50 Tips for Designing Constructable Steel Buildings Q and A.pdf

50 Tips for Designing Constructable Steel Buildings Additional Q and Q and A

AISC Live Webinar TOPIC

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QUESTION

ANSWER

connecti ctions

Would you allow welded splice (both flanges and web) at the Whether we would permit such a connection configuration would depend same location (PJP) (splicing is done without offsetting for on the magnitude of the of the load, size of the of the member and size of the of the PJP weld. fabricaton ease, fit‐up and simplicity)?

connecti ctions

AISC does not have any prohibitions against the specification of double of double Is it permissible to specify double nuts for high‐strength bolts nuts. However, the available thread length on structural fasteners is for dyanamic load applicationns (crane, machinery)? This is to typically less than the thickness of 2 of 2 heavy‐hex nuts. This may be a ensure that bolts do not get loosened. consideration when selecting nuts.

connecti ctions

How does multiple bolt sizes improve economy versus using only one size bolt for the primary connections? Does this apply for 1, 2 or 3 story buildings?

One bolt size is usually used for the majority of connections. of connections. (Most fabricators use either 3/4" or 7/8" diameter bolts as their standard bolt.) A second bolt size (at least two sizes larger to avoid mix‐up in the field) is often used for connections with very heavy reactions. Bottom line: Allow the fabricator to select the bolt type and size that they find to be most economical.

Does pretension means snug tight?

No. Snug‐tight is the basic installation method for connections using high‐ strength bolts. Pretensioning of bolts of bolts is required for connections specified as pretensionsed or slip critical. RCSC Specification Sections 4.2 and 4.3 list applications that require pretensioned and slip‐critical connections, respectively. AISC Specification Section J1.10 lists applications where pretensioned connections are required.

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connections

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connecti ctions

Where are the minimum gage requirements given based on bolt sizes?

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connecti ctions

Do you suggest to use any particular software to design the connections?

Copyright (C) 2012 American Institute of Steel of Steel Construction

Workable gage for W‐shapes is listed in AISC Manual Table 1‐1 and those for angles are listed in Table 1‐7a. We primarily use spreadsheets written in‐house for connection design.

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50 Tips for Designing Constructable Steel Buildings Additional Q and A


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connections (SSL)

connections 8 (SSL)

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connections (SSL)

QUESTION

ANSWER

If the bolts are pre‐tensioned (as is required by AISC 360‐10 for buildings Tip #8 ‐ I see the use of snug tight short slotted holes more taller than 125') then you will get some slip resistance parallel to the slot and more. I don't like it from the fact that you don't get a which will help the beams brace the columns. The floor slab as well as minimum amount of axial capacity out of the connection and members framing perpendicular to the direction of column buckling will possibly are not bracing the column out of plane. Agree? also help brace the columns.

On slides 18‐20, tip #5 for SSL holes, bolts must have to be pre tensionned and on page 23, tip #8, it is possible to use snug tight. What is the diffeence between the 2 notations?

AISC 360‐10, Section J1.10 specifies situations where pre‐tensioned bolts MUST be used. RCSC Sections 4.2 and 4.3 addresses joints that are required to be PT or SC. The intent of tip #5 was to illustrate that bearing bolt shear strength values can usually be used when SSL holes with loads perpendicular to the slot or Standard size holes are used, but that these bolts may, in certain situations have to be pre‐tensioned. When the situations listed in Section J1.10 are not applicable, then snug‐tight bolts can be used. Bearing bolt shear strength values are the same for both pre‐ tensioned bolts and snug‐tightened bolts.

Often most or all perimeter roof beams are either diaphragm chords or drag struts. They carry significant axial load, in addition to vertical shear. Is it appropriate to use SSL holes with bearing bolts in this case? Won't the accumulated deflection and impact as the bolts move in the slots be detrimental to structure performance under lateral load?

This is an engineering judgement decision that the EOR must make. Specifying that the beam‐to‐column connections be detailed with pre‐ tensioned bolts and SSL holes may be appropriate. Pre‐tensioned bolts will offer some slip resistance. At the other extreme, if the diaphragm chord forces are substantial, then relying on shear connections alone to resist axial forces may be inappropriate. In such situations slip‐critical bolted connections may be appropriate if the forces are substantial.

Copyright (C) 2012 American Institute of Steel Construction

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QUESTION

ANSWER

connections

Personally I'm always afraid to connect a beam to the flange of columns. So I add a Tee, exactly what you state not to do because the connection would be more expensive.

Many steel‐framed buildings have been successfully constructed with beams framing to column flanges using a variety of connections. I know of no problems occuring due to such connections.

connections

The message that I was trying to convey was that approximately 50% of the cost of a steel structure is related in one way or another to the You stated that 50 % cost of steel is in connection which connections ‐ thus connection cost is a substantial percentage of the total appears to be high. Regardless, it is unlikely that the cost. Each fabricator has their own method of determining their bid price. contractor prices the steel structure separately, the cost of Some may use an average cost per ton. That cost includes costs associated main members and connections prior to the bidding. Is it not with the connections. The cost that fabricators associate with connections true that the contractor prices steel based on tonnage with is not arbitrary. It is one based upon experience. Projects with many small certain percentage for connection arbitrarily determined by members and many connections will cost more per ton than projects with previous experience? fewer members, heavier members and fewer connections. Fabricators do not price projects based upon tonnage alone.

connections

I am not sure that I understand your question. You mention one consideration that results from OSHA regulations. There was a May 2001 Are there any OSHA regulations that impact bolted Modern Steel Construction article that summarized the OSHA regulations. connection design such as the use of single sided connections You can find it here: or connections that share the same bolts? http://www.modernsteel.com/Uploads/Issues/May_2001/0105_02_OSHA .pdf. Please send any additional questions to [email protected]


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QUESTION

What is the cutoff between a full‐pen weld and fillet weld (economically)?

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connections

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What about specifying shop welding and field bolting for connections connection? Good idea?

Don't fabricators routinely consider increasing the member sizes to simplify the connections?

ANSWER

Different fabricators may have different responses to this question. The answer will also vary depending on the thickness of the element being welded. We have designed fillet welds as big as 3/4" rather than use groove welds. The advantage of fillet welds is that they require no preparation or beveling of the elements to be welded. Chapter 14 of AISC Design Guide 21 discusses aspects of welding economy. It does not give a cutoff, but provides discussion of things that can affect the economy of CJP, PJP and fillet welds.

We leave this decision on whether to bolt or weld up to the fabricator. Some shops are set up primarily for welding. Other shops are set up primarily for bolting.

Fabricators may occasionally request to increase a beam size to facilitate a connection detail, however to my knowledge, this practice is rare. It is best that the EOR consider during design whether a member should be increased in size to simplify the connections.

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connections

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You are correct. All things equal I would orient the columns so that the girders frame to the flanges. I probably should have turned the columns in FYI: the sketch in Tip #38 has the columns oriented 90 connections that illustration. That said, girder connections to column webs are not degrees from the recommendation in Tip #10. Please clarify. uncommon. The intent of slide #38 was to discuss the benefits of increasing slab span to reduce the number of beams.


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partial restr. Connections

partial restr. connections

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delegation

QUESTION

ANSWER

Can practicing engineers design semi‐rigid connections? If yes, how and to what degree of fixed connections?

This is the paper on partially restrained beams that Mr. Schwinger was referring to: Engineering Journal: A Simplified Look at Partially Restrained Beams by Louis F. Geschwindner. http://www.aisc.org/store/p‐808‐a‐ simplified‐look‐at‐partially‐restrained‐beams.aspx. As well, please check out this recorded presentation by Dr. Geschwindner as well. Free to view. http://www.aisc.org/content.aspx?id=19860

What paper did you just mention written by Dr. Geschwindner? How can I retrieve that paper?

This is the paper on partially restrained beams that Mr. Schwinger was referring to: Engineering Journal: A Simplified Look at Partially Restrained Beams by Louis F. Geschwindner. http://www.aisc.org/store/p‐808‐a‐ simplified‐look‐at‐partially‐restrained‐beams.aspx. As well, please check out this recorded presentation by Dr. Geschwindner as well. Free to view. http://www.aisc.org/content.aspx?id=19860

Is there any statistical method to derive contructability factor I am not aware of any such statistical analysis. or in other words cost benefit analysis?

Do you advocate delegation of ordinary connections to the fabricator?


Yes. The majority of connections in steel‐framed buildings are standard shear connections. The bulk of the connection cost is in these ordinary connections. It is therefore best for the EOR to show the reactions and permit the fabricator to design and detail their preferred (most economical) connection.

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QUESTION

ANSWER

You are correct. Fortunately connection designer usually only needs to know the maximum moment when the EOR determines whether column stiffeners or web doubler plates are required (as they should per AISC 303‐ 10). The problem occurs when engineers require connection designers to use the full member strength to design moment connections and when Providing loads for all load cases for moment connections can they require the connections to be designed to accomodate the moments delegation get quite lengthy. When delegating moment connection acting in any direction for determining if stiffeners or doubler plates are (moment conn) design, what is the most efficient way to convey the design required. If the EOR is going to require the connection designer to check if loads? stiffeners and doublers are required (not recommended) then they must provide the individual dead, live, wind and seismic moments so that the connection designer can check the various load combinations. This is a lot of work for both the EOR as well as the connection designer. The extra work can be avoided if the EOR determines if stiffeners and doubler plates are required (and sizes them accordingly if they are required).


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QUESTION

Is it standard for EOR's to delegate moment connection delegation design, when the configuration and/or behavior of that (moment conn) connection can directly impact the size/performance of the members being joined...?


ANSWER

When moment connection design is delegated to the fabricator's connection design engineer, the EOR must provide any limitations and restrictions pertaining to the connection design so that the structural framing will perform in the manner expected by the EOR. If a certain type of moment connection is not acceptable to the EOR, then that information must be conveyed to the connection designer. For example if the EOR requires that moment connections be fully rigid connections (versus semi‐ rigid), then that requirement must be explicitly stated. If, for example the EOR wants end‐plate moment connections at beam‐to‐column connections, then that requirement must be explicitly stated. If flange‐ bolted moment connections might be used, then the EOR must make sure that the holes in the beam flanges at the connections will not reduce the available member strength and if it might be an issue, then the EOR must specify a maximum permitted bolt diameter, etc. or the EOR must make certain that the flanges are sufficiently wide so that strength at the net section (at the holes) will not be the limit state for beam flexural strength.

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delegation

delegation

QUESTION

ANSWER

Is it common for the EOR to delegate the design of special connections to the fabricator without specifying the actual design forces? Say the design dead, live and wind loads are only specified on the drawings.

The AISC Code of Standard Practice (COSP) requires that the “designated representative for design shall provide…Data concerning the loads, including shears, moments, axial forces and transfer forces, that are to be resisted by the individual members and their connections , sufficient to allow the selection, completion, or design of the connection details while preparing the shop and erection drawings .” It does not get into particulars about how this must be done. If connection design is being delegated to be designed by an engineer working for the fabricator, then it is acceptable for the connection engineer to be expected to calculate load combinations based on load cases given in the contract documents. If connections are delegated to be "selected" connections and do not require a licensed engineer providing a sealed design, it is not appropriate for the fabricator/detailer to be expected to calculate loads per ASCE 7.

For "Delegated connection design" option, is it acceptable if the fabricator is providing connection without using the services of a professional engineer?

There are 3 options given in the AISC Code of Standard Practice Section 3.1.2: (1) The complete connection design shall be shown in the structural design drawings, (2) In the structural design drawings or specifications, the connection shall be designated to be selected or completed by an experienced steel detailer, or (3) In the structural design drawings or specifications, the connection shall be designated to be designed by a licensed professional engineer working for the fabricator. In option (2), the "selection" of a connection from provided or specified tables does not require a licensed engineer. In item (3) the design of connections does require a licensed engineer.


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delegation

QUESTION

ANSWER

How do you factor in contractor/erector competence? For example with the bolts all being the same size, I see this being Specifying an AISC Certified company will ensure that the fabricator and done by some engineers to ensure the right parts are used in erector have a comprehensive QA/QC program. the right places. Or even switching between bolt steel grades.

delegation

If connection design is delegated and there is a failure, who gets sued?

I am not an attorney, so I do not know the legal answer to this question. Suffice to say that everyone will be sued if there is a structural failure. In my opinion, delegating connection design does not relieve the EOR of ultimate responsibility for properly designing the structure (including the connections). When delegating connection design, the EOR must follow a standard of care to insure that the connections were properly designed by the fabricator's connection design engineer. Structural failures are almost never the result of one error. It is typically a series of errors that leads to a failure.

delegation

How practical is it to require the delegated connection design engineer to indicate the capacity of the connection provided for each connection on each beam on the piece sheets of the shop drawings?

Your sugggestion is both practical and is a good idea. We require fabricators to indicate both the required connection strength (shown on the drawings) and the provided connection strength (computed by the fabricator's connection design engineer) adjacent to each connection on the piece drawings. Most fabricators can accommodate this requirement with their detailing software.

delegation

If you do have drawings with these notes, how does a fabricator request reactions, request single sided connections, etc., to the EOR?

The EOR must follow the the procedures outlined in AISC 303‐10, Section 3.1.2. indicating the reactions on the drawings and stating any specific restrictions regarding connection design. If no restrictions are stated, then the fabricator is free to provide any type of shear connection they desire.


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delegation

QUESTION

ANSWER

When contractor designs the connections, is he expected to have a connection design engineer stamp the shop drawings and calculations and submit them to the engineer of record?

It depends on what is specified in the contract documents. This is discussed in the Commentary to AISC Code of Standard Practice Section 3.1.2, which states, "The owner’s designated representative for design may require that the substantiating connection information be signed and sealed for option (3). The signing and sealing of the cover letter transmitting the shop and erection drawings and substantiating connection information may suffice. This signing and sealing indicates that a professional engineer performed the work but does not replace the approval process provided in Section 4.4. A requirement to sign and seal each sheet of the shop and erection drawings is discouraged as it may serve to confuse the design responsibility between the owner’s designated representative for design and the licensed professional engineer’s work in performing the connection design."


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QUESTION

ANSWER

delegation

The EOR must provide sufficient information to permit the connection design engineer to consider the full envelope of loads for which connections are subjected. When a connection design is governed only by Would you discuss axial loads you have mentioned in laterally the maximum value of the load, then the EOR needs only to provide that loaded frames versus gravity loading only? Can you discuss maximum value. When a connection design may be influenced by loads how these, along with shear and moment loads are shown on acting in different directions then the EOR must provide the maximum and drawings? Do you have a good example of this? minimum shears, moments or axial loads acting on the connection. Most connections are governed only by the maximum loads applied. For those members for which connection design may be affected by maximum or minimum loads, both values should be indicated.

delegation

There are 3 options given in the AISC Code of Standard Practice Section 3.1.2: (1) The complete connection design shall be shown in the structural design drawings, (2) In the structural design drawings or specifications, the connection shall be designated to be selected or completed by an experienced steel detailer, or (3) In the structural design drawings or specifications, the connection shall be designated to be designed by a licensed professional engineer working for the fabricator. In option (2), the "selection" of a connection from provided or specified tables does not require a licensed engineer. In item (3) the design of connections does require a licensed engineer.

For "Delegated connection design" option is it acceptable if the fabricator is providing connections without using the services of a professional engineer?


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QUESTION

ANSWER

delegation

If we are delegating connection design to the fabricator's engineer, how are we supposed to avoid the need for stiffener and doubler plates. Seems like the issue will be discovered too late in order to avoid these costly connections.

The need for stiffeners should be investigated by the EOR during design (before the drawings are issued for bid). Some design and analysis software has the ability to analyze and size columns so that stiffeners are not required. Manual calcualations to investigate if stiffeners are required can be facilitated using information provided in Table 4‐1 in the AISC Steel Construction Manual.

delegation

I disagree. All engineering firms should have an in‐house Quality Assurance process to reduce the likelihood of mistakes. Specifying that connections be designed for strengths far in excess of the actual reactions is wasteful When all the connections are deligated to fabricator, we need and usually not in the best interest of the owner. Engineering firms should to provide the reactions for each and every connections. strive to establish fees that will allow them to produce efficient designs Therefore I believe that , it may lead to more mistakes. Do with sufficient time to produce high quality contract documents that have you agree? been checked and reviewed for mistakes. The additional cost to the owner for producing high quality structural drawings will be less than the cost of providing oversized connections.

details

I recently had a problem with my column caps plates being a problem for beam erection due to some osha requirements. Are column caps plates necessary for anything other than deck support and what are others doing to support deck at columns if cap plates are a problem?

Column cap plates may not always be necessary.

As a structural engineer would you allow unity check of 1.03 general design (Loads/Capacity) since calculations are done using computers I would strive to keep the strength ratio to 1.0 or less. very precisely?


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QUESTION

ANSWER

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The design process is often a fluid one and design loads are occasionally Do design loads always remain constant? It seems to me that revised during the design process. Hence the need for structural engineers general design the design loads often change with the design progress and to be proactive in communicating with their clients and other consultants. engineer/architect continued interaction. Document all decisions regarding the agreed up design loads, so that if they later change the design can be revised accordingly.

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In designing tube to tube connections with heavier moments, should the EOR identify needed location for interior tube Yes. AISC 303‐10 requires the EOR to determine the locations and sizes of stiffeners to at least enable reasonable bidding since the EOR column stiffeners. controls member size (ie options to avoid interior stiffeners)

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HSS

members

members

Do you have a rule of thumb for when to use a deep beam instead of a fabricated truss?

No. The optimal span/depth ratio for a truss is about 10. The decision on whether to use a truss versus a plate girder depends on the available structure depth, coordination issues with MEP systems and cost considerations. The decision of whether or not to use plate girders versus trusses is not a decision to be made by the EOR alone.

For supporting slabs, beams or open web steel joists? Which is better?

There are many variables to be considered when selecting the optimal floor framing system. Issues such as fire ratings, ceiling construction, vibration criteria, availability of materials, available structure depth, future flexibility, foundation loads, siesmic loads, mechanical systems above the ceiling, and sound transmission are but some of the variables that may affect the decision to go with joists versus composite beams. There are advantages and disadvantages to all systems.


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members

members

R=3

QUESTION

ANSWER

In tip #38 spacing the beams save cost on steel but what about the cost of the extra concrete due to the added thickness? Is it still more efficient this way?

We find that it is often most cost efficient to thicken the slab on metal deck in order to increase the slab span and reduce the number of beams. The thickened slab is accomplished by using deeper floor deck. Increasing the deck from 1.5" to 3" adds the equivalent of only 0.75" of additonal slab thickness. Exceptions to this rule of thumb may be in buildings where the structure depth must be minimized to the absolute mimimum and structures where floors are being added on top of existing buildings and the structure self ‐weight must be minimized.

Does tip #38 apply to steel trusses with steel deck?

Not necessarily. The decision whether to use closely spaced joists with a thin slab, versus composite beams supporting a thicker slab is not one to be made by the EOR alone. Variables such as joist cost, vibration considerations, structure depth, mechanical systems and fire rating issues may influence determination of the most appropriate floor framing system.

In high seismic zones, is it allowable to designate a braced frame as "R=3" and design for the higher forces but save on overstrength connections, etc? Where is that code reference? Thanks.

ASCE 7 Table 12.2‐1 lists systems allowed in various seismic design categories. System H in this table (the R=3 system) is not permitted in SDC D, E or F.


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R=3

QUESTION

ANSWER

What R can be used for high seismic applications in order to avoid special connection and member design provisions?

In seismic design categories B or C, System H in ASCE 7 table 12.2‐1 (the R=3) system will have the least detailing requirements. In Seismic Design Categories D, E and F, the least amount of seismic detailing is associated with the "ordinary" systems. However, there are many restrictions on their use (such as height). Typically there is not an advantage to IMF over SMF in seismic controlled systems because the connection requirements are nearly identical.

Because R=3 systems are not permitted in seismic design categories D, E or F by ASCE 7 Table 12.2‐1.

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R=3

Why would anyone design "Ordinary" seismic frames when "R=3" does not result in significantly higher seismic forces (R=3 versus R=3.25 or 3.5)?

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R=3

What section of ASCE 7 or AISC 341 permits R=3 for an OMF ?

R=3 is not allowed for an OMF. R factors and limitations on system usage are found in ASCE 7 Table 12.2‐1.

R=3

Even if you design arbitraily for R=3 for seismic design categories D, E and F, don't you have to detail the connections per high seismic risk design detail procedures.?

The R=3 system (System H in ASCE 7 Table 12.2‐1) is not permitted in seismic design categories D, E and F. In these SDC's the appropriate R‐ factor, design and detailing requirements are those associated with the system being used.

R=3

You mentioned you can use an R of 3 for braced frames. Where does ASCE 7 allow this?

ASCE 7 Table 12.2‐1 system H (R=3 system). It refers to ASCE 7 Section 14.1 for detailing requirements. The last sentence of ASCE 7 Section 14.1.2 allows the use of system H without AISC 341.

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50 Tips for Designing Constructable Steel Buildings Q and A.pdf

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