this survey seem to think these anchors are not necessary.

2) One engineer told me that over-stiffening the cripple wall can actually damage a house. I quote him "Over-stiffening the cripple wall can damage the house.  There is no research that will prove this, but I know it is true!"  If that is the case, then how should we define over-stiffening?  Maybe sheathing over 50% of the wall should be considered over-stiffening.  Maybe the shearwalls should be nailed 6 inches on center on the edges rather than at 4 inches so that they are not so stiff?  The policy for most retrofit contractors, as well as for the existing retrofit building codes, is to nail up the strongest most rigid shear panels possible.  Maybe this is an error. Maybe they should be nailed at only 6 inches on center to reduce the stiffness differential between the floor diaphragm and the shearwall. I was told that the dissimilar rigidities between the floor diaphragm and the plywood shearwall limit the ability of earthquake forces to transfer from the less rigid floor diaphragm into the stiffer shearwall. Is this true?

The question is:

Can over-stiffening a cripple wall damage a house?

 ____Yes     Ke, Ke, L, R, S, T,B No

comment from Loar- Over-stiffening a cripple wall can damage a house if the opposing wall is not equally braced and nearly as stiff.  This can cause the house to rotate around the stiffer wall and end up damaging the house.

RELEVANCE TO CODE: Not a great deal of relevance except that this is a common misconception in the engineering and contracting community.  The importance of installing an equal amount of plywood on each cripple wall, especially cripple walls on opposing sides of the house, and how much of a differential should be allowed, may require further investigation. Existing site conditions often make the installation of equal lengths of shearwalling impossible. It also means minimum standard for bracing can be set and that additional bracing probably will not compromise the effectiveness of the retrofit.

3) Section A604.3.2 of Appendix Chapter 6 of the 1997 UCBC states "Chemical anchors or expansion bolts shall be placed within 12 inches but not less than 9 inches from the ends of the sill plates." In retrofit, this is often not possible to do. Some engineers believe that all the earthquake forces go to the stiffest element, (in this case the shear panel is the stiffest element), and therefore only the bolts that are attached to the shear panel absorb any force.

The question is:

Is it true that all the earthquake force goes to the stiffest element and therefore only to the shear panels?  Or, as Nels Roselund puts it: most of the force goes to the stiffest element; slightly less force goes to a slightly less stiff element.

 Ke, Ke, L, R, S, T Yes  _____ No  B Indeterminate

RELEVANCE TO CODE: This is extremely relevant, as it means bolting outside of shear panel locations is probably not necessary and also very expensive.   It appears that installing bolts that are not connected to the shear panels, while adding greatly to the cost of a retrofit, offer little or no protection.

4a) If the retrofit design makes sure that the shear capacity of the bolts attaching the shear panel to the foundation matches the capacity of the shear panel itself, is it still important to put bolts between 8 and 12 inches of the mudsill ends as currently required by the UBC and UCBC?

____Yes    Ke, Ke, L, R, S, T, B No

comments from Roselund, Tipping, and Kennedy - Bolts at the end of mudsills are not necessary so long as each section of mudsill attached to a shearwall has at least two bolts to prevent out of plane rotation

Relevance to code: This information is relevant because it means it is not necessary to maintain UBC bolt end distances when doing retrofit work.  This is important because it is often not possible to maintain these distances due to lack of access.  The critical thing appears to be in maintaining equality in strength capacities between the shearwalling and the bolts attached to the shearwall.

4b) If it is true that most of the force goes to the stiffest element; slightly less force goes to a slightly less stiff element, how much earthquake load is transferred to the part of the cripple wall that has no shearwalling?

from Loar, Komendan t -Only a negligible amount of earthquake force transfers to the unbraced sections of cripplewall.

from Sonntag, Tipping, Kennedy ,- No shear forces will travel to the sections of cripple wall that have no plywood shear-walling. It is therefore not necessary to install bolts along the mudsill at non-shearwall locations.  Whatever is currently holding the mudsill in place at non-shearwall locations is probably enough.

from Schmid - Indeterminate, but assume the stucco on exterior sheathing offers some resistance.

from Roselund - Very little shear forces will  travel to the sections of cripple wall that have no plywood shear-walling, depending on the existing wall sheathing – stucco in good condition will carry a lot of shear, but once the nails have corroded, it's not worth much.

4c) In a retrofit, is it necessary to bolt the sections of wall currently braced with horizontal siding or stucco that have no additional plywood  shearwalling?

_B Yes     Ke, Ke, L, R, S, T No

Relevance to code: This information is relevant because all existing retrofit building codes (UCBC,GREB,San Leandro, and Los Angeles) now recommend installing bolts 6 feet on center regardless of the placement of the shear walls. This results in the installation of many redundant bolts and unnecessarily increases the cost of the retrofit.

5) A new single story UBC foundation with a 12-inch footing provides 501plf of shear resistance in the soil (167plf per square foot of surface area times 3). Many older foundations either have no footing, or the footings only extend 4 inches into the ground.  Technically, these foundations with no footing provide approximately 167 plf of shear resistance. If an older 25foot long foundation and cripple wall were retrofitted with new 535 plf shearwalls on 100% of its length, it would have only 4175 plf of foundation (25 x 167) carrying 13,375 plf of shearwall .  This does not include the static resistance caused by the weight of the building.  I am asking this question because one engineer who works for many of the realtors in the East Bay tells his clients that the foundation need to be replaced if the footing is shallow because it cannot resist earthquakes. This is important to know since the California Dept. of Insurance often replaces foundations because they are cracked or slightly rotated in their retrofit grant program.

comments from Roselund, Loar, Komendant, Sonntag, Tipping, Kennedy, Schmidt -The shallow footings will make no difference in keeping the house attached to the foundation and this is the purpose of retrofit work.  It is not important if the footings are shallow.  At the very worst the foundations may move slightly on top of the ground, but again, the important thing is to keep the house on the foundation.

comment from Sonntag - I guess the question is:  Is it necessary to retrofit the foundation? No.

Relevance to code: There are a lot of misconceptions concerning the function of foundations in retrofit work.  Many engineers and contractors recommend foundation replacements unnecessarily because the foundations are cracked or have shallow footing.  The function and purpose of foundations should be addressed in the commentary section of the code.  The city of Berkeley authorizes the retrofit transfer tax retrofit rebate to be used for any and all foundation work and probably shouldn't.  The American Society of Home Inspectors would like to know the significance of many of the problems they find in older foundations.

6) Should the principle of rotation be allowed in retrofit work? Use of the principle of rotation in retrofit and in new construction was disallowed in L.A. after the Northridge earthquake. The most common application of this principle is found in Section 2315 of the 1997 code. This is used to protect a room above a garage by shearwalling the back wall of the garage.

The question are:

1) Should the application of the principle of rotation as found in Section 2315 be allowed in retrofit work to protect a room above a garage?

 Ke, Ke, L, R, T, B Yes  S No

2) If using the principle of rotation is the only retrofit option available, should it be used or will it probably cause more harm than good.

Ke, Ke, L, R,  T, B Should be used    S Should not be used

comment from Schmidt - Only use this if designed by a structural engineer.

comment from Roselund - It will still do a house a lot of good even if only three sides of the house can be retrofitted, this is true even if the minimum aspect ratios required by the code cannot be met. The principle of rotation will still protect the unbraced wall.  In this case the braced  wall opposite the unbraced wall should have twice as much bracing installed as it normally would.

comment from Schmid and Sonntag - Don't use it where there is a room above a garage.

Relevance to code: This is relevant because it is often not possible to retrofit a building with a weak story without installing a very expensive steel frame, which most homeowners cannot afford. The commentary by Mr. Roselund is important because it implies that a retrofit is still worth doing even if one of the four cripple walls cannot be braced as per the requirements of the new code so long as the opposing cripple wall can be braced. If this is done, Mr. Roselund recommends that this opposing wall should be braced with twice as much shearwalling as the new code would normally require.  Mr. Kennedy stated that this principle of rotation will only work in this case if it involves the longitudinal walls and not the transverse walls.

7) The new retrofit code may be based on a minimum base shear value. In the 1997 UBC, the base shear formula in seismic zone 4, with the fault proximity multiplier, is V=0.26W.

The questions are:

1) Does designing the new retrofit code around this base shear formula provide adequate protection?

Ke, Ke, L, R, S, T,B Yes    ___No

2) Should the new retrofit code be designed to give greater protection than the base shear formula provides?

____Yes    Ke, Ke, L, R, S, T,B No

Relevance to Code: This is relevant because the base shear formula should probably form the engineering background of the new code.

3) Is 75% of the base shear formula still adequate?

comment from Schmid - Design for 75% of base shear formula.

comment from Roselund - The 75% is allowed for historic buildings.

comment from Sonntag - I would design a retrofit at 50% of the base shear formula.

Relevance to Code: This is relevant because the base shear formula should probably form the engineering background of the new code.  It may even be that even 75% of the formula is sufficient.

8)- Another problem we frequently see in the field is that of loose nuts on the bolts. This happens because green lumber was used when the house was built, the nuts were tightened down on this green lumber; then the lumber dried out, shrunk and the nuts are no longer tight. We have found that most existing houses have nuts that can be removed from the bolts with your fingers .

The questions are:

1) Is this a significant problem?

 ____Yes   Ke, Ke, L, R, S, T,B No

2) Is the simple procedure of installing plate washers and tightening the nuts worth the $1,500.00 expense for an average house without a cripple wall?

____Yes    Ke, Ke, L, R, S, T,B No

comments from Schmid and Roselund: So long as the house is adequately bolted, and there is not a cripple wall, the only damage would occur where the mudsill meets the stucco or horizontal siding. The house would not slide off of the foundation. This should be explained to the homeowner so that he can decide for himself if this work is still worth doing.

Relevance to Code: This is important when dealing with houses that do not have cripple walls in order to determine whether or not it is still cost effective to go back and install plate washers on the existing bolts.

9)- Many houses have additions.  These additions are typically on the back of the house. The horizontal sheathing or stucco is usually stripped from the rear cripple wall of the house when the addition is added.  One end joist of the new addition is nailed to the end joist of the old house.  This provides vertical support of the addition on one side. New foundation is then installed on the remaining three sides of the addition.  These additions can often only be identified beneath the house because the flooring is different. For example, the floor of the old house will be diagonally sheathed while the new addition will be strait sheathing.  According to all existing retrofit building codes, the way to approach this retrofit would be to retrofit all four sides of the house.  In this case that would be the rear cripplewall of the addition, the two side cripplewalls of the main house and the addition, and the front cripple wall of the main house.  The problem is that where the addition is attached to the main house, we have two floors that are only marginally connected. A shear wall placed here has to be a high capacity shearwall since it has to resist forces generated by both floors. These connections must absorb earthquake forces generated by the main house as well as those forces generated by the addition.

The questions are:

1) If a prescriptive retrofit plan is used, is it important to connect these two floors together so that the two of them act as one floor diaphragm?

 Ke, Ke, L, R, S, T,B Yes  ____No

2) Should the connection at the two floors be able to work in tension as well as in shear, or only in shear?

 Ke, Ke, L, R, S, T,B Tension and shear  ____Shear only  ___Both, but mostly in shear

3) Is it only necessary to connect the two floors together and then brace the rear of the addition, or is it also necessary to brace the cripple wall where the old floor and the new floor meet?

_R_ Connect floors only    Ke, Ke, L, R, S, T,B Connect floors plus brace cripple wall.

4) Should the connection of separately framed floor diaphragms be considered in the new retrofit building code?

Ke, Ke, L, R, S, T ,B Yes  ____No

comment from Roselund - If this is done, it is not necessary to brace the rear addition if the addition is small. The principle of rotation will take care of that.

comment from Tipping - It is still a good idea to brace the addition given that the added cost is minimal. Even though the principal of rotation should probably take care of it, I would rather rely on cripple wall bracing than the principle of rotation.  However, I would only add one tension tie on each end of the connecting floors to hold the two floors togehter.

Relevance to Code: Additions are often found on the back of older homes.  The new code should probably address the problem of having two differently framed floor diaphragms and how the two should be connected.

10)- It is my understanding that all four cripplewalls must resist the same amount of force. Therefore all four sides of the house must equally restrain this movement by firmly attaching the floor diaphragm to the foundation on all four sides of the house. All three existing retrofit building codes, (San Leandro, L.A., and the UCBC) are designed in such a way that they call for a percentage of the cripple wall to be braced with plywood; depending on the number of stories (weight) of the house.  Using the percentages in these codes on a 24' by 48' one-story house, the transverse walls would have plywood bracing on 12 linear feet of the cripple walls and the longitudinal walls would have 24 linear feet of bracing on the cripple walls. Not only might it be redundant, but it also increases the cost of the average retrofit by one third.

The questions are:

1) In the example given above, are 12 linear feet of the 24 feet of bracing on the longitudinal walls redundant?

 Ke, Ke, L, S, T, R, B Yes  ____No

comment from Roselund - I favor sticking with the arbitrary requirements since we do no know which is more nearly correct for a given house, the longer or the shorter.  The arbitrary length-of-bracing-panel requirements of the retrofit code are modeled after bracing rules of the Conventional Light-Frame Construction provisions of the UBC.  The rules are arbitrary, and in the case you cite, can not be consistent with the expected results of an engineering analysis.  However, if we do not base the bracing panel design on a lateral analysis, we should stick with the arbitrary rules – at least we are being consistent.  To arbitrarily install no more in the long direction than would be required by the arbitrary rules for the short direction may result in inadequate bracing in both directions – it is trying to engineer a better design without an engineer. We are putting the retrofit code together for use by a homeowner and/or a small contractor to use without engineering, and at reasonable cost. It will provide an effective strengthening, but may not provide the certainty of effectiveness that an engineered design would.  For a large house, an unusual house or a homeowner who wants greater certainty about the effectiveness of the retrofit, an engineer should be hired.

2) Would the amount of cripple wall bracing on each wall be better determined by the weight of the building rather than by the percentage of the wall?

 Ke, Ke, L, R, S, T,B Yes  ____No

3) Should a retrofit code keep the cost of implementation of this code in mind by eliminating redundancy whenever possible?

 Ke, Ke, L, R, S, T,B Yes ____No

comment from Schmid - Our L.A. SEAOSC commentary says each shear wall should have equal length to match maximum required by percentage of length.

Relevance to Code: All existing retrofit building codes determine the amount of plywood cripple wall bracing on a percentage basis.  The result is that twice as much bracing ends up on the transverse walls as on the longitudinal walls.  For example, the San Leandro Code recommends that 50% of the cripple wall be braced on a single story home.  From an engineering point of view this is ineffective. It also greatly increases the cost by installing cripple wall bracing that is not necessary.  This discouraging people from retrofitting their homes.   It appears that the engineering community believes a method based on the weight of the building should be used rather than using the percentage of the length of the wall. A possibility for the code would be to require possibly 10 linear feet of shearwalling on each wall any house less than 1200sf. Anything over 1200sf but less than 2000sf could require 14 linear feet of shearwalling.  The amount of shearwalling would thus increase as the squarefootage increases and therefore as the weight of the building increases.

12) There are some disparities in capacity that arise out of the current retrofit methods found in all the prescriptive standards. For example, using the San Leandro Prescriptive Standard, a 40-foot long cripple wall would have 32 framing anchors with a total capacity of 18,500 pounds of shear resistance. It would have twelve ½ inch bolts with a capacity of 10,080 pounds of shear resistance.  And finally it would have 20 linear feet of plywood shear wall with about 8000 pounds of capacity. In addition, for a 25 by 40 foot house, the base shear formula of V=0.26W each wall would only require 3,400 pounds of shear resistance on each of the connections that the San Leandro standard addresses.  This is one of the reasons no one uses the San Leandro standard.

The question is:

Should the new code try to make sure that the bolting, framing anchors, and shearwalls have an equal capacity?

 Ke, Ke, L, R, S, T,B Yes  _S_ No

comment from Sonntag - Not necessarily. You should have bolts 6' o.c. to prevent warping of the sill plates no matter what the load is.

Relevance to code: I think the relevance of this to the code should be obvious. The method used in new construction and in all existing retrofit building codes that recommends bolts, plywood, and framing anchors be installed so many inches on center, is probably a poor way to design a retrofit building code. This method results in a serious and unnecessary disparity in capacities in the three areas of a house that are the most likely to fail.  The installation of redundant shearwalling and hardware also increases the cost of the retrofit.  Increased cost is a disincentive to retrofitting.

12)- Until the 1997 code was adopted, many homes were built with 28 inch wide walls on either side of the garage door opening, even when there is a room above the garage.  On many homes the walls are not even this wide. Many contractors install bolts, holdowns, and plywood on these walls at great expense to the homeowner and the homeowner then believe that this very vulnerable part of the house is now protected. The cost to install these narrow shear walls, bolts, and holdowns is probably around $4,500.00. The concrete is usually not reinforced and it is difficult to keep the required edge distance from the holddown bolt to the end of the concrete.

The questions are:

1) Is retrofitting these narrow walls still a good idea?

Ke, Ke, L, R, S, T,B Yes ____No

2) In this situation, how much good will the holdowns do?  PHD5 holddowns cost about $140.00 each to install.

from Sonntag and Tipping - A lot, limiting deflection.

3) Will the protection afforded to do this for $4500.00 be a good investment?

 Ke, Ke, L, R, S, T Good protection for cost __ Marginal protection _B_  Little or no protection

comment from Roselund -Don't do this without an engineer.

comment from Loar - It would be better to shear the back wall of the garage, better yet, do both.

comment from Schmid - The frame per the attached copy is easy to install, but does require design by an engineer. The good news is that the welds are shop fabricated and base plate bolts are easy to install and typically, the existing slab can be used.

Relevance to code: Many older homes built before the 1997 code came into effect are built with very narrow sections of wall on either side of the garage.  These garages often support a room above them. This is a weak part of the house and the code should address the cost effectiveness of retrofitting narrow walls on either side of the garage door opening to protect the living area above it.

13) Many contractors use angle irons to bolt a house to the foundation.  Use of angle irons is a local technique that is mainly found in the Bay Area. The angle iron has three holes in it; two holes attach it to the concrete and the other hole allows it to be bolted to a floor joist. My understanding is that if the distance from the top bolt in the concrete to the bolt in the joist is 12 inches, and if the distance between the two bolts in the concrete is only 6 inches, then 1000 pounds of force in the F1 direction will produce 2000 pounds of force on the lower concrete bolt and 3000 pounds of force on the upper concrete bolt and cause it to shear off.  My understanding is that angle irons were looked at by the committees that wrote the three existing retrofit building codes and that they were rejected as a viable retrofit method.  The answer to this question is very important since literally thousands of homes in the Bay Area have been retrofitted and are still being retrofitted in this way. Please think carefully about this as this is a method used by many contractors and engineers in the Bay Area.  If you do not recommend this technique, and if most engineers participating in this survey agree with you, your answer will be immediately given to the Building Official for the city of Berkeley and other Bay Area municipalities.

The questions are:

1) Are angle irons a viable retrofit method when trying to resist lateral loads? Ke, R, Yes        L, T ,B, S, Ko No

2) Should angle irons be allowed in the new retrofit code?

 Ke, R, Yes, but only with adequate bolt spacing    L, T ,B, S, Ko No

3) If they are allowed, should there be some minimum distance ratio required for the bolts?

Ke, Ko, L, R, S, T, B Yes      ____No

comment from Tipping - This is a convoluted load path that will probably break down when trying to get the load into the angle iron.

comment from Loar - This is a poor method because of torsion that will be created at the connections, no matter what the bolt spacing is.

comment from Schmidt - It is too difficult to develop necessary blocking and shear clips to carry load.

Relevance to code: This technique for attaching the floor of a house to the foundation was developed many years ago and is still extensively used throughout the Bay Area.  Thousands of houses have been retrofitted in the Bay Area using this method. Ben Schmid, S.E., of Southern California, once told me that this method was evaluated by L.A.'s retrofit code committee and rejected as a viable retrofit method.  It is important for the commentary section of the code to address this retrofit method because it has been so widely used in the Bay Area.  It is also important for homeowner's who have already had this type of work done on their homes to know whether or not it will be effective. The American Society of Home Inspectors is also quite interested in knowing what they should tell their clients when they see this method used in the field.  Given the variety of opinions on this technique, further engineering analysis appears to be needed.

14) Concerning the installation of 2 x 4 blocks on top of the 2 x redwood mudsill so that the bottom edge of the plywood has something to nail to; the blocks on the mudsill may not be necessary.  The edge of the 2 by mudsill can be cut flush with the studs so that the edge of the mudsill is on the same plane as the studs and topplate.  This eliminates the need for blocking. When flush-cutting no more than 1 ½ inch of the redwood mudsill is ever removed. I have always been a little concerned about blocking based on the following observations:

A) The short blocks split easily when you drive nails through the tops of them.  They are especially prone to splitting when even more nails are driven through the side of the block when the plywood is installed. When nailing the plywood you cannot see where in the block the nails are landing.  Any nail less than an inch from the edge of the block will certainly split the block.  Many contractors use full length 10-penny nails because they believe they make a stronger shearwall. These nails will certainly split out the blocks.

B) There is an engineering problem when the blocks are used. In a shear wall four feet long, the three 13 ½ inch long 2x4 blocks are nailed to a total of 40-1/2 inches of mudsill. The 2 inch wide studs take up the other 7-1/2 inches.  Each block can hold four 16d nails without splitting.  Each block therefore provides 400 pounds of shear resistance to the mudsill. This is based on the values given on Table 4 of NER 272 that gives 100 pounds for each 16d sinker (94x1.33x0.8 for redwood) with 1-1/2 inches of penetration.  The three blocks with 4 nails therefore provide 400 plf of attachment to the mudsill.

The questions are:

1) Is it too much to ask a 13½inch 2 x 4 block to pick up and transfer 400 pounds of force?

           Yes   Ke, Ko, L, R, S, T,B No

2) Will nailing the bottom edge of the plywood shearwall directly into the mudsill allow the load to be better distributed along the mudsill?

 Ke, Ko, L, R, S, T, B   Yes  __S__ No

C) Since most contractors will use green lumber when making these blocks, the nuts and nails will loosen up once the blocks lose moisture and shrink.

The questions are:

1) Is this a significant problem?

Ke, Ko, L, R, S, T Yes    S, B_ No

2) Is nailing the bottom edge of a plywood shearwall directly into an old growth redwood mudsill better than nailing it into new growth Douglas Fir blocks? In my experience, old growth redwood seems very hard relative to new growth redwood or even new growth Douglas Fir.

  Ke, Ko, L, R, S, T Yes  _S_ No

3) Will reducing the Redwood mudsill from a full dimension 2 x 5 1/2 to a full dimension 2 by 4 seriously compromise the strength of the sill and therefore of the shearwall?

_S, B Yes   Ke, Ko, L, R, S, T No

comment from Sonntag - Don't do it

comment from Tipping - Intuitively, I don't like cutting away any of the mudsill. I can't give you an engineering justification for this opinion

comment from Schmid - This does not look possible, it cannot be done with a skilsaw or a sawzall. Nailing into 2 by 4 blocks is not a problem so long as the carpenter PREDRILLS the nails.

4) Will this significantly increase damage from out of plane shear?

   S Yes    Ke, Ko, L, R, S, T No

5) Is splitting of the blocks from top and side nailing a serious concern if the 2 by 4 blocks on the mudsill are not pre-drilled for the nails?

 Ke, Ko, L, R, S, T, B Yes    ____No

comments from Schmid,  Sonntag-Redwood has a 20% reduction in allowable value for shear compared to D.F.

Relevance to Code: It is important to address the different methods that can be used to attach a shear wall to an existing mudsill.  This is especially important because thousands, perhaps tens of thousands, of Bay Area Homes have been retrofitted using the nailed block method.  Very few contractors and engineers realize that a stapler exists that shoots 2 ½ inch long 15 gauge staples or that a saw exists that can flush cut a mudsill.

15) How important is the blocking around small cutouts?  The radii are easy to do but the blocking is, again, pretty labor intensive.

The questions are:

1)When calculating capacity, can you just ignore the reduction in strength caused by the unblocked cut-out?

Ke, Ko, L, R, S, T, B Yes    ____No

comment from Roselund, Loar, Komendant, Sonntag, Tipping, Kennedy, Schmid  Small unblocked cut-outs do not significantly effect the performance of a shearwall.

2) Is drilling a few holes in the plywood at vent locations just as effective as blocking around the vent so long as the crawlspace area has no signs of ever being moist?

 Ke, Ko, L, R, S, T, B Yes         ____No

comment from Schmid: Plywood with 3" diameter holes were tested at U.C. Irvine. No reduction in strength was noted on 16" o. c. cripplewalls.

Relevance to code: This is important because it is often necessary to put small holes in shearwalls to allow for the penetration of pipes, ventilation, etc.  The official position of the American Plywood Association is that unblocked holes in plywood less that 12 inches wide are not a serious concern and do not need to be blocked. Installing blocking around small openings in shear walls takes a lot of time and requiring that it be done in the new code will unnecessarily increase the cost of the retrofit.

16)- One structural engineer told me that shear walls that are less that 24 inches in height do not require field nailing, just as a shear walls that are only 24 inches wide require no field nailing.

The questions are:

1) In the case of shear walls less than 24 inches in height, is edge nailing still necessary on the end studs?

 Ke, Ko, L, R, S, T, B   Yes ___No

comment from Loar, Komendant, Sonntag, Tipping, Kennedy -Edge nailing is not necessary as long as the cripple wall is less than 12 inches in height, even though the code will allow no edge nailing for shearwalls up to 24 inches in height

comment from Roselund: where does the Code say that?. I would require nailing on the end stud unless the stud is so short that splitting is likely. Two 8d nails in an 8" long stud does seems reasonable to get 8d @ 4" nailing.

comment from Schmid: Use 2x lumber instead of narrow pieces of plywood. The code requires that the minimum width of plywood be at least 16 inches. ( I contacted Tom Skaggs of the APA about this, and in his opinion it does not matter how narrow the plywood is so long as nail edge distances can be maintained)

comment from Roselund: add a tightly-fit 2x6 stud at the end. I would not omit the end nailing except as mentioned above.

Relevance to Code: This is relevant because it is often necessary to build shearwalls on cripple walls that are extremely short.  It is always a concern that the short studs will split. It is also often the case that the 2 by 4 topplate and mudsill are in the same plane, while the 2 by 4 studs are recessed in.  This disallows nailing the plywood onto the studs because of the air gap between the plywood and the studs.

17)- The UCBC and San Leandro retrofit codes require 2 to 3 inch ventilation holes in cripple wall sheathing.  The American Plywood Association requires 1-inch ventilation holes in their cripple wall sheathing. In my company, we use 1-1/8 inch ventilation holes. The larger 2 to 3 inch holes must be made with a hole saw or star bit and are difficult to drill and very time consuming.  The 1-1/8 inch or 1 ½ inch ventilation holes can be drilled with a spade bit and they can be drilled quickly.

The questions are:

1) How important are ventilation holes when working in a crawlspace that is obviously always dry?

_B_ Very important     Ke, Ko, L, R, S, T Not important.

comment from Roselund-Not important for ventilation, but are important for bolt inspection holes.

comment from Schmid-Do not use such little holes, what does the American Plywood Association know anyway? Don't use spade drills. Having used a sharp hole drill 3" diameter on many panels, there is no problem unless you are trying to do bad work.

2) One person on the committee is concerned that the spade bit damages the plywood when it punches through the other side. Is this damage a significant concern?

_S, B_ Yes    Ke, Ko, L, R, S, T No

18)- One recommendation may be that a single shearwall be installed in the center of the cripple wall and that any breaks in the topplates along that wall be tied together with steel straps, or the top topplate and the bottom topplate will be nailed together at splice locations, so that the top topplate is continuous.

The question is:

 In this case, how important is it to have a continuous topplates?

Ke, Ko, L, R, S, T, B Very important  __Mildly important  S_ Not important

comment from Sonntag -At the worst case, when a  cripple wall ends just off a splice on the top plate, there will be some stress transfer through the floor and back to the top plate-so don't worry.

Relevance to Code: Many older homes are built with 4 by 4 or 2 by 4 single topplates. Base on the engineering data above, it may be a good idea to have a provision that requires breaks in these topplates be spliced together with steel.

19) How important is the end blocking on the joists? The L.A. prescriptive blueprint calls for the installation of joist blocking on every other joist for a one-story building when no blocking or rim joist is already present.  If the bottoms of the joists are secured by Simpson H-10R-s that provide 580plf, and the tops of the joists are secured by the nailing of the subfloor, are these blocks really necessary? See detail at right.

The questions are:

1) Is there much of a chance that these joists will rotate?

Ke, Ko, L, R, S, T, B Yes    ____No

comment from Roselund - This is an uncertain yes.  This hardware should be tested. It will probably work but I cannot be sure. This hardware needs more testing.

comment from Sonntag-A little chance, but the blocking is for shear transfer too.

2) If the bottoms of the joists are only secured by toenails into the topplate and not by H10R's, and there is no joist blocking, is there much of a chance that these joists will rotate?

 Ke, Ko, L, R, S, T, B Yes        No

comment from Roselund-This should also be tested, I have never seen joists rotate in an earthquake.

Relevance to Code: This is relevant only because many older homes do not have any end blocking between the floor joists. Installing blocking is quite expensive compared to installing Simpson H-10R hardware. It appears from the data above that further testing of this hardware should be done.  Maybe someone can call Simpson and see what they think.

20)- When building retrofit shear panels we try to double up the studs at all vertical splices and nail the vertical 2bys four inches o.c. staggered.   In a recent seminar it was mentioned that bolting these two vertical studs together would be better.  Can you comment on this? How much better is bolting?  When is it especially important; the taller the shear panel the more the importance?

The question is:

Is bolting of the two vertical members much better than nailing?

_____ Yes    Ke, Ko, L, R, S, T, B   No

Relevance to code: The code should specify how plywood splices and doubled 2 by 4 posts required for hold downs should be addressed.

A few more observations from Ben Schmid, S.E.

Don't use green lumber!  Our jobs allow 19% maximum moisture content in Southern California.  Green lumber is not necessary when installing blocking on top of the mudsill. (I contacted Tom Skaggs of the APA about this and it was his opinion that the use of green lumber does not significantly affect the strength of the shearwalls)

PREDRILL when nailing anything into these blocks.

DO NOT USE NAIL GUNS, PERIOD.

Staples are not permitted in Southern California, Natural moisture will ruin them.

ADDITIONAL OBSERVATION

Table A-1-D on page 25 of the 1997 UCBC, which is the same as Table 4-E of the GREB, lists allowable shear values for archaic materials. Are the values listed here accurate? Many archaic materials are not listed there.  This is important when trying to determine whether it is worthwhile to install 400plf of plywood on a wall with stucco or sheetrock that may already have a value far in excess of 400plf.  Accurate values for archaic materials used in retrofit work should be tabulated in the new code in a manner similar to GREB Table 4-E.

Howard Cook

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