Q&A About MPBZ Moment Post Base

This week’s post was written by Jhalak Vasavada, Research & Development Engineer at Simpson Strong-Tie.

This past December, Simpson Strong-Tie hosted an interactive webinar in which product manager Emmet Mielbrecht and I discussed the development, testing, evaluation and applications of our new moment-resisting MPBZ moment post base. During the one-hour webinar, we explained the testing and evaluation criteria for new product development, test procedures, installation recommendations, allowable loads and the rotational stiffness of the connection. We also included a design example. In case you missed the discussion, you can watch the on-demand webinar and earn PDH and CEU credits here.

As part of the live webinar in December, Emmet and I led a lively Q&A session with the attendees. What follows is a curated selection of those questions and answers. Click here for more answers to participant questions. 

What is the most common ultimate failure mode?

It is concrete breakout.

How is wood shrinkage addressed?

We have evaluated wood shrinkage by testing, however, it is in review with ICC-ES. Additional information shall be made available upon approval from ICC-ES.

What was the actual strength of the concrete being used in the test (not the design value)?

It was 2500 psi, +/- 10 percent.

Breakout/pryout failure seems to govern allowable loads. Are there plans to test connection with adequate reinforcement to ignore breakout failure and achieve higher allowable moment loads?

Based on the overwhelming requests for higher loads we will be testing MPBX for:

  • Higher strength concrete
  • Reinforced concrete
  • Greater edge distances

Given overlap of steel, is one direction stronger than the other and Simpson uses the weak direction for tbl?

Yes, one direction is stronger than the other and the weak direction allowable loads are listed.

Technically, the stand-off tabs and side friction will also aid in the vertical load transfer, just extremely minimal.

Correct. Our tested loads are actually higher than the screw calculations. The code requires we use the lower of the two loads, so we use the SDS screws calculated capacity only.

Why is the 4×4 stiffer than 6×6?

The stiffness in the graphs is relative to the stiffness of the post. The 6×6 post is much stiffer, so the post base is less stiff as a percentage of the 6×6 post stiffness. The actual stiffness of the MPB66Z is stiffer than the MPB44Z.

Why a F1 value for wood? And you can use the higher wood values with a proper concrete design?

The F1 values listed in the allowable load tables are the lowest of concrete and wood assembly allowable loads.

Why aren’t uplift loads for the wood connection published?

The uplift loads are limited by the lesser of the wood or concrete capacity. We lumped those together under the concrete. This will be clarified in future publications.

Relative to deflection associated with rotation at the base is that considered elastic? In other words, when the load is removed will the deflection return to zero?

Yes. Deflection associated with rotation at the base due to applied loads within allowable load range is considered elastic.

Ready to learn more about MPBZ moment post base? See the rest of the Q&A questions here and you can watch the on-demand webinar and earn PDH and CEU credits here.

Watch a free MPBZ webinar.

Join Simpson Strong-Tie R&D engineer Jhalak Vasavada, P.E., and Simpson Strong-Tie product manager Emmet Mielbrecht for a lively and informative discussion of MPBZ.

How to Pick a Connector Series – Truss Hangers

In our second blog in the “How to Pick a Connector Series,” Randy Shackelford discussed the various considerations involved in selecting a joist hanger. So why is this blog post about truss hangers? A hanger is a hanger, right? Before I moved into the Engineering Department at Simpson Strong-Tie, I was the product manager for our Plated Truss product line. I can assure you that there is a bit more that goes into the selection (and design) of a truss hanger than does into selecting a joist hanger!

Of course, all of the considerations that were covered in the joist hanger blog apply to truss hangers as well. This blog post is going to discuss some additional considerations that come into play in selecting a hanger for a truss rather than a joist, and how some hangers have features designed especially for trusses.

The first (and most obvious) truss-specific consideration is the presence of webs. Because of truss webs, top-flange hangers are not as conducive to truss applications as they are to joist applications. A better alternative for trusses is an adjustable-strap hanger that can be installed as a top-flange hanger or face-mount hanger. Take the THA29, for example, Simpson’s first hanger developed specifically for the truss industry (circa 1984). It can accommodate different girder bottom chord depths, which eliminates the need for multiple SKUs, and the straps can be field-formed over the top of the girder bottom chord to reduce the number of fasteners (just like top-flange hangers). When a web member is in the way of the top-flange installation method, the straps can be attached vertically to the web in a face-mount installation instead.

Typical THA29 Installation

What if the web at that location isn’t vertical? You can still install the strap onto the web, but if any nails land in the joint lines formed by the intersection of the wood members, they cannot be considered effective. Therefore, the hanger allowable load may need to be reduced to account for ineffective header nails. This alternative installation is acceptable for any face-mount hanger located at a panel point as shown in our catalog (see detail below).


Although very versatile, not all adjustable-strap hangers can be installed on all sizes of bottom chords. Our catalog specifies a C-dimension for these hangers, which corresponds to the height of the side-nailing flanges. If that dimension exceeds the height of the bottom chord, then the straps cannot be field-formed as needed for the top-flange installation. And if the hanger isn’t located at a panel point, nailing the straps to any diagonal web that the straps can reach (see photo below) is not an acceptable option!

The wrong hanger selection for the application

The wrong hanger selection for the application

Another unique consideration that goes into the selection of a truss hanger is the heel height of the carried truss. A truss with a short heel height installed into a tall hanger will likely leave air (or “daylight,” as I call it) behind a lot of the nail holes running up the side flanges. When nail holes in a hanger have air behind them instead of wood, this equates to a reduction in hanger capacity. So when the carried truss has a heel height that is much less than the depth of the carrying member (and the hanger), it is important to use the appropriate hanger capacity for that condition and not overestimate the hanger’s capacity. Refer to our technical bulletin T-REDHEEL for allowable loads for reduced heel height conditions.

Example of a short heel installed in a tall hanger.

Because trusses are capable of carrying a lot of load –  and producing large reactions –  hangers for truss applications often require larger capacities than joist hangers. Unfortunately, there is only so much capacity that can be achieved from a hanger that fits entirely onto a girder truss bottom chord. Therefore, in order to use our highest load-rated truss hangers, a properly located vertical web is required, and the web must be wide enough for the hanger’s required face fasteners and minimum edge distances. The more capacity that is required, the more fasteners it takes, and the wider the vertical web must be. Our highest-load-rated truss hanger that installs with screws is the HTHGQ. It has a maximum download capacity of 20,735 lb., but it requires a minimum 2×10 vertical web. The THGQ/THGQH series can be installed onto as small as a 2×6 web, but the maximum possible capacity on a 2×6 web is 9,140 lb.


In addition to high-capacity hangers, truss applications often require high-capacity skewed hangers. When selecting skewed hangers, it’s important to realize that hangers with custom skew options usually have a reduction that must be applied to the hanger’s 90-degree capacity.  Another important factor that is sometimes overlooked in the selection of skewed hangers is whether the carried member is square-cut or bevel-cut. When the member is square cut – as in the case of trusses – not only does this typically result in a greater reduction in capacity, but some skewed hangers cannot be used at all with square-cut members. For example, the fastener holes on the side flange may not be located far enough away from the header to accommodate square-cut members. See the photo below for an example of what can happen if a skewed hanger that is intended for a bevel-cut member is used for a truss.

Incorrect hanger selection – this skewed hanger requires the carried member to be bevel-cut whereas the truss is square-cut.

Incorrect hanger selection – this skewed hanger requires the carried member to be bevel-cut whereas the truss is square-cut.

Not all skewed hangers can be used with square-cut members (trusses).

Not all skewed hangers can be used with square-cut members (trusses).

As discussed in the previous hanger blog, face-mount hangers offer the advantage of being installed after the joist (or truss) is installed. What if the truss is installed prior to the hanger and a gap exists between the truss and the carrying member? In that case, the best option may be to select a truss hanger that was designed with this type of installation tolerance in mind, the HTU hanger. Other face-mount truss hangers that use double-shear nailing are great when gaps are limited to ⅛” or less, but their capacities take a pretty large hit when the gap exceeds ⅛” (see our previous blog Minding the Gap in Hangers for more information). The HTU was designed to give an allowable load for up to a ½” gap between the end of the truss and the carrying member. In addition, it has built-in nailing options to accommodate short heel heights even in the taller models – definitely a truss hanger!

HTU Hanger

HTU Hanger

Finally, there is one more thing to consider when selecting a face-mount hanger for a truss application, which relates to how tall the carrying member is compared to the hanger. Assuming the bottom of the hanger will be installed flush with the bottom of the girder bottom chord, a hanger that is much shorter than the bottom chord will induce tension perpendicular to the grain in the chord. Due to wood’s inherent weakness in perpendicular-to-grain tension, a hanger that is too short may limit the amount of load that can be transferred– to something less than the hanger’s published allowable load. Therefore, it isn’t enough to check whether the hanger fits on the bottom chord; the hanger must also cover enough depth of the chord to effectively transfer the load (or else the allowable hanger load may need to be reduced to the member’s allowable cross-grain tension limit).

Cross-grain tension is not a truss-specific issue, but because it is an explicit design provision in the truss design standard (TPI 1), it is a necessary consideration to mention in a discussion about truss hanger selection. In fact, proper detailing for cross-grain tension in different wood applications could be a future topic in and of itself.

Add to all this the specialty truss hangers that can carry two, three, four, and even five trusses framing into one location, and it is no wonder that there is an entire section in our catalog that is dedicated to truss hangers. Are there any other truss hanger needs that you would like to discuss? Please let us know in the comments below!


So, What’s Behind A Structural Connector’s Allowable Load?

This is Part 1 of a four-part series I’ll be doing on how connectors, fasteners, anchors and cold-formed steel systems are load rated.

Today I did my presentation for the WoodWorks webinar on Testing and Product Evaluation of Products for Wood-framed Construction. We covered a lot of material regarding code requirements for using alternate materials or construction methods, how testing and evaluation criteria are developed, and some specifics on several Acceptance Criteria (AC’s) that are commonly used for connector evaluations. We also discussed some specific testing requirements, so I thought it would be timely to discuss some of those issues in this week’s blog post.

So, how are structural connectors for light frame wood construction load rated? What’s behind the allowable loads information published in Simpson Strong-Tie literature or wood connector evaluation reports? These are things that you might find yourself wondering while driving to the office or jobsite, or on a Sunday afternoon while enjoying your favorite iced tea or barley-based beverage.

The short answer is: testing, calculations, and of course, sound engineering judgment.

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